1======================== 2ftrace - Function Tracer 3======================== 4 5Copyright 2008 Red Hat Inc. 6 7:Author: Steven Rostedt <srostedt@redhat.com> 8:License: The GNU Free Documentation License, Version 1.2 9 (dual licensed under the GPL v2) 10:Original Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton, 11 John Kacur, and David Teigland. 12 13- Written for: 2.6.28-rc2 14- Updated for: 3.10 15- Updated for: 4.13 - Copyright 2017 VMware Inc. Steven Rostedt 16- Converted to rst format - Changbin Du <changbin.du@intel.com> 17 18Introduction 19------------ 20 21Ftrace is an internal tracer designed to help out developers and 22designers of systems to find what is going on inside the kernel. 23It can be used for debugging or analyzing latencies and 24performance issues that take place outside of user-space. 25 26Although ftrace is typically considered the function tracer, it 27is really a framework of several assorted tracing utilities. 28There's latency tracing to examine what occurs between interrupts 29disabled and enabled, as well as for preemption and from a time 30a task is woken to the task is actually scheduled in. 31 32One of the most common uses of ftrace is the event tracing. 33Throughout the kernel is hundreds of static event points that 34can be enabled via the tracefs file system to see what is 35going on in certain parts of the kernel. 36 37See events.rst for more information. 38 39 40Implementation Details 41---------------------- 42 43See Documentation/trace/ftrace-design.rst for details for arch porters and such. 44 45 46The File System 47--------------- 48 49Ftrace uses the tracefs file system to hold the control files as 50well as the files to display output. 51 52When tracefs is configured into the kernel (which selecting any ftrace 53option will do) the directory /sys/kernel/tracing will be created. To mount 54this directory, you can add to your /etc/fstab file:: 55 56 tracefs /sys/kernel/tracing tracefs defaults 0 0 57 58Or you can mount it at run time with:: 59 60 mount -t tracefs nodev /sys/kernel/tracing 61 62For quicker access to that directory you may want to make a soft link to 63it:: 64 65 ln -s /sys/kernel/tracing /tracing 66 67.. attention:: 68 69 Before 4.1, all ftrace tracing control files were within the debugfs 70 file system, which is typically located at /sys/kernel/debug/tracing. 71 For backward compatibility, when mounting the debugfs file system, 72 the tracefs file system will be automatically mounted at: 73 74 /sys/kernel/debug/tracing 75 76 All files located in the tracefs file system will be located in that 77 debugfs file system directory as well. 78 79.. attention:: 80 81 Any selected ftrace option will also create the tracefs file system. 82 The rest of the document will assume that you are in the ftrace directory 83 (cd /sys/kernel/tracing) and will only concentrate on the files within that 84 directory and not distract from the content with the extended 85 "/sys/kernel/tracing" path name. 86 87That's it! (assuming that you have ftrace configured into your kernel) 88 89After mounting tracefs you will have access to the control and output files 90of ftrace. Here is a list of some of the key files: 91 92 93 Note: all time values are in microseconds. 94 95 current_tracer: 96 97 This is used to set or display the current tracer 98 that is configured. Changing the current tracer clears 99 the ring buffer content as well as the "snapshot" buffer. 100 101 available_tracers: 102 103 This holds the different types of tracers that 104 have been compiled into the kernel. The 105 tracers listed here can be configured by 106 echoing their name into current_tracer. 107 108 tracing_on: 109 110 This sets or displays whether writing to the trace 111 ring buffer is enabled. Echo 0 into this file to disable 112 the tracer or 1 to enable it. Note, this only disables 113 writing to the ring buffer, the tracing overhead may 114 still be occurring. 115 116 The kernel function tracing_off() can be used within the 117 kernel to disable writing to the ring buffer, which will 118 set this file to "0". User space can re-enable tracing by 119 echoing "1" into the file. 120 121 Note, the function and event trigger "traceoff" will also 122 set this file to zero and stop tracing. Which can also 123 be re-enabled by user space using this file. 124 125 trace: 126 127 This file holds the output of the trace in a human 128 readable format (described below). Opening this file for 129 writing with the O_TRUNC flag clears the ring buffer content. 130 Note, this file is not a consumer. If tracing is off 131 (no tracer running, or tracing_on is zero), it will produce 132 the same output each time it is read. When tracing is on, 133 it may produce inconsistent results as it tries to read 134 the entire buffer without consuming it. 135 136 trace_pipe: 137 138 The output is the same as the "trace" file but this 139 file is meant to be streamed with live tracing. 140 Reads from this file will block until new data is 141 retrieved. Unlike the "trace" file, this file is a 142 consumer. This means reading from this file causes 143 sequential reads to display more current data. Once 144 data is read from this file, it is consumed, and 145 will not be read again with a sequential read. The 146 "trace" file is static, and if the tracer is not 147 adding more data, it will display the same 148 information every time it is read. 149 150 trace_options: 151 152 This file lets the user control the amount of data 153 that is displayed in one of the above output 154 files. Options also exist to modify how a tracer 155 or events work (stack traces, timestamps, etc). 156 157 options: 158 159 This is a directory that has a file for every available 160 trace option (also in trace_options). Options may also be set 161 or cleared by writing a "1" or "0" respectively into the 162 corresponding file with the option name. 163 164 tracing_max_latency: 165 166 Some of the tracers record the max latency. 167 For example, the maximum time that interrupts are disabled. 168 The maximum time is saved in this file. The max trace will also be 169 stored, and displayed by "trace". A new max trace will only be 170 recorded if the latency is greater than the value in this file 171 (in microseconds). 172 173 By echoing in a time into this file, no latency will be recorded 174 unless it is greater than the time in this file. 175 176 tracing_thresh: 177 178 Some latency tracers will record a trace whenever the 179 latency is greater than the number in this file. 180 Only active when the file contains a number greater than 0. 181 (in microseconds) 182 183 buffer_percent: 184 185 This is the watermark for how much the ring buffer needs to be filled 186 before a waiter is woken up. That is, if an application calls a 187 blocking read syscall on one of the per_cpu trace_pipe_raw files, it 188 will block until the given amount of data specified by buffer_percent 189 is in the ring buffer before it wakes the reader up. This also 190 controls how the splice system calls are blocked on this file:: 191 192 0 - means to wake up as soon as there is any data in the ring buffer. 193 50 - means to wake up when roughly half of the ring buffer sub-buffers 194 are full. 195 100 - means to block until the ring buffer is totally full and is 196 about to start overwriting the older data. 197 198 buffer_size_kb: 199 200 This sets or displays the number of kilobytes each CPU 201 buffer holds. By default, the trace buffers are the same size 202 for each CPU. The displayed number is the size of the 203 CPU buffer and not total size of all buffers. The 204 trace buffers are allocated in pages (blocks of memory 205 that the kernel uses for allocation, usually 4 KB in size). 206 A few extra pages may be allocated to accommodate buffer management 207 meta-data. If the last page allocated has room for more bytes 208 than requested, the rest of the page will be used, 209 making the actual allocation bigger than requested or shown. 210 ( Note, the size may not be a multiple of the page size 211 due to buffer management meta-data. ) 212 213 Buffer sizes for individual CPUs may vary 214 (see "per_cpu/cpu0/buffer_size_kb" below), and if they do 215 this file will show "X". 216 217 buffer_total_size_kb: 218 219 This displays the total combined size of all the trace buffers. 220 221 buffer_subbuf_size_kb: 222 223 This sets or displays the sub buffer size. The ring buffer is broken up 224 into several same size "sub buffers". An event can not be bigger than 225 the size of the sub buffer. Normally, the sub buffer is the size of the 226 architecture's page (4K on x86). The sub buffer also contains meta data 227 at the start which also limits the size of an event. That means when 228 the sub buffer is a page size, no event can be larger than the page 229 size minus the sub buffer meta data. 230 231 Note, the buffer_subbuf_size_kb is a way for the user to specify the 232 minimum size of the subbuffer. The kernel may make it bigger due to the 233 implementation details, or simply fail the operation if the kernel can 234 not handle the request. 235 236 Changing the sub buffer size allows for events to be larger than the 237 page size. 238 239 Note: When changing the sub-buffer size, tracing is stopped and any 240 data in the ring buffer and the snapshot buffer will be discarded. 241 242 free_buffer: 243 244 If a process is performing tracing, and the ring buffer should be 245 shrunk "freed" when the process is finished, even if it were to be 246 killed by a signal, this file can be used for that purpose. On close 247 of this file, the ring buffer will be resized to its minimum size. 248 Having a process that is tracing also open this file, when the process 249 exits its file descriptor for this file will be closed, and in doing so, 250 the ring buffer will be "freed". 251 252 It may also stop tracing if disable_on_free option is set. 253 254 tracing_cpumask: 255 256 This is a mask that lets the user only trace on specified CPUs. 257 The format is a hex string representing the CPUs. 258 259 set_ftrace_filter: 260 261 When dynamic ftrace is configured in (see the 262 section below "dynamic ftrace"), the code is dynamically 263 modified (code text rewrite) to disable calling of the 264 function profiler (mcount). This lets tracing be configured 265 in with practically no overhead in performance. This also 266 has a side effect of enabling or disabling specific functions 267 to be traced. Echoing names of functions into this file 268 will limit the trace to only those functions. 269 This influences the tracers "function" and "function_graph" 270 and thus also function profiling (see "function_profile_enabled"). 271 272 The functions listed in "available_filter_functions" are what 273 can be written into this file. 274 275 This interface also allows for commands to be used. See the 276 "Filter commands" section for more details. 277 278 As a speed up, since processing strings can be quite expensive 279 and requires a check of all functions registered to tracing, instead 280 an index can be written into this file. A number (starting with "1") 281 written will instead select the same corresponding at the line position 282 of the "available_filter_functions" file. 283 284 set_ftrace_notrace: 285 286 This has an effect opposite to that of 287 set_ftrace_filter. Any function that is added here will not 288 be traced. If a function exists in both set_ftrace_filter 289 and set_ftrace_notrace, the function will _not_ be traced. 290 291 set_ftrace_pid: 292 293 Have the function tracer only trace the threads whose PID are 294 listed in this file. 295 296 If the "function-fork" option is set, then when a task whose 297 PID is listed in this file forks, the child's PID will 298 automatically be added to this file, and the child will be 299 traced by the function tracer as well. This option will also 300 cause PIDs of tasks that exit to be removed from the file. 301 302 set_ftrace_notrace_pid: 303 304 Have the function tracer ignore threads whose PID are listed in 305 this file. 306 307 If the "function-fork" option is set, then when a task whose 308 PID is listed in this file forks, the child's PID will 309 automatically be added to this file, and the child will not be 310 traced by the function tracer as well. This option will also 311 cause PIDs of tasks that exit to be removed from the file. 312 313 If a PID is in both this file and "set_ftrace_pid", then this 314 file takes precedence, and the thread will not be traced. 315 316 set_event_pid: 317 318 Have the events only trace a task with a PID listed in this file. 319 Note, sched_switch and sched_wake_up will also trace events 320 listed in this file. 321 322 To have the PIDs of children of tasks with their PID in this file 323 added on fork, enable the "event-fork" option. That option will also 324 cause the PIDs of tasks to be removed from this file when the task 325 exits. 326 327 set_event_notrace_pid: 328 329 Have the events not trace a task with a PID listed in this file. 330 Note, sched_switch and sched_wakeup will trace threads not listed 331 in this file, even if a thread's PID is in the file if the 332 sched_switch or sched_wakeup events also trace a thread that should 333 be traced. 334 335 To have the PIDs of children of tasks with their PID in this file 336 added on fork, enable the "event-fork" option. That option will also 337 cause the PIDs of tasks to be removed from this file when the task 338 exits. 339 340 set_graph_function: 341 342 Functions listed in this file will cause the function graph 343 tracer to only trace these functions and the functions that 344 they call. (See the section "dynamic ftrace" for more details). 345 Note, set_ftrace_filter and set_ftrace_notrace still affects 346 what functions are being traced. 347 348 set_graph_notrace: 349 350 Similar to set_graph_function, but will disable function graph 351 tracing when the function is hit until it exits the function. 352 This makes it possible to ignore tracing functions that are called 353 by a specific function. 354 355 available_filter_functions: 356 357 This lists the functions that ftrace has processed and can trace. 358 These are the function names that you can pass to 359 "set_ftrace_filter", "set_ftrace_notrace", 360 "set_graph_function", or "set_graph_notrace". 361 (See the section "dynamic ftrace" below for more details.) 362 363 available_filter_functions_addrs: 364 365 Similar to available_filter_functions, but with address displayed 366 for each function. The displayed address is the patch-site address 367 and can differ from /proc/kallsyms address. 368 369 dyn_ftrace_total_info: 370 371 This file is for debugging purposes. The number of functions that 372 have been converted to nops and are available to be traced. 373 374 enabled_functions: 375 376 This file is more for debugging ftrace, but can also be useful 377 in seeing if any function has a callback attached to it. 378 Not only does the trace infrastructure use ftrace function 379 trace utility, but other subsystems might too. This file 380 displays all functions that have a callback attached to them 381 as well as the number of callbacks that have been attached. 382 Note, a callback may also call multiple functions which will 383 not be listed in this count. 384 385 If the callback registered to be traced by a function with 386 the "save regs" attribute (thus even more overhead), a 'R' 387 will be displayed on the same line as the function that 388 is returning registers. 389 390 If the callback registered to be traced by a function with 391 the "ip modify" attribute (thus the regs->ip can be changed), 392 an 'I' will be displayed on the same line as the function that 393 can be overridden. 394 395 If a non ftrace trampoline is attached (BPF) a 'D' will be displayed. 396 Note, normal ftrace trampolines can also be attached, but only one 397 "direct" trampoline can be attached to a given function at a time. 398 399 Some architectures can not call direct trampolines, but instead have 400 the ftrace ops function located above the function entry point. In 401 such cases an 'O' will be displayed. 402 403 If a function had either the "ip modify" or a "direct" call attached to 404 it in the past, a 'M' will be shown. This flag is never cleared. It is 405 used to know if a function was every modified by the ftrace infrastructure, 406 and can be used for debugging. 407 408 If the architecture supports it, it will also show what callback 409 is being directly called by the function. If the count is greater 410 than 1 it most likely will be ftrace_ops_list_func(). 411 412 If the callback of a function jumps to a trampoline that is 413 specific to the callback and which is not the standard trampoline, 414 its address will be printed as well as the function that the 415 trampoline calls. 416 417 touched_functions: 418 419 This file contains all the functions that ever had a function callback 420 to it via the ftrace infrastructure. It has the same format as 421 enabled_functions but shows all functions that have every been 422 traced. 423 424 To see any function that has every been modified by "ip modify" or a 425 direct trampoline, one can perform the following command: 426 427 grep ' M ' /sys/kernel/tracing/touched_functions 428 429 function_profile_enabled: 430 431 When set it will enable all functions with either the function 432 tracer, or if configured, the function graph tracer. It will 433 keep a histogram of the number of functions that were called 434 and if the function graph tracer was configured, it will also keep 435 track of the time spent in those functions. The histogram 436 content can be displayed in the files: 437 438 trace_stat/function<cpu> ( function0, function1, etc). 439 440 trace_stat: 441 442 A directory that holds different tracing stats. 443 444 kprobe_events: 445 446 Enable dynamic trace points. See kprobetrace.rst. 447 448 kprobe_profile: 449 450 Dynamic trace points stats. See kprobetrace.rst. 451 452 max_graph_depth: 453 454 Used with the function graph tracer. This is the max depth 455 it will trace into a function. Setting this to a value of 456 one will show only the first kernel function that is called 457 from user space. 458 459 printk_formats: 460 461 This is for tools that read the raw format files. If an event in 462 the ring buffer references a string, only a pointer to the string 463 is recorded into the buffer and not the string itself. This prevents 464 tools from knowing what that string was. This file displays the string 465 and address for the string allowing tools to map the pointers to what 466 the strings were. 467 468 saved_cmdlines: 469 470 Only the pid of the task is recorded in a trace event unless 471 the event specifically saves the task comm as well. Ftrace 472 makes a cache of pid mappings to comms to try to display 473 comms for events. If a pid for a comm is not listed, then 474 "<...>" is displayed in the output. 475 476 If the option "record-cmd" is set to "0", then comms of tasks 477 will not be saved during recording. By default, it is enabled. 478 479 saved_cmdlines_size: 480 481 By default, 128 comms are saved (see "saved_cmdlines" above). To 482 increase or decrease the amount of comms that are cached, echo 483 the number of comms to cache into this file. 484 485 saved_tgids: 486 487 If the option "record-tgid" is set, on each scheduling context switch 488 the Task Group ID of a task is saved in a table mapping the PID of 489 the thread to its TGID. By default, the "record-tgid" option is 490 disabled. 491 492 snapshot: 493 494 This displays the "snapshot" buffer and also lets the user 495 take a snapshot of the current running trace. 496 See the "Snapshot" section below for more details. 497 498 stack_max_size: 499 500 When the stack tracer is activated, this will display the 501 maximum stack size it has encountered. 502 See the "Stack Trace" section below. 503 504 stack_trace: 505 506 This displays the stack back trace of the largest stack 507 that was encountered when the stack tracer is activated. 508 See the "Stack Trace" section below. 509 510 stack_trace_filter: 511 512 This is similar to "set_ftrace_filter" but it limits what 513 functions the stack tracer will check. 514 515 trace_clock: 516 517 Whenever an event is recorded into the ring buffer, a 518 "timestamp" is added. This stamp comes from a specified 519 clock. By default, ftrace uses the "local" clock. This 520 clock is very fast and strictly per cpu, but on some 521 systems it may not be monotonic with respect to other 522 CPUs. In other words, the local clocks may not be in sync 523 with local clocks on other CPUs. 524 525 Usual clocks for tracing:: 526 527 # cat trace_clock 528 [local] global counter x86-tsc 529 530 The clock with the square brackets around it is the one in effect. 531 532 local: 533 Default clock, but may not be in sync across CPUs 534 535 global: 536 This clock is in sync with all CPUs but may 537 be a bit slower than the local clock. 538 539 counter: 540 This is not a clock at all, but literally an atomic 541 counter. It counts up one by one, but is in sync 542 with all CPUs. This is useful when you need to 543 know exactly the order events occurred with respect to 544 each other on different CPUs. 545 546 uptime: 547 This uses the jiffies counter and the time stamp 548 is relative to the time since boot up. 549 550 perf: 551 This makes ftrace use the same clock that perf uses. 552 Eventually perf will be able to read ftrace buffers 553 and this will help out in interleaving the data. 554 555 x86-tsc: 556 Architectures may define their own clocks. For 557 example, x86 uses its own TSC cycle clock here. 558 559 ppc-tb: 560 This uses the powerpc timebase register value. 561 This is in sync across CPUs and can also be used 562 to correlate events across hypervisor/guest if 563 tb_offset is known. 564 565 mono: 566 This uses the fast monotonic clock (CLOCK_MONOTONIC) 567 which is monotonic and is subject to NTP rate adjustments. 568 569 mono_raw: 570 This is the raw monotonic clock (CLOCK_MONOTONIC_RAW) 571 which is monotonic but is not subject to any rate adjustments 572 and ticks at the same rate as the hardware clocksource. 573 574 boot: 575 This is the boot clock (CLOCK_BOOTTIME) and is based on the 576 fast monotonic clock, but also accounts for time spent in 577 suspend. Since the clock access is designed for use in 578 tracing in the suspend path, some side effects are possible 579 if clock is accessed after the suspend time is accounted before 580 the fast mono clock is updated. In this case, the clock update 581 appears to happen slightly sooner than it normally would have. 582 Also on 32-bit systems, it's possible that the 64-bit boot offset 583 sees a partial update. These effects are rare and post 584 processing should be able to handle them. See comments in the 585 ktime_get_boot_fast_ns() function for more information. 586 587 tai: 588 This is the tai clock (CLOCK_TAI) and is derived from the wall- 589 clock time. However, this clock does not experience 590 discontinuities and backwards jumps caused by NTP inserting leap 591 seconds. Since the clock access is designed for use in tracing, 592 side effects are possible. The clock access may yield wrong 593 readouts in case the internal TAI offset is updated e.g., caused 594 by setting the system time or using adjtimex() with an offset. 595 These effects are rare and post processing should be able to 596 handle them. See comments in the ktime_get_tai_fast_ns() 597 function for more information. 598 599 To set a clock, simply echo the clock name into this file:: 600 601 # echo global > trace_clock 602 603 Setting a clock clears the ring buffer content as well as the 604 "snapshot" buffer. 605 606 trace_marker: 607 608 This is a very useful file for synchronizing user space 609 with events happening in the kernel. Writing strings into 610 this file will be written into the ftrace buffer. 611 612 It is useful in applications to open this file at the start 613 of the application and just reference the file descriptor 614 for the file:: 615 616 void trace_write(const char *fmt, ...) 617 { 618 va_list ap; 619 char buf[256]; 620 int n; 621 622 if (trace_fd < 0) 623 return; 624 625 va_start(ap, fmt); 626 n = vsnprintf(buf, 256, fmt, ap); 627 va_end(ap); 628 629 write(trace_fd, buf, n); 630 } 631 632 start:: 633 634 trace_fd = open("trace_marker", O_WRONLY); 635 636 Note: Writing into the trace_marker file can also initiate triggers 637 that are written into /sys/kernel/tracing/events/ftrace/print/trigger 638 See "Event triggers" in Documentation/trace/events.rst and an 639 example in Documentation/trace/histogram.rst (Section 3.) 640 641 trace_marker_raw: 642 643 This is similar to trace_marker above, but is meant for binary data 644 to be written to it, where a tool can be used to parse the data 645 from trace_pipe_raw. 646 647 uprobe_events: 648 649 Add dynamic tracepoints in programs. 650 See uprobetracer.rst 651 652 uprobe_profile: 653 654 Uprobe statistics. See uprobetrace.txt 655 656 instances: 657 658 This is a way to make multiple trace buffers where different 659 events can be recorded in different buffers. 660 See "Instances" section below. 661 662 events: 663 664 This is the trace event directory. It holds event tracepoints 665 (also known as static tracepoints) that have been compiled 666 into the kernel. It shows what event tracepoints exist 667 and how they are grouped by system. There are "enable" 668 files at various levels that can enable the tracepoints 669 when a "1" is written to them. 670 671 See events.rst for more information. 672 673 set_event: 674 675 By echoing in the event into this file, will enable that event. 676 677 See events.rst for more information. 678 679 available_events: 680 681 A list of events that can be enabled in tracing. 682 683 See events.rst for more information. 684 685 timestamp_mode: 686 687 Certain tracers may change the timestamp mode used when 688 logging trace events into the event buffer. Events with 689 different modes can coexist within a buffer but the mode in 690 effect when an event is logged determines which timestamp mode 691 is used for that event. The default timestamp mode is 692 'delta'. 693 694 Usual timestamp modes for tracing: 695 696 # cat timestamp_mode 697 [delta] absolute 698 699 The timestamp mode with the square brackets around it is the 700 one in effect. 701 702 delta: Default timestamp mode - timestamp is a delta against 703 a per-buffer timestamp. 704 705 absolute: The timestamp is a full timestamp, not a delta 706 against some other value. As such it takes up more 707 space and is less efficient. 708 709 hwlat_detector: 710 711 Directory for the Hardware Latency Detector. 712 See "Hardware Latency Detector" section below. 713 714 per_cpu: 715 716 This is a directory that contains the trace per_cpu information. 717 718 per_cpu/cpu0/buffer_size_kb: 719 720 The ftrace buffer is defined per_cpu. That is, there's a separate 721 buffer for each CPU to allow writes to be done atomically, 722 and free from cache bouncing. These buffers may have different 723 size buffers. This file is similar to the buffer_size_kb 724 file, but it only displays or sets the buffer size for the 725 specific CPU. (here cpu0). 726 727 per_cpu/cpu0/trace: 728 729 This is similar to the "trace" file, but it will only display 730 the data specific for the CPU. If written to, it only clears 731 the specific CPU buffer. 732 733 per_cpu/cpu0/trace_pipe 734 735 This is similar to the "trace_pipe" file, and is a consuming 736 read, but it will only display (and consume) the data specific 737 for the CPU. 738 739 per_cpu/cpu0/trace_pipe_raw 740 741 For tools that can parse the ftrace ring buffer binary format, 742 the trace_pipe_raw file can be used to extract the data 743 from the ring buffer directly. With the use of the splice() 744 system call, the buffer data can be quickly transferred to 745 a file or to the network where a server is collecting the 746 data. 747 748 Like trace_pipe, this is a consuming reader, where multiple 749 reads will always produce different data. 750 751 per_cpu/cpu0/snapshot: 752 753 This is similar to the main "snapshot" file, but will only 754 snapshot the current CPU (if supported). It only displays 755 the content of the snapshot for a given CPU, and if 756 written to, only clears this CPU buffer. 757 758 per_cpu/cpu0/snapshot_raw: 759 760 Similar to the trace_pipe_raw, but will read the binary format 761 from the snapshot buffer for the given CPU. 762 763 per_cpu/cpu0/stats: 764 765 This displays certain stats about the ring buffer: 766 767 entries: 768 The number of events that are still in the buffer. 769 770 overrun: 771 The number of lost events due to overwriting when 772 the buffer was full. 773 774 commit overrun: 775 Should always be zero. 776 This gets set if so many events happened within a nested 777 event (ring buffer is re-entrant), that it fills the 778 buffer and starts dropping events. 779 780 bytes: 781 Bytes actually read (not overwritten). 782 783 oldest event ts: 784 The oldest timestamp in the buffer 785 786 now ts: 787 The current timestamp 788 789 dropped events: 790 Events lost due to overwrite option being off. 791 792 read events: 793 The number of events read. 794 795The Tracers 796----------- 797 798Here is the list of current tracers that may be configured. 799 800 "function" 801 802 Function call tracer to trace all kernel functions. 803 804 "function_graph" 805 806 Similar to the function tracer except that the 807 function tracer probes the functions on their entry 808 whereas the function graph tracer traces on both entry 809 and exit of the functions. It then provides the ability 810 to draw a graph of function calls similar to C code 811 source. 812 813 Note that the function graph calculates the timings of when the 814 function starts and returns internally and for each instance. If 815 there are two instances that run function graph tracer and traces 816 the same functions, the length of the timings may be slightly off as 817 each read the timestamp separately and not at the same time. 818 819 "blk" 820 821 The block tracer. The tracer used by the blktrace user 822 application. 823 824 "hwlat" 825 826 The Hardware Latency tracer is used to detect if the hardware 827 produces any latency. See "Hardware Latency Detector" section 828 below. 829 830 "irqsoff" 831 832 Traces the areas that disable interrupts and saves 833 the trace with the longest max latency. 834 See tracing_max_latency. When a new max is recorded, 835 it replaces the old trace. It is best to view this 836 trace with the latency-format option enabled, which 837 happens automatically when the tracer is selected. 838 839 "preemptoff" 840 841 Similar to irqsoff but traces and records the amount of 842 time for which preemption is disabled. 843 844 "preemptirqsoff" 845 846 Similar to irqsoff and preemptoff, but traces and 847 records the largest time for which irqs and/or preemption 848 is disabled. 849 850 "wakeup" 851 852 Traces and records the max latency that it takes for 853 the highest priority task to get scheduled after 854 it has been woken up. 855 Traces all tasks as an average developer would expect. 856 857 "wakeup_rt" 858 859 Traces and records the max latency that it takes for just 860 RT tasks (as the current "wakeup" does). This is useful 861 for those interested in wake up timings of RT tasks. 862 863 "wakeup_dl" 864 865 Traces and records the max latency that it takes for 866 a SCHED_DEADLINE task to be woken (as the "wakeup" and 867 "wakeup_rt" does). 868 869 "mmiotrace" 870 871 A special tracer that is used to trace binary module. 872 It will trace all the calls that a module makes to the 873 hardware. Everything it writes and reads from the I/O 874 as well. 875 876 "branch" 877 878 This tracer can be configured when tracing likely/unlikely 879 calls within the kernel. It will trace when a likely and 880 unlikely branch is hit and if it was correct in its prediction 881 of being correct. 882 883 "nop" 884 885 This is the "trace nothing" tracer. To remove all 886 tracers from tracing simply echo "nop" into 887 current_tracer. 888 889Error conditions 890---------------- 891 892 For most ftrace commands, failure modes are obvious and communicated 893 using standard return codes. 894 895 For other more involved commands, extended error information may be 896 available via the tracing/error_log file. For the commands that 897 support it, reading the tracing/error_log file after an error will 898 display more detailed information about what went wrong, if 899 information is available. The tracing/error_log file is a circular 900 error log displaying a small number (currently, 8) of ftrace errors 901 for the last (8) failed commands. 902 903 The extended error information and usage takes the form shown in 904 this example:: 905 906 # echo xxx > /sys/kernel/tracing/events/sched/sched_wakeup/trigger 907 echo: write error: Invalid argument 908 909 # cat /sys/kernel/tracing/error_log 910 [ 5348.887237] location: error: Couldn't yyy: zzz 911 Command: xxx 912 ^ 913 [ 7517.023364] location: error: Bad rrr: sss 914 Command: ppp qqq 915 ^ 916 917 To clear the error log, echo the empty string into it:: 918 919 # echo > /sys/kernel/tracing/error_log 920 921Examples of using the tracer 922---------------------------- 923 924Here are typical examples of using the tracers when controlling 925them only with the tracefs interface (without using any 926user-land utilities). 927 928Output format: 929-------------- 930 931Here is an example of the output format of the file "trace":: 932 933 # tracer: function 934 # 935 # entries-in-buffer/entries-written: 140080/250280 #P:4 936 # 937 # _-----=> irqs-off 938 # / _----=> need-resched 939 # | / _---=> hardirq/softirq 940 # || / _--=> preempt-depth 941 # ||| / delay 942 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 943 # | | | |||| | | 944 bash-1977 [000] .... 17284.993652: sys_close <-system_call_fastpath 945 bash-1977 [000] .... 17284.993653: __close_fd <-sys_close 946 bash-1977 [000] .... 17284.993653: _raw_spin_lock <-__close_fd 947 sshd-1974 [003] .... 17284.993653: __srcu_read_unlock <-fsnotify 948 bash-1977 [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock 949 bash-1977 [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd 950 bash-1977 [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock 951 bash-1977 [000] .... 17284.993657: filp_close <-__close_fd 952 bash-1977 [000] .... 17284.993657: dnotify_flush <-filp_close 953 sshd-1974 [003] .... 17284.993658: sys_select <-system_call_fastpath 954 .... 955 956A header is printed with the tracer name that is represented by 957the trace. In this case the tracer is "function". Then it shows the 958number of events in the buffer as well as the total number of entries 959that were written. The difference is the number of entries that were 960lost due to the buffer filling up (250280 - 140080 = 110200 events 961lost). 962 963The header explains the content of the events. Task name "bash", the task 964PID "1977", the CPU that it was running on "000", the latency format 965(explained below), the timestamp in <secs>.<usecs> format, the 966function name that was traced "sys_close" and the parent function that 967called this function "system_call_fastpath". The timestamp is the time 968at which the function was entered. 969 970Latency trace format 971-------------------- 972 973When the latency-format option is enabled or when one of the latency 974tracers is set, the trace file gives somewhat more information to see 975why a latency happened. Here is a typical trace:: 976 977 # tracer: irqsoff 978 # 979 # irqsoff latency trace v1.1.5 on 3.8.0-test+ 980 # -------------------------------------------------------------------- 981 # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 982 # ----------------- 983 # | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0) 984 # ----------------- 985 # => started at: __lock_task_sighand 986 # => ended at: _raw_spin_unlock_irqrestore 987 # 988 # 989 # _------=> CPU# 990 # / _-----=> irqs-off 991 # | / _----=> need-resched 992 # || / _---=> hardirq/softirq 993 # ||| / _--=> preempt-depth 994 # |||| / delay 995 # cmd pid ||||| time | caller 996 # \ / ||||| \ | / 997 ps-6143 2d... 0us!: trace_hardirqs_off <-__lock_task_sighand 998 ps-6143 2d..1 259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore 999 ps-6143 2d..1 263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore 1000 ps-6143 2d..1 306us : <stack trace> 1001 => trace_hardirqs_on_caller 1002 => trace_hardirqs_on 1003 => _raw_spin_unlock_irqrestore 1004 => do_task_stat 1005 => proc_tgid_stat 1006 => proc_single_show 1007 => seq_read 1008 => vfs_read 1009 => sys_read 1010 => system_call_fastpath 1011 1012 1013This shows that the current tracer is "irqsoff" tracing the time 1014for which interrupts were disabled. It gives the trace version (which 1015never changes) and the version of the kernel upon which this was executed on 1016(3.8). Then it displays the max latency in microseconds (259 us). The number 1017of trace entries displayed and the total number (both are four: #4/4). 1018VP, KP, SP, and HP are always zero and are reserved for later use. 1019#P is the number of online CPUs (#P:4). 1020 1021The task is the process that was running when the latency 1022occurred. (ps pid: 6143). 1023 1024The start and stop (the functions in which the interrupts were 1025disabled and enabled respectively) that caused the latencies: 1026 1027 - __lock_task_sighand is where the interrupts were disabled. 1028 - _raw_spin_unlock_irqrestore is where they were enabled again. 1029 1030The next lines after the header are the trace itself. The header 1031explains which is which. 1032 1033 cmd: The name of the process in the trace. 1034 1035 pid: The PID of that process. 1036 1037 CPU#: The CPU which the process was running on. 1038 1039 irqs-off: 'd' interrupts are disabled. '.' otherwise. 1040 1041 need-resched: 1042 - 'B' all, TIF_NEED_RESCHED, PREEMPT_NEED_RESCHED and TIF_RESCHED_LAZY is set, 1043 - 'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set, 1044 - 'n' only TIF_NEED_RESCHED is set, 1045 - 'p' only PREEMPT_NEED_RESCHED is set, 1046 - 'L' both PREEMPT_NEED_RESCHED and TIF_RESCHED_LAZY is set, 1047 - 'b' both TIF_NEED_RESCHED and TIF_RESCHED_LAZY is set, 1048 - 'l' only TIF_RESCHED_LAZY is set 1049 - '.' otherwise. 1050 1051 hardirq/softirq: 1052 - 'Z' - NMI occurred inside a hardirq 1053 - 'z' - NMI is running 1054 - 'H' - hard irq occurred inside a softirq. 1055 - 'h' - hard irq is running 1056 - 's' - soft irq is running 1057 - '.' - normal context. 1058 1059 preempt-depth: The level of preempt_disabled 1060 1061The above is mostly meaningful for kernel developers. 1062 1063 time: 1064 When the latency-format option is enabled, the trace file 1065 output includes a timestamp relative to the start of the 1066 trace. This differs from the output when latency-format 1067 is disabled, which includes an absolute timestamp. 1068 1069 delay: 1070 This is just to help catch your eye a bit better. And 1071 needs to be fixed to be only relative to the same CPU. 1072 The marks are determined by the difference between this 1073 current trace and the next trace. 1074 1075 - '$' - greater than 1 second 1076 - '@' - greater than 100 millisecond 1077 - '*' - greater than 10 millisecond 1078 - '#' - greater than 1000 microsecond 1079 - '!' - greater than 100 microsecond 1080 - '+' - greater than 10 microsecond 1081 - ' ' - less than or equal to 10 microsecond. 1082 1083 The rest is the same as the 'trace' file. 1084 1085 Note, the latency tracers will usually end with a back trace 1086 to easily find where the latency occurred. 1087 1088trace_options 1089------------- 1090 1091The trace_options file (or the options directory) is used to control 1092what gets printed in the trace output, or manipulate the tracers. 1093To see what is available, simply cat the file:: 1094 1095 cat trace_options 1096 print-parent 1097 nosym-offset 1098 nosym-addr 1099 noverbose 1100 noraw 1101 nohex 1102 nobin 1103 noblock 1104 nofields 1105 trace_printk 1106 annotate 1107 nouserstacktrace 1108 nosym-userobj 1109 noprintk-msg-only 1110 context-info 1111 nolatency-format 1112 record-cmd 1113 norecord-tgid 1114 overwrite 1115 nodisable_on_free 1116 irq-info 1117 markers 1118 noevent-fork 1119 function-trace 1120 nofunction-fork 1121 nodisplay-graph 1122 nostacktrace 1123 nobranch 1124 1125To disable one of the options, echo in the option prepended with 1126"no":: 1127 1128 echo noprint-parent > trace_options 1129 1130To enable an option, leave off the "no":: 1131 1132 echo sym-offset > trace_options 1133 1134Here are the available options: 1135 1136 print-parent 1137 On function traces, display the calling (parent) 1138 function as well as the function being traced. 1139 :: 1140 1141 print-parent: 1142 bash-4000 [01] 1477.606694: simple_strtoul <-kstrtoul 1143 1144 noprint-parent: 1145 bash-4000 [01] 1477.606694: simple_strtoul 1146 1147 1148 sym-offset 1149 Display not only the function name, but also the 1150 offset in the function. For example, instead of 1151 seeing just "ktime_get", you will see 1152 "ktime_get+0xb/0x20". 1153 :: 1154 1155 sym-offset: 1156 bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0 1157 1158 sym-addr 1159 This will also display the function address as well 1160 as the function name. 1161 :: 1162 1163 sym-addr: 1164 bash-4000 [01] 1477.606694: simple_strtoul <c0339346> 1165 1166 verbose 1167 This deals with the trace file when the 1168 latency-format option is enabled. 1169 :: 1170 1171 bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \ 1172 (+0.000ms): simple_strtoul (kstrtoul) 1173 1174 raw 1175 This will display raw numbers. This option is best for 1176 use with user applications that can translate the raw 1177 numbers better than having it done in the kernel. 1178 1179 hex 1180 Similar to raw, but the numbers will be in a hexadecimal format. 1181 1182 bin 1183 This will print out the formats in raw binary. 1184 1185 block 1186 When set, reading trace_pipe will not block when polled. 1187 1188 fields 1189 Print the fields as described by their types. This is a better 1190 option than using hex, bin or raw, as it gives a better parsing 1191 of the content of the event. 1192 1193 trace_printk 1194 Can disable trace_printk() from writing into the buffer. 1195 1196 trace_printk_dest 1197 Set to have trace_printk() and similar internal tracing functions 1198 write into this instance. Note, only one trace instance can have 1199 this set. By setting this flag, it clears the trace_printk_dest flag 1200 of the instance that had it set previously. By default, the top 1201 level trace has this set, and will get it set again if another 1202 instance has it set then clears it. 1203 1204 This flag cannot be cleared by the top level instance, as it is the 1205 default instance. The only way the top level instance has this flag 1206 cleared, is by it being set in another instance. 1207 1208 annotate 1209 It is sometimes confusing when the CPU buffers are full 1210 and one CPU buffer had a lot of events recently, thus 1211 a shorter time frame, were another CPU may have only had 1212 a few events, which lets it have older events. When 1213 the trace is reported, it shows the oldest events first, 1214 and it may look like only one CPU ran (the one with the 1215 oldest events). When the annotate option is set, it will 1216 display when a new CPU buffer started:: 1217 1218 <idle>-0 [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on 1219 <idle>-0 [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on 1220 <idle>-0 [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore 1221 ##### CPU 2 buffer started #### 1222 <idle>-0 [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle 1223 <idle>-0 [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog 1224 <idle>-0 [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock 1225 1226 userstacktrace 1227 This option changes the trace. It records a 1228 stacktrace of the current user space thread after 1229 each trace event. 1230 1231 sym-userobj 1232 when user stacktrace are enabled, look up which 1233 object the address belongs to, and print a 1234 relative address. This is especially useful when 1235 ASLR is on, otherwise you don't get a chance to 1236 resolve the address to object/file/line after 1237 the app is no longer running 1238 1239 The lookup is performed when you read 1240 trace,trace_pipe. Example:: 1241 1242 a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0 1243 x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6] 1244 1245 1246 printk-msg-only 1247 When set, trace_printk()s will only show the format 1248 and not their parameters (if trace_bprintk() or 1249 trace_bputs() was used to save the trace_printk()). 1250 1251 context-info 1252 Show only the event data. Hides the comm, PID, 1253 timestamp, CPU, and other useful data. 1254 1255 latency-format 1256 This option changes the trace output. When it is enabled, 1257 the trace displays additional information about the 1258 latency, as described in "Latency trace format". 1259 1260 pause-on-trace 1261 When set, opening the trace file for read, will pause 1262 writing to the ring buffer (as if tracing_on was set to zero). 1263 This simulates the original behavior of the trace file. 1264 When the file is closed, tracing will be enabled again. 1265 1266 hash-ptr 1267 When set, "%p" in the event printk format displays the 1268 hashed pointer value instead of real address. 1269 This will be useful if you want to find out which hashed 1270 value is corresponding to the real value in trace log. 1271 1272 record-cmd 1273 When any event or tracer is enabled, a hook is enabled 1274 in the sched_switch trace point to fill comm cache 1275 with mapped pids and comms. But this may cause some 1276 overhead, and if you only care about pids, and not the 1277 name of the task, disabling this option can lower the 1278 impact of tracing. See "saved_cmdlines". 1279 1280 record-tgid 1281 When any event or tracer is enabled, a hook is enabled 1282 in the sched_switch trace point to fill the cache of 1283 mapped Thread Group IDs (TGID) mapping to pids. See 1284 "saved_tgids". 1285 1286 overwrite 1287 This controls what happens when the trace buffer is 1288 full. If "1" (default), the oldest events are 1289 discarded and overwritten. If "0", then the newest 1290 events are discarded. 1291 (see per_cpu/cpu0/stats for overrun and dropped) 1292 1293 disable_on_free 1294 When the free_buffer is closed, tracing will 1295 stop (tracing_on set to 0). 1296 1297 irq-info 1298 Shows the interrupt, preempt count, need resched data. 1299 When disabled, the trace looks like:: 1300 1301 # tracer: function 1302 # 1303 # entries-in-buffer/entries-written: 144405/9452052 #P:4 1304 # 1305 # TASK-PID CPU# TIMESTAMP FUNCTION 1306 # | | | | | 1307 <idle>-0 [002] 23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up 1308 <idle>-0 [002] 23636.756054: activate_task <-ttwu_do_activate.constprop.89 1309 <idle>-0 [002] 23636.756055: enqueue_task <-activate_task 1310 1311 1312 markers 1313 When set, the trace_marker is writable (only by root). 1314 When disabled, the trace_marker will error with EINVAL 1315 on write. 1316 1317 event-fork 1318 When set, tasks with PIDs listed in set_event_pid will have 1319 the PIDs of their children added to set_event_pid when those 1320 tasks fork. Also, when tasks with PIDs in set_event_pid exit, 1321 their PIDs will be removed from the file. 1322 1323 This affects PIDs listed in set_event_notrace_pid as well. 1324 1325 function-trace 1326 The latency tracers will enable function tracing 1327 if this option is enabled (default it is). When 1328 it is disabled, the latency tracers do not trace 1329 functions. This keeps the overhead of the tracer down 1330 when performing latency tests. 1331 1332 function-fork 1333 When set, tasks with PIDs listed in set_ftrace_pid will 1334 have the PIDs of their children added to set_ftrace_pid 1335 when those tasks fork. Also, when tasks with PIDs in 1336 set_ftrace_pid exit, their PIDs will be removed from the 1337 file. 1338 1339 This affects PIDs in set_ftrace_notrace_pid as well. 1340 1341 display-graph 1342 When set, the latency tracers (irqsoff, wakeup, etc) will 1343 use function graph tracing instead of function tracing. 1344 1345 stacktrace 1346 When set, a stack trace is recorded after any trace event 1347 is recorded. 1348 1349 branch 1350 Enable branch tracing with the tracer. This enables branch 1351 tracer along with the currently set tracer. Enabling this 1352 with the "nop" tracer is the same as just enabling the 1353 "branch" tracer. 1354 1355.. tip:: Some tracers have their own options. They only appear in this 1356 file when the tracer is active. They always appear in the 1357 options directory. 1358 1359 1360Here are the per tracer options: 1361 1362Options for function tracer: 1363 1364 func_stack_trace 1365 When set, a stack trace is recorded after every 1366 function that is recorded. NOTE! Limit the functions 1367 that are recorded before enabling this, with 1368 "set_ftrace_filter" otherwise the system performance 1369 will be critically degraded. Remember to disable 1370 this option before clearing the function filter. 1371 1372Options for function_graph tracer: 1373 1374 Since the function_graph tracer has a slightly different output 1375 it has its own options to control what is displayed. 1376 1377 funcgraph-overrun 1378 When set, the "overrun" of the graph stack is 1379 displayed after each function traced. The 1380 overrun, is when the stack depth of the calls 1381 is greater than what is reserved for each task. 1382 Each task has a fixed array of functions to 1383 trace in the call graph. If the depth of the 1384 calls exceeds that, the function is not traced. 1385 The overrun is the number of functions missed 1386 due to exceeding this array. 1387 1388 funcgraph-cpu 1389 When set, the CPU number of the CPU where the trace 1390 occurred is displayed. 1391 1392 funcgraph-overhead 1393 When set, if the function takes longer than 1394 A certain amount, then a delay marker is 1395 displayed. See "delay" above, under the 1396 header description. 1397 1398 funcgraph-proc 1399 Unlike other tracers, the process' command line 1400 is not displayed by default, but instead only 1401 when a task is traced in and out during a context 1402 switch. Enabling this options has the command 1403 of each process displayed at every line. 1404 1405 funcgraph-duration 1406 At the end of each function (the return) 1407 the duration of the amount of time in the 1408 function is displayed in microseconds. 1409 1410 funcgraph-abstime 1411 When set, the timestamp is displayed at each line. 1412 1413 funcgraph-irqs 1414 When disabled, functions that happen inside an 1415 interrupt will not be traced. 1416 1417 funcgraph-tail 1418 When set, the return event will include the function 1419 that it represents. By default this is off, and 1420 only a closing curly bracket "}" is displayed for 1421 the return of a function. 1422 1423 funcgraph-retval 1424 When set, the return value of each traced function 1425 will be printed after an equal sign "=". By default 1426 this is off. 1427 1428 funcgraph-retval-hex 1429 When set, the return value will always be printed 1430 in hexadecimal format. If the option is not set and 1431 the return value is an error code, it will be printed 1432 in signed decimal format; otherwise it will also be 1433 printed in hexadecimal format. By default, this option 1434 is off. 1435 1436 sleep-time 1437 When running function graph tracer, to include 1438 the time a task schedules out in its function. 1439 When enabled, it will account time the task has been 1440 scheduled out as part of the function call. 1441 1442 graph-time 1443 When running function profiler with function graph tracer, 1444 to include the time to call nested functions. When this is 1445 not set, the time reported for the function will only 1446 include the time the function itself executed for, not the 1447 time for functions that it called. 1448 1449Options for blk tracer: 1450 1451 blk_classic 1452 Shows a more minimalistic output. 1453 1454 1455irqsoff 1456------- 1457 1458When interrupts are disabled, the CPU can not react to any other 1459external event (besides NMIs and SMIs). This prevents the timer 1460interrupt from triggering or the mouse interrupt from letting 1461the kernel know of a new mouse event. The result is a latency 1462with the reaction time. 1463 1464The irqsoff tracer tracks the time for which interrupts are 1465disabled. When a new maximum latency is hit, the tracer saves 1466the trace leading up to that latency point so that every time a 1467new maximum is reached, the old saved trace is discarded and the 1468new trace is saved. 1469 1470To reset the maximum, echo 0 into tracing_max_latency. Here is 1471an example:: 1472 1473 # echo 0 > options/function-trace 1474 # echo irqsoff > current_tracer 1475 # echo 1 > tracing_on 1476 # echo 0 > tracing_max_latency 1477 # ls -ltr 1478 [...] 1479 # echo 0 > tracing_on 1480 # cat trace 1481 # tracer: irqsoff 1482 # 1483 # irqsoff latency trace v1.1.5 on 3.8.0-test+ 1484 # -------------------------------------------------------------------- 1485 # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1486 # ----------------- 1487 # | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0) 1488 # ----------------- 1489 # => started at: run_timer_softirq 1490 # => ended at: run_timer_softirq 1491 # 1492 # 1493 # _------=> CPU# 1494 # / _-----=> irqs-off 1495 # | / _----=> need-resched 1496 # || / _---=> hardirq/softirq 1497 # ||| / _--=> preempt-depth 1498 # |||| / delay 1499 # cmd pid ||||| time | caller 1500 # \ / ||||| \ | / 1501 <idle>-0 0d.s2 0us+: _raw_spin_lock_irq <-run_timer_softirq 1502 <idle>-0 0dNs3 17us : _raw_spin_unlock_irq <-run_timer_softirq 1503 <idle>-0 0dNs3 17us+: trace_hardirqs_on <-run_timer_softirq 1504 <idle>-0 0dNs3 25us : <stack trace> 1505 => _raw_spin_unlock_irq 1506 => run_timer_softirq 1507 => __do_softirq 1508 => call_softirq 1509 => do_softirq 1510 => irq_exit 1511 => smp_apic_timer_interrupt 1512 => apic_timer_interrupt 1513 => rcu_idle_exit 1514 => cpu_idle 1515 => rest_init 1516 => start_kernel 1517 => x86_64_start_reservations 1518 => x86_64_start_kernel 1519 1520Here we see that we had a latency of 16 microseconds (which is 1521very good). The _raw_spin_lock_irq in run_timer_softirq disabled 1522interrupts. The difference between the 16 and the displayed 1523timestamp 25us occurred because the clock was incremented 1524between the time of recording the max latency and the time of 1525recording the function that had that latency. 1526 1527Note the above example had function-trace not set. If we set 1528function-trace, we get a much larger output:: 1529 1530 with echo 1 > options/function-trace 1531 1532 # tracer: irqsoff 1533 # 1534 # irqsoff latency trace v1.1.5 on 3.8.0-test+ 1535 # -------------------------------------------------------------------- 1536 # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1537 # ----------------- 1538 # | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0) 1539 # ----------------- 1540 # => started at: ata_scsi_queuecmd 1541 # => ended at: ata_scsi_queuecmd 1542 # 1543 # 1544 # _------=> CPU# 1545 # / _-----=> irqs-off 1546 # | / _----=> need-resched 1547 # || / _---=> hardirq/softirq 1548 # ||| / _--=> preempt-depth 1549 # |||| / delay 1550 # cmd pid ||||| time | caller 1551 # \ / ||||| \ | / 1552 bash-2042 3d... 0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd 1553 bash-2042 3d... 0us : add_preempt_count <-_raw_spin_lock_irqsave 1554 bash-2042 3d..1 1us : ata_scsi_find_dev <-ata_scsi_queuecmd 1555 bash-2042 3d..1 1us : __ata_scsi_find_dev <-ata_scsi_find_dev 1556 bash-2042 3d..1 2us : ata_find_dev.part.14 <-__ata_scsi_find_dev 1557 bash-2042 3d..1 2us : ata_qc_new_init <-__ata_scsi_queuecmd 1558 bash-2042 3d..1 3us : ata_sg_init <-__ata_scsi_queuecmd 1559 bash-2042 3d..1 4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd 1560 bash-2042 3d..1 4us : ata_build_rw_tf <-ata_scsi_rw_xlat 1561 [...] 1562 bash-2042 3d..1 67us : delay_tsc <-__delay 1563 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc 1564 bash-2042 3d..2 67us : sub_preempt_count <-delay_tsc 1565 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc 1566 bash-2042 3d..2 68us : sub_preempt_count <-delay_tsc 1567 bash-2042 3d..1 68us+: ata_bmdma_start <-ata_bmdma_qc_issue 1568 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd 1569 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd 1570 bash-2042 3d..1 72us+: trace_hardirqs_on <-ata_scsi_queuecmd 1571 bash-2042 3d..1 120us : <stack trace> 1572 => _raw_spin_unlock_irqrestore 1573 => ata_scsi_queuecmd 1574 => scsi_dispatch_cmd 1575 => scsi_request_fn 1576 => __blk_run_queue_uncond 1577 => __blk_run_queue 1578 => blk_queue_bio 1579 => submit_bio_noacct 1580 => submit_bio 1581 => submit_bh 1582 => __ext3_get_inode_loc 1583 => ext3_iget 1584 => ext3_lookup 1585 => lookup_real 1586 => __lookup_hash 1587 => walk_component 1588 => lookup_last 1589 => path_lookupat 1590 => filename_lookup 1591 => user_path_at_empty 1592 => user_path_at 1593 => vfs_fstatat 1594 => vfs_stat 1595 => sys_newstat 1596 => system_call_fastpath 1597 1598 1599Here we traced a 71 microsecond latency. But we also see all the 1600functions that were called during that time. Note that by 1601enabling function tracing, we incur an added overhead. This 1602overhead may extend the latency times. But nevertheless, this 1603trace has provided some very helpful debugging information. 1604 1605If we prefer function graph output instead of function, we can set 1606display-graph option:: 1607 1608 with echo 1 > options/display-graph 1609 1610 # tracer: irqsoff 1611 # 1612 # irqsoff latency trace v1.1.5 on 4.20.0-rc6+ 1613 # -------------------------------------------------------------------- 1614 # latency: 3751 us, #274/274, CPU#0 | (M:desktop VP:0, KP:0, SP:0 HP:0 #P:4) 1615 # ----------------- 1616 # | task: bash-1507 (uid:0 nice:0 policy:0 rt_prio:0) 1617 # ----------------- 1618 # => started at: free_debug_processing 1619 # => ended at: return_to_handler 1620 # 1621 # 1622 # _-----=> irqs-off 1623 # / _----=> need-resched 1624 # | / _---=> hardirq/softirq 1625 # || / _--=> preempt-depth 1626 # ||| / 1627 # REL TIME CPU TASK/PID |||| DURATION FUNCTION CALLS 1628 # | | | | |||| | | | | | | 1629 0 us | 0) bash-1507 | d... | 0.000 us | _raw_spin_lock_irqsave(); 1630 0 us | 0) bash-1507 | d..1 | 0.378 us | do_raw_spin_trylock(); 1631 1 us | 0) bash-1507 | d..2 | | set_track() { 1632 2 us | 0) bash-1507 | d..2 | | save_stack_trace() { 1633 2 us | 0) bash-1507 | d..2 | | __save_stack_trace() { 1634 3 us | 0) bash-1507 | d..2 | | __unwind_start() { 1635 3 us | 0) bash-1507 | d..2 | | get_stack_info() { 1636 3 us | 0) bash-1507 | d..2 | 0.351 us | in_task_stack(); 1637 4 us | 0) bash-1507 | d..2 | 1.107 us | } 1638 [...] 1639 3750 us | 0) bash-1507 | d..1 | 0.516 us | do_raw_spin_unlock(); 1640 3750 us | 0) bash-1507 | d..1 | 0.000 us | _raw_spin_unlock_irqrestore(); 1641 3764 us | 0) bash-1507 | d..1 | 0.000 us | tracer_hardirqs_on(); 1642 bash-1507 0d..1 3792us : <stack trace> 1643 => free_debug_processing 1644 => __slab_free 1645 => kmem_cache_free 1646 => vm_area_free 1647 => remove_vma 1648 => exit_mmap 1649 => mmput 1650 => begin_new_exec 1651 => load_elf_binary 1652 => search_binary_handler 1653 => __do_execve_file.isra.32 1654 => __x64_sys_execve 1655 => do_syscall_64 1656 => entry_SYSCALL_64_after_hwframe 1657 1658preemptoff 1659---------- 1660 1661When preemption is disabled, we may be able to receive 1662interrupts but the task cannot be preempted and a higher 1663priority task must wait for preemption to be enabled again 1664before it can preempt a lower priority task. 1665 1666The preemptoff tracer traces the places that disable preemption. 1667Like the irqsoff tracer, it records the maximum latency for 1668which preemption was disabled. The control of preemptoff tracer 1669is much like the irqsoff tracer. 1670:: 1671 1672 # echo 0 > options/function-trace 1673 # echo preemptoff > current_tracer 1674 # echo 1 > tracing_on 1675 # echo 0 > tracing_max_latency 1676 # ls -ltr 1677 [...] 1678 # echo 0 > tracing_on 1679 # cat trace 1680 # tracer: preemptoff 1681 # 1682 # preemptoff latency trace v1.1.5 on 3.8.0-test+ 1683 # -------------------------------------------------------------------- 1684 # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1685 # ----------------- 1686 # | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0) 1687 # ----------------- 1688 # => started at: do_IRQ 1689 # => ended at: do_IRQ 1690 # 1691 # 1692 # _------=> CPU# 1693 # / _-----=> irqs-off 1694 # | / _----=> need-resched 1695 # || / _---=> hardirq/softirq 1696 # ||| / _--=> preempt-depth 1697 # |||| / delay 1698 # cmd pid ||||| time | caller 1699 # \ / ||||| \ | / 1700 sshd-1991 1d.h. 0us+: irq_enter <-do_IRQ 1701 sshd-1991 1d..1 46us : irq_exit <-do_IRQ 1702 sshd-1991 1d..1 47us+: trace_preempt_on <-do_IRQ 1703 sshd-1991 1d..1 52us : <stack trace> 1704 => sub_preempt_count 1705 => irq_exit 1706 => do_IRQ 1707 => ret_from_intr 1708 1709 1710This has some more changes. Preemption was disabled when an 1711interrupt came in (notice the 'h'), and was enabled on exit. 1712But we also see that interrupts have been disabled when entering 1713the preempt off section and leaving it (the 'd'). We do not know if 1714interrupts were enabled in the mean time or shortly after this 1715was over. 1716:: 1717 1718 # tracer: preemptoff 1719 # 1720 # preemptoff latency trace v1.1.5 on 3.8.0-test+ 1721 # -------------------------------------------------------------------- 1722 # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1723 # ----------------- 1724 # | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0) 1725 # ----------------- 1726 # => started at: wake_up_new_task 1727 # => ended at: task_rq_unlock 1728 # 1729 # 1730 # _------=> CPU# 1731 # / _-----=> irqs-off 1732 # | / _----=> need-resched 1733 # || / _---=> hardirq/softirq 1734 # ||| / _--=> preempt-depth 1735 # |||| / delay 1736 # cmd pid ||||| time | caller 1737 # \ / ||||| \ | / 1738 bash-1994 1d..1 0us : _raw_spin_lock_irqsave <-wake_up_new_task 1739 bash-1994 1d..1 0us : select_task_rq_fair <-select_task_rq 1740 bash-1994 1d..1 1us : __rcu_read_lock <-select_task_rq_fair 1741 bash-1994 1d..1 1us : source_load <-select_task_rq_fair 1742 bash-1994 1d..1 1us : source_load <-select_task_rq_fair 1743 [...] 1744 bash-1994 1d..1 12us : irq_enter <-smp_apic_timer_interrupt 1745 bash-1994 1d..1 12us : rcu_irq_enter <-irq_enter 1746 bash-1994 1d..1 13us : add_preempt_count <-irq_enter 1747 bash-1994 1d.h1 13us : exit_idle <-smp_apic_timer_interrupt 1748 bash-1994 1d.h1 13us : hrtimer_interrupt <-smp_apic_timer_interrupt 1749 bash-1994 1d.h1 13us : _raw_spin_lock <-hrtimer_interrupt 1750 bash-1994 1d.h1 14us : add_preempt_count <-_raw_spin_lock 1751 bash-1994 1d.h2 14us : ktime_get_update_offsets <-hrtimer_interrupt 1752 [...] 1753 bash-1994 1d.h1 35us : lapic_next_event <-clockevents_program_event 1754 bash-1994 1d.h1 35us : irq_exit <-smp_apic_timer_interrupt 1755 bash-1994 1d.h1 36us : sub_preempt_count <-irq_exit 1756 bash-1994 1d..2 36us : do_softirq <-irq_exit 1757 bash-1994 1d..2 36us : __do_softirq <-call_softirq 1758 bash-1994 1d..2 36us : __local_bh_disable <-__do_softirq 1759 bash-1994 1d.s2 37us : add_preempt_count <-_raw_spin_lock_irq 1760 bash-1994 1d.s3 38us : _raw_spin_unlock <-run_timer_softirq 1761 bash-1994 1d.s3 39us : sub_preempt_count <-_raw_spin_unlock 1762 bash-1994 1d.s2 39us : call_timer_fn <-run_timer_softirq 1763 [...] 1764 bash-1994 1dNs2 81us : cpu_needs_another_gp <-rcu_process_callbacks 1765 bash-1994 1dNs2 82us : __local_bh_enable <-__do_softirq 1766 bash-1994 1dNs2 82us : sub_preempt_count <-__local_bh_enable 1767 bash-1994 1dN.2 82us : idle_cpu <-irq_exit 1768 bash-1994 1dN.2 83us : rcu_irq_exit <-irq_exit 1769 bash-1994 1dN.2 83us : sub_preempt_count <-irq_exit 1770 bash-1994 1.N.1 84us : _raw_spin_unlock_irqrestore <-task_rq_unlock 1771 bash-1994 1.N.1 84us+: trace_preempt_on <-task_rq_unlock 1772 bash-1994 1.N.1 104us : <stack trace> 1773 => sub_preempt_count 1774 => _raw_spin_unlock_irqrestore 1775 => task_rq_unlock 1776 => wake_up_new_task 1777 => do_fork 1778 => sys_clone 1779 => stub_clone 1780 1781 1782The above is an example of the preemptoff trace with 1783function-trace set. Here we see that interrupts were not disabled 1784the entire time. The irq_enter code lets us know that we entered 1785an interrupt 'h'. Before that, the functions being traced still 1786show that it is not in an interrupt, but we can see from the 1787functions themselves that this is not the case. 1788 1789preemptirqsoff 1790-------------- 1791 1792Knowing the locations that have interrupts disabled or 1793preemption disabled for the longest times is helpful. But 1794sometimes we would like to know when either preemption and/or 1795interrupts are disabled. 1796 1797Consider the following code:: 1798 1799 local_irq_disable(); 1800 call_function_with_irqs_off(); 1801 preempt_disable(); 1802 call_function_with_irqs_and_preemption_off(); 1803 local_irq_enable(); 1804 call_function_with_preemption_off(); 1805 preempt_enable(); 1806 1807The irqsoff tracer will record the total length of 1808call_function_with_irqs_off() and 1809call_function_with_irqs_and_preemption_off(). 1810 1811The preemptoff tracer will record the total length of 1812call_function_with_irqs_and_preemption_off() and 1813call_function_with_preemption_off(). 1814 1815But neither will trace the time that interrupts and/or 1816preemption is disabled. This total time is the time that we can 1817not schedule. To record this time, use the preemptirqsoff 1818tracer. 1819 1820Again, using this trace is much like the irqsoff and preemptoff 1821tracers. 1822:: 1823 1824 # echo 0 > options/function-trace 1825 # echo preemptirqsoff > current_tracer 1826 # echo 1 > tracing_on 1827 # echo 0 > tracing_max_latency 1828 # ls -ltr 1829 [...] 1830 # echo 0 > tracing_on 1831 # cat trace 1832 # tracer: preemptirqsoff 1833 # 1834 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+ 1835 # -------------------------------------------------------------------- 1836 # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1837 # ----------------- 1838 # | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0) 1839 # ----------------- 1840 # => started at: ata_scsi_queuecmd 1841 # => ended at: ata_scsi_queuecmd 1842 # 1843 # 1844 # _------=> CPU# 1845 # / _-----=> irqs-off 1846 # | / _----=> need-resched 1847 # || / _---=> hardirq/softirq 1848 # ||| / _--=> preempt-depth 1849 # |||| / delay 1850 # cmd pid ||||| time | caller 1851 # \ / ||||| \ | / 1852 ls-2230 3d... 0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd 1853 ls-2230 3...1 100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd 1854 ls-2230 3...1 101us+: trace_preempt_on <-ata_scsi_queuecmd 1855 ls-2230 3...1 111us : <stack trace> 1856 => sub_preempt_count 1857 => _raw_spin_unlock_irqrestore 1858 => ata_scsi_queuecmd 1859 => scsi_dispatch_cmd 1860 => scsi_request_fn 1861 => __blk_run_queue_uncond 1862 => __blk_run_queue 1863 => blk_queue_bio 1864 => submit_bio_noacct 1865 => submit_bio 1866 => submit_bh 1867 => ext3_bread 1868 => ext3_dir_bread 1869 => htree_dirblock_to_tree 1870 => ext3_htree_fill_tree 1871 => ext3_readdir 1872 => vfs_readdir 1873 => sys_getdents 1874 => system_call_fastpath 1875 1876 1877The trace_hardirqs_off_thunk is called from assembly on x86 when 1878interrupts are disabled in the assembly code. Without the 1879function tracing, we do not know if interrupts were enabled 1880within the preemption points. We do see that it started with 1881preemption enabled. 1882 1883Here is a trace with function-trace set:: 1884 1885 # tracer: preemptirqsoff 1886 # 1887 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+ 1888 # -------------------------------------------------------------------- 1889 # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1890 # ----------------- 1891 # | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0) 1892 # ----------------- 1893 # => started at: schedule 1894 # => ended at: mutex_unlock 1895 # 1896 # 1897 # _------=> CPU# 1898 # / _-----=> irqs-off 1899 # | / _----=> need-resched 1900 # || / _---=> hardirq/softirq 1901 # ||| / _--=> preempt-depth 1902 # |||| / delay 1903 # cmd pid ||||| time | caller 1904 # \ / ||||| \ | / 1905 kworker/-59 3...1 0us : __schedule <-schedule 1906 kworker/-59 3d..1 0us : rcu_preempt_qs <-rcu_note_context_switch 1907 kworker/-59 3d..1 1us : add_preempt_count <-_raw_spin_lock_irq 1908 kworker/-59 3d..2 1us : deactivate_task <-__schedule 1909 kworker/-59 3d..2 1us : dequeue_task <-deactivate_task 1910 kworker/-59 3d..2 2us : update_rq_clock <-dequeue_task 1911 kworker/-59 3d..2 2us : dequeue_task_fair <-dequeue_task 1912 kworker/-59 3d..2 2us : update_curr <-dequeue_task_fair 1913 kworker/-59 3d..2 2us : update_min_vruntime <-update_curr 1914 kworker/-59 3d..2 3us : cpuacct_charge <-update_curr 1915 kworker/-59 3d..2 3us : __rcu_read_lock <-cpuacct_charge 1916 kworker/-59 3d..2 3us : __rcu_read_unlock <-cpuacct_charge 1917 kworker/-59 3d..2 3us : update_cfs_rq_blocked_load <-dequeue_task_fair 1918 kworker/-59 3d..2 4us : clear_buddies <-dequeue_task_fair 1919 kworker/-59 3d..2 4us : account_entity_dequeue <-dequeue_task_fair 1920 kworker/-59 3d..2 4us : update_min_vruntime <-dequeue_task_fair 1921 kworker/-59 3d..2 4us : update_cfs_shares <-dequeue_task_fair 1922 kworker/-59 3d..2 5us : hrtick_update <-dequeue_task_fair 1923 kworker/-59 3d..2 5us : wq_worker_sleeping <-__schedule 1924 kworker/-59 3d..2 5us : kthread_data <-wq_worker_sleeping 1925 kworker/-59 3d..2 5us : put_prev_task_fair <-__schedule 1926 kworker/-59 3d..2 6us : pick_next_task_fair <-pick_next_task 1927 kworker/-59 3d..2 6us : clear_buddies <-pick_next_task_fair 1928 kworker/-59 3d..2 6us : set_next_entity <-pick_next_task_fair 1929 kworker/-59 3d..2 6us : update_stats_wait_end <-set_next_entity 1930 ls-2269 3d..2 7us : finish_task_switch <-__schedule 1931 ls-2269 3d..2 7us : _raw_spin_unlock_irq <-finish_task_switch 1932 ls-2269 3d..2 8us : do_IRQ <-ret_from_intr 1933 ls-2269 3d..2 8us : irq_enter <-do_IRQ 1934 ls-2269 3d..2 8us : rcu_irq_enter <-irq_enter 1935 ls-2269 3d..2 9us : add_preempt_count <-irq_enter 1936 ls-2269 3d.h2 9us : exit_idle <-do_IRQ 1937 [...] 1938 ls-2269 3d.h3 20us : sub_preempt_count <-_raw_spin_unlock 1939 ls-2269 3d.h2 20us : irq_exit <-do_IRQ 1940 ls-2269 3d.h2 21us : sub_preempt_count <-irq_exit 1941 ls-2269 3d..3 21us : do_softirq <-irq_exit 1942 ls-2269 3d..3 21us : __do_softirq <-call_softirq 1943 ls-2269 3d..3 21us+: __local_bh_disable <-__do_softirq 1944 ls-2269 3d.s4 29us : sub_preempt_count <-_local_bh_enable_ip 1945 ls-2269 3d.s5 29us : sub_preempt_count <-_local_bh_enable_ip 1946 ls-2269 3d.s5 31us : do_IRQ <-ret_from_intr 1947 ls-2269 3d.s5 31us : irq_enter <-do_IRQ 1948 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter 1949 [...] 1950 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter 1951 ls-2269 3d.s5 32us : add_preempt_count <-irq_enter 1952 ls-2269 3d.H5 32us : exit_idle <-do_IRQ 1953 ls-2269 3d.H5 32us : handle_irq <-do_IRQ 1954 ls-2269 3d.H5 32us : irq_to_desc <-handle_irq 1955 ls-2269 3d.H5 33us : handle_fasteoi_irq <-handle_irq 1956 [...] 1957 ls-2269 3d.s5 158us : _raw_spin_unlock_irqrestore <-rtl8139_poll 1958 ls-2269 3d.s3 158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action 1959 ls-2269 3d.s3 159us : __local_bh_enable <-__do_softirq 1960 ls-2269 3d.s3 159us : sub_preempt_count <-__local_bh_enable 1961 ls-2269 3d..3 159us : idle_cpu <-irq_exit 1962 ls-2269 3d..3 159us : rcu_irq_exit <-irq_exit 1963 ls-2269 3d..3 160us : sub_preempt_count <-irq_exit 1964 ls-2269 3d... 161us : __mutex_unlock_slowpath <-mutex_unlock 1965 ls-2269 3d... 162us+: trace_hardirqs_on <-mutex_unlock 1966 ls-2269 3d... 186us : <stack trace> 1967 => __mutex_unlock_slowpath 1968 => mutex_unlock 1969 => process_output 1970 => n_tty_write 1971 => tty_write 1972 => vfs_write 1973 => sys_write 1974 => system_call_fastpath 1975 1976This is an interesting trace. It started with kworker running and 1977scheduling out and ls taking over. But as soon as ls released the 1978rq lock and enabled interrupts (but not preemption) an interrupt 1979triggered. When the interrupt finished, it started running softirqs. 1980But while the softirq was running, another interrupt triggered. 1981When an interrupt is running inside a softirq, the annotation is 'H'. 1982 1983 1984wakeup 1985------ 1986 1987One common case that people are interested in tracing is the 1988time it takes for a task that is woken to actually wake up. 1989Now for non Real-Time tasks, this can be arbitrary. But tracing 1990it nonetheless can be interesting. 1991 1992Without function tracing:: 1993 1994 # echo 0 > options/function-trace 1995 # echo wakeup > current_tracer 1996 # echo 1 > tracing_on 1997 # echo 0 > tracing_max_latency 1998 # chrt -f 5 sleep 1 1999 # echo 0 > tracing_on 2000 # cat trace 2001 # tracer: wakeup 2002 # 2003 # wakeup latency trace v1.1.5 on 3.8.0-test+ 2004 # -------------------------------------------------------------------- 2005 # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 2006 # ----------------- 2007 # | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0) 2008 # ----------------- 2009 # 2010 # _------=> CPU# 2011 # / _-----=> irqs-off 2012 # | / _----=> need-resched 2013 # || / _---=> hardirq/softirq 2014 # ||| / _--=> preempt-depth 2015 # |||| / delay 2016 # cmd pid ||||| time | caller 2017 # \ / ||||| \ | / 2018 <idle>-0 3dNs7 0us : 0:120:R + [003] 312:100:R kworker/3:1H 2019 <idle>-0 3dNs7 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up 2020 <idle>-0 3d..3 15us : __schedule <-schedule 2021 <idle>-0 3d..3 15us : 0:120:R ==> [003] 312:100:R kworker/3:1H 2022 2023The tracer only traces the highest priority task in the system 2024to avoid tracing the normal circumstances. Here we see that 2025the kworker with a nice priority of -20 (not very nice), took 2026just 15 microseconds from the time it woke up, to the time it 2027ran. 2028 2029Non Real-Time tasks are not that interesting. A more interesting 2030trace is to concentrate only on Real-Time tasks. 2031 2032wakeup_rt 2033--------- 2034 2035In a Real-Time environment it is very important to know the 2036wakeup time it takes for the highest priority task that is woken 2037up to the time that it executes. This is also known as "schedule 2038latency". I stress the point that this is about RT tasks. It is 2039also important to know the scheduling latency of non-RT tasks, 2040but the average schedule latency is better for non-RT tasks. 2041Tools like LatencyTop are more appropriate for such 2042measurements. 2043 2044Real-Time environments are interested in the worst case latency. 2045That is the longest latency it takes for something to happen, 2046and not the average. We can have a very fast scheduler that may 2047only have a large latency once in a while, but that would not 2048work well with Real-Time tasks. The wakeup_rt tracer was designed 2049to record the worst case wakeups of RT tasks. Non-RT tasks are 2050not recorded because the tracer only records one worst case and 2051tracing non-RT tasks that are unpredictable will overwrite the 2052worst case latency of RT tasks (just run the normal wakeup 2053tracer for a while to see that effect). 2054 2055Since this tracer only deals with RT tasks, we will run this 2056slightly differently than we did with the previous tracers. 2057Instead of performing an 'ls', we will run 'sleep 1' under 2058'chrt' which changes the priority of the task. 2059:: 2060 2061 # echo 0 > options/function-trace 2062 # echo wakeup_rt > current_tracer 2063 # echo 1 > tracing_on 2064 # echo 0 > tracing_max_latency 2065 # chrt -f 5 sleep 1 2066 # echo 0 > tracing_on 2067 # cat trace 2068 # tracer: wakeup 2069 # 2070 # tracer: wakeup_rt 2071 # 2072 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ 2073 # -------------------------------------------------------------------- 2074 # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 2075 # ----------------- 2076 # | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5) 2077 # ----------------- 2078 # 2079 # _------=> CPU# 2080 # / _-----=> irqs-off 2081 # | / _----=> need-resched 2082 # || / _---=> hardirq/softirq 2083 # ||| / _--=> preempt-depth 2084 # |||| / delay 2085 # cmd pid ||||| time | caller 2086 # \ / ||||| \ | / 2087 <idle>-0 3d.h4 0us : 0:120:R + [003] 2389: 94:R sleep 2088 <idle>-0 3d.h4 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up 2089 <idle>-0 3d..3 5us : __schedule <-schedule 2090 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep 2091 2092 2093Running this on an idle system, we see that it only took 5 microseconds 2094to perform the task switch. Note, since the trace point in the schedule 2095is before the actual "switch", we stop the tracing when the recorded task 2096is about to schedule in. This may change if we add a new marker at the 2097end of the scheduler. 2098 2099Notice that the recorded task is 'sleep' with the PID of 2389 2100and it has an rt_prio of 5. This priority is user-space priority 2101and not the internal kernel priority. The policy is 1 for 2102SCHED_FIFO and 2 for SCHED_RR. 2103 2104Note, that the trace data shows the internal priority (99 - rtprio). 2105:: 2106 2107 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep 2108 2109The 0:120:R means idle was running with a nice priority of 0 (120 - 120) 2110and in the running state 'R'. The sleep task was scheduled in with 21112389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94) 2112and it too is in the running state. 2113 2114Doing the same with chrt -r 5 and function-trace set. 2115:: 2116 2117 echo 1 > options/function-trace 2118 2119 # tracer: wakeup_rt 2120 # 2121 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ 2122 # -------------------------------------------------------------------- 2123 # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 2124 # ----------------- 2125 # | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5) 2126 # ----------------- 2127 # 2128 # _------=> CPU# 2129 # / _-----=> irqs-off 2130 # | / _----=> need-resched 2131 # || / _---=> hardirq/softirq 2132 # ||| / _--=> preempt-depth 2133 # |||| / delay 2134 # cmd pid ||||| time | caller 2135 # \ / ||||| \ | / 2136 <idle>-0 3d.h4 1us+: 0:120:R + [003] 2448: 94:R sleep 2137 <idle>-0 3d.h4 2us : ttwu_do_activate.constprop.87 <-try_to_wake_up 2138 <idle>-0 3d.h3 3us : check_preempt_curr <-ttwu_do_wakeup 2139 <idle>-0 3d.h3 3us : resched_curr <-check_preempt_curr 2140 <idle>-0 3dNh3 4us : task_woken_rt <-ttwu_do_wakeup 2141 <idle>-0 3dNh3 4us : _raw_spin_unlock <-try_to_wake_up 2142 <idle>-0 3dNh3 4us : sub_preempt_count <-_raw_spin_unlock 2143 <idle>-0 3dNh2 5us : ttwu_stat <-try_to_wake_up 2144 <idle>-0 3dNh2 5us : _raw_spin_unlock_irqrestore <-try_to_wake_up 2145 <idle>-0 3dNh2 6us : sub_preempt_count <-_raw_spin_unlock_irqrestore 2146 <idle>-0 3dNh1 6us : _raw_spin_lock <-__run_hrtimer 2147 <idle>-0 3dNh1 6us : add_preempt_count <-_raw_spin_lock 2148 <idle>-0 3dNh2 7us : _raw_spin_unlock <-hrtimer_interrupt 2149 <idle>-0 3dNh2 7us : sub_preempt_count <-_raw_spin_unlock 2150 <idle>-0 3dNh1 7us : tick_program_event <-hrtimer_interrupt 2151 <idle>-0 3dNh1 7us : clockevents_program_event <-tick_program_event 2152 <idle>-0 3dNh1 8us : ktime_get <-clockevents_program_event 2153 <idle>-0 3dNh1 8us : lapic_next_event <-clockevents_program_event 2154 <idle>-0 3dNh1 8us : irq_exit <-smp_apic_timer_interrupt 2155 <idle>-0 3dNh1 9us : sub_preempt_count <-irq_exit 2156 <idle>-0 3dN.2 9us : idle_cpu <-irq_exit 2157 <idle>-0 3dN.2 9us : rcu_irq_exit <-irq_exit 2158 <idle>-0 3dN.2 10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit 2159 <idle>-0 3dN.2 10us : sub_preempt_count <-irq_exit 2160 <idle>-0 3.N.1 11us : rcu_idle_exit <-cpu_idle 2161 <idle>-0 3dN.1 11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit 2162 <idle>-0 3.N.1 11us : tick_nohz_idle_exit <-cpu_idle 2163 <idle>-0 3dN.1 12us : menu_hrtimer_cancel <-tick_nohz_idle_exit 2164 <idle>-0 3dN.1 12us : ktime_get <-tick_nohz_idle_exit 2165 <idle>-0 3dN.1 12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit 2166 <idle>-0 3dN.1 13us : cpu_load_update_nohz <-tick_nohz_idle_exit 2167 <idle>-0 3dN.1 13us : _raw_spin_lock <-cpu_load_update_nohz 2168 <idle>-0 3dN.1 13us : add_preempt_count <-_raw_spin_lock 2169 <idle>-0 3dN.2 13us : __cpu_load_update <-cpu_load_update_nohz 2170 <idle>-0 3dN.2 14us : sched_avg_update <-__cpu_load_update 2171 <idle>-0 3dN.2 14us : _raw_spin_unlock <-cpu_load_update_nohz 2172 <idle>-0 3dN.2 14us : sub_preempt_count <-_raw_spin_unlock 2173 <idle>-0 3dN.1 15us : calc_load_nohz_stop <-tick_nohz_idle_exit 2174 <idle>-0 3dN.1 15us : touch_softlockup_watchdog <-tick_nohz_idle_exit 2175 <idle>-0 3dN.1 15us : hrtimer_cancel <-tick_nohz_idle_exit 2176 <idle>-0 3dN.1 15us : hrtimer_try_to_cancel <-hrtimer_cancel 2177 <idle>-0 3dN.1 16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel 2178 <idle>-0 3dN.1 16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18 2179 <idle>-0 3dN.1 16us : add_preempt_count <-_raw_spin_lock_irqsave 2180 <idle>-0 3dN.2 17us : __remove_hrtimer <-remove_hrtimer.part.16 2181 <idle>-0 3dN.2 17us : hrtimer_force_reprogram <-__remove_hrtimer 2182 <idle>-0 3dN.2 17us : tick_program_event <-hrtimer_force_reprogram 2183 <idle>-0 3dN.2 18us : clockevents_program_event <-tick_program_event 2184 <idle>-0 3dN.2 18us : ktime_get <-clockevents_program_event 2185 <idle>-0 3dN.2 18us : lapic_next_event <-clockevents_program_event 2186 <idle>-0 3dN.2 19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel 2187 <idle>-0 3dN.2 19us : sub_preempt_count <-_raw_spin_unlock_irqrestore 2188 <idle>-0 3dN.1 19us : hrtimer_forward <-tick_nohz_idle_exit 2189 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward 2190 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward 2191 <idle>-0 3dN.1 20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11 2192 <idle>-0 3dN.1 20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns 2193 <idle>-0 3dN.1 21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns 2194 <idle>-0 3dN.1 21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18 2195 <idle>-0 3dN.1 21us : add_preempt_count <-_raw_spin_lock_irqsave 2196 <idle>-0 3dN.2 22us : ktime_add_safe <-__hrtimer_start_range_ns 2197 <idle>-0 3dN.2 22us : enqueue_hrtimer <-__hrtimer_start_range_ns 2198 <idle>-0 3dN.2 22us : tick_program_event <-__hrtimer_start_range_ns 2199 <idle>-0 3dN.2 23us : clockevents_program_event <-tick_program_event 2200 <idle>-0 3dN.2 23us : ktime_get <-clockevents_program_event 2201 <idle>-0 3dN.2 23us : lapic_next_event <-clockevents_program_event 2202 <idle>-0 3dN.2 24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns 2203 <idle>-0 3dN.2 24us : sub_preempt_count <-_raw_spin_unlock_irqrestore 2204 <idle>-0 3dN.1 24us : account_idle_ticks <-tick_nohz_idle_exit 2205 <idle>-0 3dN.1 24us : account_idle_time <-account_idle_ticks 2206 <idle>-0 3.N.1 25us : sub_preempt_count <-cpu_idle 2207 <idle>-0 3.N.. 25us : schedule <-cpu_idle 2208 <idle>-0 3.N.. 25us : __schedule <-preempt_schedule 2209 <idle>-0 3.N.. 26us : add_preempt_count <-__schedule 2210 <idle>-0 3.N.1 26us : rcu_note_context_switch <-__schedule 2211 <idle>-0 3.N.1 26us : rcu_sched_qs <-rcu_note_context_switch 2212 <idle>-0 3dN.1 27us : rcu_preempt_qs <-rcu_note_context_switch 2213 <idle>-0 3.N.1 27us : _raw_spin_lock_irq <-__schedule 2214 <idle>-0 3dN.1 27us : add_preempt_count <-_raw_spin_lock_irq 2215 <idle>-0 3dN.2 28us : put_prev_task_idle <-__schedule 2216 <idle>-0 3dN.2 28us : pick_next_task_stop <-pick_next_task 2217 <idle>-0 3dN.2 28us : pick_next_task_rt <-pick_next_task 2218 <idle>-0 3dN.2 29us : dequeue_pushable_task <-pick_next_task_rt 2219 <idle>-0 3d..3 29us : __schedule <-preempt_schedule 2220 <idle>-0 3d..3 30us : 0:120:R ==> [003] 2448: 94:R sleep 2221 2222This isn't that big of a trace, even with function tracing enabled, 2223so I included the entire trace. 2224 2225The interrupt went off while when the system was idle. Somewhere 2226before task_woken_rt() was called, the NEED_RESCHED flag was set, 2227this is indicated by the first occurrence of the 'N' flag. 2228 2229Latency tracing and events 2230-------------------------- 2231As function tracing can induce a much larger latency, but without 2232seeing what happens within the latency it is hard to know what 2233caused it. There is a middle ground, and that is with enabling 2234events. 2235:: 2236 2237 # echo 0 > options/function-trace 2238 # echo wakeup_rt > current_tracer 2239 # echo 1 > events/enable 2240 # echo 1 > tracing_on 2241 # echo 0 > tracing_max_latency 2242 # chrt -f 5 sleep 1 2243 # echo 0 > tracing_on 2244 # cat trace 2245 # tracer: wakeup_rt 2246 # 2247 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ 2248 # -------------------------------------------------------------------- 2249 # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 2250 # ----------------- 2251 # | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5) 2252 # ----------------- 2253 # 2254 # _------=> CPU# 2255 # / _-----=> irqs-off 2256 # | / _----=> need-resched 2257 # || / _---=> hardirq/softirq 2258 # ||| / _--=> preempt-depth 2259 # |||| / delay 2260 # cmd pid ||||| time | caller 2261 # \ / ||||| \ | / 2262 <idle>-0 2d.h4 0us : 0:120:R + [002] 5882: 94:R sleep 2263 <idle>-0 2d.h4 0us : ttwu_do_activate.constprop.87 <-try_to_wake_up 2264 <idle>-0 2d.h4 1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002 2265 <idle>-0 2dNh2 1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8 2266 <idle>-0 2.N.2 2us : power_end: cpu_id=2 2267 <idle>-0 2.N.2 3us : cpu_idle: state=4294967295 cpu_id=2 2268 <idle>-0 2dN.3 4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0 2269 <idle>-0 2dN.3 4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000 2270 <idle>-0 2.N.2 5us : rcu_utilization: Start context switch 2271 <idle>-0 2.N.2 5us : rcu_utilization: End context switch 2272 <idle>-0 2d..3 6us : __schedule <-schedule 2273 <idle>-0 2d..3 6us : 0:120:R ==> [002] 5882: 94:R sleep 2274 2275 2276Hardware Latency Detector 2277------------------------- 2278 2279The hardware latency detector is executed by enabling the "hwlat" tracer. 2280 2281NOTE, this tracer will affect the performance of the system as it will 2282periodically make a CPU constantly busy with interrupts disabled. 2283:: 2284 2285 # echo hwlat > current_tracer 2286 # sleep 100 2287 # cat trace 2288 # tracer: hwlat 2289 # 2290 # entries-in-buffer/entries-written: 13/13 #P:8 2291 # 2292 # _-----=> irqs-off 2293 # / _----=> need-resched 2294 # | / _---=> hardirq/softirq 2295 # || / _--=> preempt-depth 2296 # ||| / delay 2297 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 2298 # | | | |||| | | 2299 <...>-1729 [001] d... 678.473449: #1 inner/outer(us): 11/12 ts:1581527483.343962693 count:6 2300 <...>-1729 [004] d... 689.556542: #2 inner/outer(us): 16/9 ts:1581527494.889008092 count:1 2301 <...>-1729 [005] d... 714.756290: #3 inner/outer(us): 16/16 ts:1581527519.678961629 count:5 2302 <...>-1729 [001] d... 718.788247: #4 inner/outer(us): 9/17 ts:1581527523.889012713 count:1 2303 <...>-1729 [002] d... 719.796341: #5 inner/outer(us): 13/9 ts:1581527524.912872606 count:1 2304 <...>-1729 [006] d... 844.787091: #6 inner/outer(us): 9/12 ts:1581527649.889048502 count:2 2305 <...>-1729 [003] d... 849.827033: #7 inner/outer(us): 18/9 ts:1581527654.889013793 count:1 2306 <...>-1729 [007] d... 853.859002: #8 inner/outer(us): 9/12 ts:1581527658.889065736 count:1 2307 <...>-1729 [001] d... 855.874978: #9 inner/outer(us): 9/11 ts:1581527660.861991877 count:1 2308 <...>-1729 [001] d... 863.938932: #10 inner/outer(us): 9/11 ts:1581527668.970010500 count:1 nmi-total:7 nmi-count:1 2309 <...>-1729 [007] d... 878.050780: #11 inner/outer(us): 9/12 ts:1581527683.385002600 count:1 nmi-total:5 nmi-count:1 2310 <...>-1729 [007] d... 886.114702: #12 inner/outer(us): 9/12 ts:1581527691.385001600 count:1 2311 2312 2313The above output is somewhat the same in the header. All events will have 2314interrupts disabled 'd'. Under the FUNCTION title there is: 2315 2316 #1 2317 This is the count of events recorded that were greater than the 2318 tracing_threshold (See below). 2319 2320 inner/outer(us): 11/11 2321 2322 This shows two numbers as "inner latency" and "outer latency". The test 2323 runs in a loop checking a timestamp twice. The latency detected within 2324 the two timestamps is the "inner latency" and the latency detected 2325 after the previous timestamp and the next timestamp in the loop is 2326 the "outer latency". 2327 2328 ts:1581527483.343962693 2329 2330 The absolute timestamp that the first latency was recorded in the window. 2331 2332 count:6 2333 2334 The number of times a latency was detected during the window. 2335 2336 nmi-total:7 nmi-count:1 2337 2338 On architectures that support it, if an NMI comes in during the 2339 test, the time spent in NMI is reported in "nmi-total" (in 2340 microseconds). 2341 2342 All architectures that have NMIs will show the "nmi-count" if an 2343 NMI comes in during the test. 2344 2345hwlat files: 2346 2347 tracing_threshold 2348 This gets automatically set to "10" to represent 10 2349 microseconds. This is the threshold of latency that 2350 needs to be detected before the trace will be recorded. 2351 2352 Note, when hwlat tracer is finished (another tracer is 2353 written into "current_tracer"), the original value for 2354 tracing_threshold is placed back into this file. 2355 2356 hwlat_detector/width 2357 The length of time the test runs with interrupts disabled. 2358 2359 hwlat_detector/window 2360 The length of time of the window which the test 2361 runs. That is, the test will run for "width" 2362 microseconds per "window" microseconds 2363 2364 tracing_cpumask 2365 When the test is started. A kernel thread is created that 2366 runs the test. This thread will alternate between CPUs 2367 listed in the tracing_cpumask between each period 2368 (one "window"). To limit the test to specific CPUs 2369 set the mask in this file to only the CPUs that the test 2370 should run on. 2371 2372function 2373-------- 2374 2375This tracer is the function tracer. Enabling the function tracer 2376can be done from the debug file system. Make sure the 2377ftrace_enabled is set; otherwise this tracer is a nop. 2378See the "ftrace_enabled" section below. 2379:: 2380 2381 # sysctl kernel.ftrace_enabled=1 2382 # echo function > current_tracer 2383 # echo 1 > tracing_on 2384 # usleep 1 2385 # echo 0 > tracing_on 2386 # cat trace 2387 # tracer: function 2388 # 2389 # entries-in-buffer/entries-written: 24799/24799 #P:4 2390 # 2391 # _-----=> irqs-off 2392 # / _----=> need-resched 2393 # | / _---=> hardirq/softirq 2394 # || / _--=> preempt-depth 2395 # ||| / delay 2396 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 2397 # | | | |||| | | 2398 bash-1994 [002] .... 3082.063030: mutex_unlock <-rb_simple_write 2399 bash-1994 [002] .... 3082.063031: __mutex_unlock_slowpath <-mutex_unlock 2400 bash-1994 [002] .... 3082.063031: __fsnotify_parent <-fsnotify_modify 2401 bash-1994 [002] .... 3082.063032: fsnotify <-fsnotify_modify 2402 bash-1994 [002] .... 3082.063032: __srcu_read_lock <-fsnotify 2403 bash-1994 [002] .... 3082.063032: add_preempt_count <-__srcu_read_lock 2404 bash-1994 [002] ...1 3082.063032: sub_preempt_count <-__srcu_read_lock 2405 bash-1994 [002] .... 3082.063033: __srcu_read_unlock <-fsnotify 2406 [...] 2407 2408 2409Note: function tracer uses ring buffers to store the above 2410entries. The newest data may overwrite the oldest data. 2411Sometimes using echo to stop the trace is not sufficient because 2412the tracing could have overwritten the data that you wanted to 2413record. For this reason, it is sometimes better to disable 2414tracing directly from a program. This allows you to stop the 2415tracing at the point that you hit the part that you are 2416interested in. To disable the tracing directly from a C program, 2417something like following code snippet can be used:: 2418 2419 int trace_fd; 2420 [...] 2421 int main(int argc, char *argv[]) { 2422 [...] 2423 trace_fd = open(tracing_file("tracing_on"), O_WRONLY); 2424 [...] 2425 if (condition_hit()) { 2426 write(trace_fd, "0", 1); 2427 } 2428 [...] 2429 } 2430 2431 2432Single thread tracing 2433--------------------- 2434 2435By writing into set_ftrace_pid you can trace a 2436single thread. For example:: 2437 2438 # cat set_ftrace_pid 2439 no pid 2440 # echo 3111 > set_ftrace_pid 2441 # cat set_ftrace_pid 2442 3111 2443 # echo function > current_tracer 2444 # cat trace | head 2445 # tracer: function 2446 # 2447 # TASK-PID CPU# TIMESTAMP FUNCTION 2448 # | | | | | 2449 yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return 2450 yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range 2451 yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel 2452 yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel 2453 yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll 2454 yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll 2455 # echo > set_ftrace_pid 2456 # cat trace |head 2457 # tracer: function 2458 # 2459 # TASK-PID CPU# TIMESTAMP FUNCTION 2460 # | | | | | 2461 ##### CPU 3 buffer started #### 2462 yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait 2463 yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry 2464 yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry 2465 yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit 2466 yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit 2467 2468If you want to trace a function when executing, you could use 2469something like this simple program. 2470:: 2471 2472 #include <stdio.h> 2473 #include <stdlib.h> 2474 #include <sys/types.h> 2475 #include <sys/stat.h> 2476 #include <fcntl.h> 2477 #include <unistd.h> 2478 #include <string.h> 2479 2480 #define _STR(x) #x 2481 #define STR(x) _STR(x) 2482 #define MAX_PATH 256 2483 2484 const char *find_tracefs(void) 2485 { 2486 static char tracefs[MAX_PATH+1]; 2487 static int tracefs_found; 2488 char type[100]; 2489 FILE *fp; 2490 2491 if (tracefs_found) 2492 return tracefs; 2493 2494 if ((fp = fopen("/proc/mounts","r")) == NULL) { 2495 perror("/proc/mounts"); 2496 return NULL; 2497 } 2498 2499 while (fscanf(fp, "%*s %" 2500 STR(MAX_PATH) 2501 "s %99s %*s %*d %*d\n", 2502 tracefs, type) == 2) { 2503 if (strcmp(type, "tracefs") == 0) 2504 break; 2505 } 2506 fclose(fp); 2507 2508 if (strcmp(type, "tracefs") != 0) { 2509 fprintf(stderr, "tracefs not mounted"); 2510 return NULL; 2511 } 2512 2513 strcat(tracefs, "/tracing/"); 2514 tracefs_found = 1; 2515 2516 return tracefs; 2517 } 2518 2519 const char *tracing_file(const char *file_name) 2520 { 2521 static char trace_file[MAX_PATH+1]; 2522 snprintf(trace_file, MAX_PATH, "%s/%s", find_tracefs(), file_name); 2523 return trace_file; 2524 } 2525 2526 int main (int argc, char **argv) 2527 { 2528 if (argc < 1) 2529 exit(-1); 2530 2531 if (fork() > 0) { 2532 int fd, ffd; 2533 char line[64]; 2534 int s; 2535 2536 ffd = open(tracing_file("current_tracer"), O_WRONLY); 2537 if (ffd < 0) 2538 exit(-1); 2539 write(ffd, "nop", 3); 2540 2541 fd = open(tracing_file("set_ftrace_pid"), O_WRONLY); 2542 s = sprintf(line, "%d\n", getpid()); 2543 write(fd, line, s); 2544 2545 write(ffd, "function", 8); 2546 2547 close(fd); 2548 close(ffd); 2549 2550 execvp(argv[1], argv+1); 2551 } 2552 2553 return 0; 2554 } 2555 2556Or this simple script! 2557:: 2558 2559 #!/bin/bash 2560 2561 tracefs=`sed -ne 's/^tracefs \(.*\) tracefs.*/\1/p' /proc/mounts` 2562 echo 0 > $tracefs/tracing_on 2563 echo $$ > $tracefs/set_ftrace_pid 2564 echo function > $tracefs/current_tracer 2565 echo 1 > $tracefs/tracing_on 2566 exec "$@" 2567 2568 2569function graph tracer 2570--------------------------- 2571 2572This tracer is similar to the function tracer except that it 2573probes a function on its entry and its exit. This is done by 2574using a dynamically allocated stack of return addresses in each 2575task_struct. On function entry the tracer overwrites the return 2576address of each function traced to set a custom probe. Thus the 2577original return address is stored on the stack of return address 2578in the task_struct. 2579 2580Probing on both ends of a function leads to special features 2581such as: 2582 2583- measure of a function's time execution 2584- having a reliable call stack to draw function calls graph 2585 2586This tracer is useful in several situations: 2587 2588- you want to find the reason of a strange kernel behavior and 2589 need to see what happens in detail on any areas (or specific 2590 ones). 2591 2592- you are experiencing weird latencies but it's difficult to 2593 find its origin. 2594 2595- you want to find quickly which path is taken by a specific 2596 function 2597 2598- you just want to peek inside a working kernel and want to see 2599 what happens there. 2600 2601:: 2602 2603 # tracer: function_graph 2604 # 2605 # CPU DURATION FUNCTION CALLS 2606 # | | | | | | | 2607 2608 0) | sys_open() { 2609 0) | do_sys_open() { 2610 0) | getname() { 2611 0) | kmem_cache_alloc() { 2612 0) 1.382 us | __might_sleep(); 2613 0) 2.478 us | } 2614 0) | strncpy_from_user() { 2615 0) | might_fault() { 2616 0) 1.389 us | __might_sleep(); 2617 0) 2.553 us | } 2618 0) 3.807 us | } 2619 0) 7.876 us | } 2620 0) | alloc_fd() { 2621 0) 0.668 us | _spin_lock(); 2622 0) 0.570 us | expand_files(); 2623 0) 0.586 us | _spin_unlock(); 2624 2625 2626There are several columns that can be dynamically 2627enabled/disabled. You can use every combination of options you 2628want, depending on your needs. 2629 2630- The cpu number on which the function executed is default 2631 enabled. It is sometimes better to only trace one cpu (see 2632 tracing_cpumask file) or you might sometimes see unordered 2633 function calls while cpu tracing switch. 2634 2635 - hide: echo nofuncgraph-cpu > trace_options 2636 - show: echo funcgraph-cpu > trace_options 2637 2638- The duration (function's time of execution) is displayed on 2639 the closing bracket line of a function or on the same line 2640 than the current function in case of a leaf one. It is default 2641 enabled. 2642 2643 - hide: echo nofuncgraph-duration > trace_options 2644 - show: echo funcgraph-duration > trace_options 2645 2646- The overhead field precedes the duration field in case of 2647 reached duration thresholds. 2648 2649 - hide: echo nofuncgraph-overhead > trace_options 2650 - show: echo funcgraph-overhead > trace_options 2651 - depends on: funcgraph-duration 2652 2653 ie:: 2654 2655 3) # 1837.709 us | } /* __switch_to */ 2656 3) | finish_task_switch() { 2657 3) 0.313 us | _raw_spin_unlock_irq(); 2658 3) 3.177 us | } 2659 3) # 1889.063 us | } /* __schedule */ 2660 3) ! 140.417 us | } /* __schedule */ 2661 3) # 2034.948 us | } /* schedule */ 2662 3) * 33998.59 us | } /* schedule_preempt_disabled */ 2663 2664 [...] 2665 2666 1) 0.260 us | msecs_to_jiffies(); 2667 1) 0.313 us | __rcu_read_unlock(); 2668 1) + 61.770 us | } 2669 1) + 64.479 us | } 2670 1) 0.313 us | rcu_bh_qs(); 2671 1) 0.313 us | __local_bh_enable(); 2672 1) ! 217.240 us | } 2673 1) 0.365 us | idle_cpu(); 2674 1) | rcu_irq_exit() { 2675 1) 0.417 us | rcu_eqs_enter_common.isra.47(); 2676 1) 3.125 us | } 2677 1) ! 227.812 us | } 2678 1) ! 457.395 us | } 2679 1) @ 119760.2 us | } 2680 2681 [...] 2682 2683 2) | handle_IPI() { 2684 1) 6.979 us | } 2685 2) 0.417 us | scheduler_ipi(); 2686 1) 9.791 us | } 2687 1) + 12.917 us | } 2688 2) 3.490 us | } 2689 1) + 15.729 us | } 2690 1) + 18.542 us | } 2691 2) $ 3594274 us | } 2692 2693Flags:: 2694 2695 + means that the function exceeded 10 usecs. 2696 ! means that the function exceeded 100 usecs. 2697 # means that the function exceeded 1000 usecs. 2698 * means that the function exceeded 10 msecs. 2699 @ means that the function exceeded 100 msecs. 2700 $ means that the function exceeded 1 sec. 2701 2702 2703- The task/pid field displays the thread cmdline and pid which 2704 executed the function. It is default disabled. 2705 2706 - hide: echo nofuncgraph-proc > trace_options 2707 - show: echo funcgraph-proc > trace_options 2708 2709 ie:: 2710 2711 # tracer: function_graph 2712 # 2713 # CPU TASK/PID DURATION FUNCTION CALLS 2714 # | | | | | | | | | 2715 0) sh-4802 | | d_free() { 2716 0) sh-4802 | | call_rcu() { 2717 0) sh-4802 | | __call_rcu() { 2718 0) sh-4802 | 0.616 us | rcu_process_gp_end(); 2719 0) sh-4802 | 0.586 us | check_for_new_grace_period(); 2720 0) sh-4802 | 2.899 us | } 2721 0) sh-4802 | 4.040 us | } 2722 0) sh-4802 | 5.151 us | } 2723 0) sh-4802 | + 49.370 us | } 2724 2725 2726- The absolute time field is an absolute timestamp given by the 2727 system clock since it started. A snapshot of this time is 2728 given on each entry/exit of functions 2729 2730 - hide: echo nofuncgraph-abstime > trace_options 2731 - show: echo funcgraph-abstime > trace_options 2732 2733 ie:: 2734 2735 # 2736 # TIME CPU DURATION FUNCTION CALLS 2737 # | | | | | | | | 2738 360.774522 | 1) 0.541 us | } 2739 360.774522 | 1) 4.663 us | } 2740 360.774523 | 1) 0.541 us | __wake_up_bit(); 2741 360.774524 | 1) 6.796 us | } 2742 360.774524 | 1) 7.952 us | } 2743 360.774525 | 1) 9.063 us | } 2744 360.774525 | 1) 0.615 us | journal_mark_dirty(); 2745 360.774527 | 1) 0.578 us | __brelse(); 2746 360.774528 | 1) | reiserfs_prepare_for_journal() { 2747 360.774528 | 1) | unlock_buffer() { 2748 360.774529 | 1) | wake_up_bit() { 2749 360.774529 | 1) | bit_waitqueue() { 2750 360.774530 | 1) 0.594 us | __phys_addr(); 2751 2752 2753The function name is always displayed after the closing bracket 2754for a function if the start of that function is not in the 2755trace buffer. 2756 2757Display of the function name after the closing bracket may be 2758enabled for functions whose start is in the trace buffer, 2759allowing easier searching with grep for function durations. 2760It is default disabled. 2761 2762 - hide: echo nofuncgraph-tail > trace_options 2763 - show: echo funcgraph-tail > trace_options 2764 2765 Example with nofuncgraph-tail (default):: 2766 2767 0) | putname() { 2768 0) | kmem_cache_free() { 2769 0) 0.518 us | __phys_addr(); 2770 0) 1.757 us | } 2771 0) 2.861 us | } 2772 2773 Example with funcgraph-tail:: 2774 2775 0) | putname() { 2776 0) | kmem_cache_free() { 2777 0) 0.518 us | __phys_addr(); 2778 0) 1.757 us | } /* kmem_cache_free() */ 2779 0) 2.861 us | } /* putname() */ 2780 2781The return value of each traced function can be displayed after 2782an equal sign "=". When encountering system call failures, it 2783can be very helpful to quickly locate the function that first 2784returns an error code. 2785 2786 - hide: echo nofuncgraph-retval > trace_options 2787 - show: echo funcgraph-retval > trace_options 2788 2789 Example with funcgraph-retval:: 2790 2791 1) | cgroup_migrate() { 2792 1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */ 2793 1) | cgroup_migrate_execute() { 2794 1) | cpu_cgroup_can_attach() { 2795 1) | cgroup_taskset_first() { 2796 1) 0.732 us | cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */ 2797 1) 1.232 us | } /* cgroup_taskset_first = 0xffff93fc8fb20000 */ 2798 1) 0.380 us | sched_rt_can_attach(); /* = 0x0 */ 2799 1) 2.335 us | } /* cpu_cgroup_can_attach = -22 */ 2800 1) 4.369 us | } /* cgroup_migrate_execute = -22 */ 2801 1) 7.143 us | } /* cgroup_migrate = -22 */ 2802 2803The above example shows that the function cpu_cgroup_can_attach 2804returned the error code -22 firstly, then we can read the code 2805of this function to get the root cause. 2806 2807When the option funcgraph-retval-hex is not set, the return value can 2808be displayed in a smart way. Specifically, if it is an error code, 2809it will be printed in signed decimal format, otherwise it will 2810printed in hexadecimal format. 2811 2812 - smart: echo nofuncgraph-retval-hex > trace_options 2813 - hexadecimal: echo funcgraph-retval-hex > trace_options 2814 2815 Example with funcgraph-retval-hex:: 2816 2817 1) | cgroup_migrate() { 2818 1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */ 2819 1) | cgroup_migrate_execute() { 2820 1) | cpu_cgroup_can_attach() { 2821 1) | cgroup_taskset_first() { 2822 1) 0.732 us | cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */ 2823 1) 1.232 us | } /* cgroup_taskset_first = 0xffff93fc8fb20000 */ 2824 1) 0.380 us | sched_rt_can_attach(); /* = 0x0 */ 2825 1) 2.335 us | } /* cpu_cgroup_can_attach = 0xffffffea */ 2826 1) 4.369 us | } /* cgroup_migrate_execute = 0xffffffea */ 2827 1) 7.143 us | } /* cgroup_migrate = 0xffffffea */ 2828 2829At present, there are some limitations when using the funcgraph-retval 2830option, and these limitations will be eliminated in the future: 2831 2832- Even if the function return type is void, a return value will still 2833 be printed, and you can just ignore it. 2834 2835- Even if return values are stored in multiple registers, only the 2836 value contained in the first register will be recorded and printed. 2837 To illustrate, in the x86 architecture, eax and edx are used to store 2838 a 64-bit return value, with the lower 32 bits saved in eax and the 2839 upper 32 bits saved in edx. However, only the value stored in eax 2840 will be recorded and printed. 2841 2842- In certain procedure call standards, such as arm64's AAPCS64, when a 2843 type is smaller than a GPR, it is the responsibility of the consumer 2844 to perform the narrowing, and the upper bits may contain UNKNOWN values. 2845 Therefore, it is advisable to check the code for such cases. For instance, 2846 when using a u8 in a 64-bit GPR, bits [63:8] may contain arbitrary values, 2847 especially when larger types are truncated, whether explicitly or implicitly. 2848 Here are some specific cases to illustrate this point: 2849 2850 **Case One**: 2851 2852 The function narrow_to_u8 is defined as follows:: 2853 2854 u8 narrow_to_u8(u64 val) 2855 { 2856 // implicitly truncated 2857 return val; 2858 } 2859 2860 It may be compiled to:: 2861 2862 narrow_to_u8: 2863 < ... ftrace instrumentation ... > 2864 RET 2865 2866 If you pass 0x123456789abcdef to this function and want to narrow it, 2867 it may be recorded as 0x123456789abcdef instead of 0xef. 2868 2869 **Case Two**: 2870 2871 The function error_if_not_4g_aligned is defined as follows:: 2872 2873 int error_if_not_4g_aligned(u64 val) 2874 { 2875 if (val & GENMASK(31, 0)) 2876 return -EINVAL; 2877 2878 return 0; 2879 } 2880 2881 It could be compiled to:: 2882 2883 error_if_not_4g_aligned: 2884 CBNZ w0, .Lnot_aligned 2885 RET // bits [31:0] are zero, bits 2886 // [63:32] are UNKNOWN 2887 .Lnot_aligned: 2888 MOV x0, #-EINVAL 2889 RET 2890 2891 When passing 0x2_0000_0000 to it, the return value may be recorded as 2892 0x2_0000_0000 instead of 0. 2893 2894You can put some comments on specific functions by using 2895trace_printk() For example, if you want to put a comment inside 2896the __might_sleep() function, you just have to include 2897<linux/ftrace.h> and call trace_printk() inside __might_sleep():: 2898 2899 trace_printk("I'm a comment!\n") 2900 2901will produce:: 2902 2903 1) | __might_sleep() { 2904 1) | /* I'm a comment! */ 2905 1) 1.449 us | } 2906 2907 2908You might find other useful features for this tracer in the 2909following "dynamic ftrace" section such as tracing only specific 2910functions or tasks. 2911 2912dynamic ftrace 2913-------------- 2914 2915If CONFIG_DYNAMIC_FTRACE is set, the system will run with 2916virtually no overhead when function tracing is disabled. The way 2917this works is the mcount function call (placed at the start of 2918every kernel function, produced by the -pg switch in gcc), 2919starts of pointing to a simple return. (Enabling FTRACE will 2920include the -pg switch in the compiling of the kernel.) 2921 2922At compile time every C file object is run through the 2923recordmcount program (located in the scripts directory). This 2924program will parse the ELF headers in the C object to find all 2925the locations in the .text section that call mcount. Starting 2926with gcc version 4.6, the -mfentry has been added for x86, which 2927calls "__fentry__" instead of "mcount". Which is called before 2928the creation of the stack frame. 2929 2930Note, not all sections are traced. They may be prevented by either 2931a notrace, or blocked another way and all inline functions are not 2932traced. Check the "available_filter_functions" file to see what functions 2933can be traced. 2934 2935A section called "__mcount_loc" is created that holds 2936references to all the mcount/fentry call sites in the .text section. 2937The recordmcount program re-links this section back into the 2938original object. The final linking stage of the kernel will add all these 2939references into a single table. 2940 2941On boot up, before SMP is initialized, the dynamic ftrace code 2942scans this table and updates all the locations into nops. It 2943also records the locations, which are added to the 2944available_filter_functions list. Modules are processed as they 2945are loaded and before they are executed. When a module is 2946unloaded, it also removes its functions from the ftrace function 2947list. This is automatic in the module unload code, and the 2948module author does not need to worry about it. 2949 2950When tracing is enabled, the process of modifying the function 2951tracepoints is dependent on architecture. The old method is to use 2952kstop_machine to prevent races with the CPUs executing code being 2953modified (which can cause the CPU to do undesirable things, especially 2954if the modified code crosses cache (or page) boundaries), and the nops are 2955patched back to calls. But this time, they do not call mcount 2956(which is just a function stub). They now call into the ftrace 2957infrastructure. 2958 2959The new method of modifying the function tracepoints is to place 2960a breakpoint at the location to be modified, sync all CPUs, modify 2961the rest of the instruction not covered by the breakpoint. Sync 2962all CPUs again, and then remove the breakpoint with the finished 2963version to the ftrace call site. 2964 2965Some archs do not even need to monkey around with the synchronization, 2966and can just slap the new code on top of the old without any 2967problems with other CPUs executing it at the same time. 2968 2969One special side-effect to the recording of the functions being 2970traced is that we can now selectively choose which functions we 2971wish to trace and which ones we want the mcount calls to remain 2972as nops. 2973 2974Two files are used, one for enabling and one for disabling the 2975tracing of specified functions. They are: 2976 2977 set_ftrace_filter 2978 2979and 2980 2981 set_ftrace_notrace 2982 2983A list of available functions that you can add to these files is 2984listed in: 2985 2986 available_filter_functions 2987 2988:: 2989 2990 # cat available_filter_functions 2991 put_prev_task_idle 2992 kmem_cache_create 2993 pick_next_task_rt 2994 cpus_read_lock 2995 pick_next_task_fair 2996 mutex_lock 2997 [...] 2998 2999If I am only interested in sys_nanosleep and hrtimer_interrupt:: 3000 3001 # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter 3002 # echo function > current_tracer 3003 # echo 1 > tracing_on 3004 # usleep 1 3005 # echo 0 > tracing_on 3006 # cat trace 3007 # tracer: function 3008 # 3009 # entries-in-buffer/entries-written: 5/5 #P:4 3010 # 3011 # _-----=> irqs-off 3012 # / _----=> need-resched 3013 # | / _---=> hardirq/softirq 3014 # || / _--=> preempt-depth 3015 # ||| / delay 3016 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3017 # | | | |||| | | 3018 usleep-2665 [001] .... 4186.475355: sys_nanosleep <-system_call_fastpath 3019 <idle>-0 [001] d.h1 4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt 3020 usleep-2665 [001] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt 3021 <idle>-0 [003] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt 3022 <idle>-0 [002] d.h1 4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt 3023 3024To see which functions are being traced, you can cat the file: 3025:: 3026 3027 # cat set_ftrace_filter 3028 hrtimer_interrupt 3029 sys_nanosleep 3030 3031 3032Perhaps this is not enough. The filters also allow glob(7) matching. 3033 3034 ``<match>*`` 3035 will match functions that begin with <match> 3036 ``*<match>`` 3037 will match functions that end with <match> 3038 ``*<match>*`` 3039 will match functions that have <match> in it 3040 ``<match1>*<match2>`` 3041 will match functions that begin with <match1> and end with <match2> 3042 3043.. note:: 3044 It is better to use quotes to enclose the wild cards, 3045 otherwise the shell may expand the parameters into names 3046 of files in the local directory. 3047 3048:: 3049 3050 # echo 'hrtimer_*' > set_ftrace_filter 3051 3052Produces:: 3053 3054 # tracer: function 3055 # 3056 # entries-in-buffer/entries-written: 897/897 #P:4 3057 # 3058 # _-----=> irqs-off 3059 # / _----=> need-resched 3060 # | / _---=> hardirq/softirq 3061 # || / _--=> preempt-depth 3062 # ||| / delay 3063 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3064 # | | | |||| | | 3065 <idle>-0 [003] dN.1 4228.547803: hrtimer_cancel <-tick_nohz_idle_exit 3066 <idle>-0 [003] dN.1 4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel 3067 <idle>-0 [003] dN.2 4228.547805: hrtimer_force_reprogram <-__remove_hrtimer 3068 <idle>-0 [003] dN.1 4228.547805: hrtimer_forward <-tick_nohz_idle_exit 3069 <idle>-0 [003] dN.1 4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11 3070 <idle>-0 [003] d..1 4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt 3071 <idle>-0 [003] d..1 4228.547859: hrtimer_start <-__tick_nohz_idle_enter 3072 <idle>-0 [003] d..2 4228.547860: hrtimer_force_reprogram <-__rem 3073 3074Notice that we lost the sys_nanosleep. 3075:: 3076 3077 # cat set_ftrace_filter 3078 hrtimer_run_queues 3079 hrtimer_run_pending 3080 hrtimer_setup 3081 hrtimer_cancel 3082 hrtimer_try_to_cancel 3083 hrtimer_forward 3084 hrtimer_start 3085 hrtimer_reprogram 3086 hrtimer_force_reprogram 3087 hrtimer_get_next_event 3088 hrtimer_interrupt 3089 hrtimer_nanosleep 3090 hrtimer_wakeup 3091 hrtimer_get_remaining 3092 hrtimer_get_res 3093 hrtimer_init_sleeper 3094 3095 3096This is because the '>' and '>>' act just like they do in bash. 3097To rewrite the filters, use '>' 3098To append to the filters, use '>>' 3099 3100To clear out a filter so that all functions will be recorded 3101again:: 3102 3103 # echo > set_ftrace_filter 3104 # cat set_ftrace_filter 3105 # 3106 3107Again, now we want to append. 3108 3109:: 3110 3111 # echo sys_nanosleep > set_ftrace_filter 3112 # cat set_ftrace_filter 3113 sys_nanosleep 3114 # echo 'hrtimer_*' >> set_ftrace_filter 3115 # cat set_ftrace_filter 3116 hrtimer_run_queues 3117 hrtimer_run_pending 3118 hrtimer_setup 3119 hrtimer_cancel 3120 hrtimer_try_to_cancel 3121 hrtimer_forward 3122 hrtimer_start 3123 hrtimer_reprogram 3124 hrtimer_force_reprogram 3125 hrtimer_get_next_event 3126 hrtimer_interrupt 3127 sys_nanosleep 3128 hrtimer_nanosleep 3129 hrtimer_wakeup 3130 hrtimer_get_remaining 3131 hrtimer_get_res 3132 hrtimer_init_sleeper 3133 3134 3135The set_ftrace_notrace prevents those functions from being 3136traced. 3137:: 3138 3139 # echo '*preempt*' '*lock*' > set_ftrace_notrace 3140 3141Produces:: 3142 3143 # tracer: function 3144 # 3145 # entries-in-buffer/entries-written: 39608/39608 #P:4 3146 # 3147 # _-----=> irqs-off 3148 # / _----=> need-resched 3149 # | / _---=> hardirq/softirq 3150 # || / _--=> preempt-depth 3151 # ||| / delay 3152 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3153 # | | | |||| | | 3154 bash-1994 [000] .... 4342.324896: file_ra_state_init <-do_dentry_open 3155 bash-1994 [000] .... 4342.324897: open_check_o_direct <-do_last 3156 bash-1994 [000] .... 4342.324897: ima_file_check <-do_last 3157 bash-1994 [000] .... 4342.324898: process_measurement <-ima_file_check 3158 bash-1994 [000] .... 4342.324898: ima_get_action <-process_measurement 3159 bash-1994 [000] .... 4342.324898: ima_match_policy <-ima_get_action 3160 bash-1994 [000] .... 4342.324899: do_truncate <-do_last 3161 bash-1994 [000] .... 4342.324899: setattr_should_drop_suidgid <-do_truncate 3162 bash-1994 [000] .... 4342.324899: notify_change <-do_truncate 3163 bash-1994 [000] .... 4342.324900: current_fs_time <-notify_change 3164 bash-1994 [000] .... 4342.324900: current_kernel_time <-current_fs_time 3165 bash-1994 [000] .... 4342.324900: timespec_trunc <-current_fs_time 3166 3167We can see that there's no more lock or preempt tracing. 3168 3169Selecting function filters via index 3170------------------------------------ 3171 3172Because processing of strings is expensive (the address of the function 3173needs to be looked up before comparing to the string being passed in), 3174an index can be used as well to enable functions. This is useful in the 3175case of setting thousands of specific functions at a time. By passing 3176in a list of numbers, no string processing will occur. Instead, the function 3177at the specific location in the internal array (which corresponds to the 3178functions in the "available_filter_functions" file), is selected. 3179 3180:: 3181 3182 # echo 1 > set_ftrace_filter 3183 3184Will select the first function listed in "available_filter_functions" 3185 3186:: 3187 3188 # head -1 available_filter_functions 3189 trace_initcall_finish_cb 3190 3191 # cat set_ftrace_filter 3192 trace_initcall_finish_cb 3193 3194 # head -50 available_filter_functions | tail -1 3195 x86_pmu_commit_txn 3196 3197 # echo 1 50 > set_ftrace_filter 3198 # cat set_ftrace_filter 3199 trace_initcall_finish_cb 3200 x86_pmu_commit_txn 3201 3202Dynamic ftrace with the function graph tracer 3203--------------------------------------------- 3204 3205Although what has been explained above concerns both the 3206function tracer and the function-graph-tracer, there are some 3207special features only available in the function-graph tracer. 3208 3209If you want to trace only one function and all of its children, 3210you just have to echo its name into set_graph_function:: 3211 3212 echo __do_fault > set_graph_function 3213 3214will produce the following "expanded" trace of the __do_fault() 3215function:: 3216 3217 0) | __do_fault() { 3218 0) | filemap_fault() { 3219 0) | find_lock_page() { 3220 0) 0.804 us | find_get_page(); 3221 0) | __might_sleep() { 3222 0) 1.329 us | } 3223 0) 3.904 us | } 3224 0) 4.979 us | } 3225 0) 0.653 us | _spin_lock(); 3226 0) 0.578 us | page_add_file_rmap(); 3227 0) 0.525 us | native_set_pte_at(); 3228 0) 0.585 us | _spin_unlock(); 3229 0) | unlock_page() { 3230 0) 0.541 us | page_waitqueue(); 3231 0) 0.639 us | __wake_up_bit(); 3232 0) 2.786 us | } 3233 0) + 14.237 us | } 3234 0) | __do_fault() { 3235 0) | filemap_fault() { 3236 0) | find_lock_page() { 3237 0) 0.698 us | find_get_page(); 3238 0) | __might_sleep() { 3239 0) 1.412 us | } 3240 0) 3.950 us | } 3241 0) 5.098 us | } 3242 0) 0.631 us | _spin_lock(); 3243 0) 0.571 us | page_add_file_rmap(); 3244 0) 0.526 us | native_set_pte_at(); 3245 0) 0.586 us | _spin_unlock(); 3246 0) | unlock_page() { 3247 0) 0.533 us | page_waitqueue(); 3248 0) 0.638 us | __wake_up_bit(); 3249 0) 2.793 us | } 3250 0) + 14.012 us | } 3251 3252You can also expand several functions at once:: 3253 3254 echo sys_open > set_graph_function 3255 echo sys_close >> set_graph_function 3256 3257Now if you want to go back to trace all functions you can clear 3258this special filter via:: 3259 3260 echo > set_graph_function 3261 3262 3263ftrace_enabled 3264-------------- 3265 3266Note, the proc sysctl ftrace_enable is a big on/off switch for the 3267function tracer. By default it is enabled (when function tracing is 3268enabled in the kernel). If it is disabled, all function tracing is 3269disabled. This includes not only the function tracers for ftrace, but 3270also for any other uses (perf, kprobes, stack tracing, profiling, etc). It 3271cannot be disabled if there is a callback with FTRACE_OPS_FL_PERMANENT set 3272registered. 3273 3274Please disable this with care. 3275 3276This can be disable (and enabled) with:: 3277 3278 sysctl kernel.ftrace_enabled=0 3279 sysctl kernel.ftrace_enabled=1 3280 3281 or 3282 3283 echo 0 > /proc/sys/kernel/ftrace_enabled 3284 echo 1 > /proc/sys/kernel/ftrace_enabled 3285 3286 3287Filter commands 3288--------------- 3289 3290A few commands are supported by the set_ftrace_filter interface. 3291Trace commands have the following format:: 3292 3293 <function>:<command>:<parameter> 3294 3295The following commands are supported: 3296 3297- mod: 3298 This command enables function filtering per module. The 3299 parameter defines the module. For example, if only the write* 3300 functions in the ext3 module are desired, run: 3301 3302 echo 'write*:mod:ext3' > set_ftrace_filter 3303 3304 This command interacts with the filter in the same way as 3305 filtering based on function names. Thus, adding more functions 3306 in a different module is accomplished by appending (>>) to the 3307 filter file. Remove specific module functions by prepending 3308 '!':: 3309 3310 echo '!writeback*:mod:ext3' >> set_ftrace_filter 3311 3312 Mod command supports module globbing. Disable tracing for all 3313 functions except a specific module:: 3314 3315 echo '!*:mod:!ext3' >> set_ftrace_filter 3316 3317 Disable tracing for all modules, but still trace kernel:: 3318 3319 echo '!*:mod:*' >> set_ftrace_filter 3320 3321 Enable filter only for kernel:: 3322 3323 echo '*write*:mod:!*' >> set_ftrace_filter 3324 3325 Enable filter for module globbing:: 3326 3327 echo '*write*:mod:*snd*' >> set_ftrace_filter 3328 3329- traceon/traceoff: 3330 These commands turn tracing on and off when the specified 3331 functions are hit. The parameter determines how many times the 3332 tracing system is turned on and off. If unspecified, there is 3333 no limit. For example, to disable tracing when a schedule bug 3334 is hit the first 5 times, run:: 3335 3336 echo '__schedule_bug:traceoff:5' > set_ftrace_filter 3337 3338 To always disable tracing when __schedule_bug is hit:: 3339 3340 echo '__schedule_bug:traceoff' > set_ftrace_filter 3341 3342 These commands are cumulative whether or not they are appended 3343 to set_ftrace_filter. To remove a command, prepend it by '!' 3344 and drop the parameter:: 3345 3346 echo '!__schedule_bug:traceoff:0' > set_ftrace_filter 3347 3348 The above removes the traceoff command for __schedule_bug 3349 that have a counter. To remove commands without counters:: 3350 3351 echo '!__schedule_bug:traceoff' > set_ftrace_filter 3352 3353- snapshot: 3354 Will cause a snapshot to be triggered when the function is hit. 3355 :: 3356 3357 echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter 3358 3359 To only snapshot once: 3360 :: 3361 3362 echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter 3363 3364 To remove the above commands:: 3365 3366 echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter 3367 echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter 3368 3369- enable_event/disable_event: 3370 These commands can enable or disable a trace event. Note, because 3371 function tracing callbacks are very sensitive, when these commands 3372 are registered, the trace point is activated, but disabled in 3373 a "soft" mode. That is, the tracepoint will be called, but 3374 just will not be traced. The event tracepoint stays in this mode 3375 as long as there's a command that triggers it. 3376 :: 3377 3378 echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \ 3379 set_ftrace_filter 3380 3381 The format is:: 3382 3383 <function>:enable_event:<system>:<event>[:count] 3384 <function>:disable_event:<system>:<event>[:count] 3385 3386 To remove the events commands:: 3387 3388 echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \ 3389 set_ftrace_filter 3390 echo '!schedule:disable_event:sched:sched_switch' > \ 3391 set_ftrace_filter 3392 3393- dump: 3394 When the function is hit, it will dump the contents of the ftrace 3395 ring buffer to the console. This is useful if you need to debug 3396 something, and want to dump the trace when a certain function 3397 is hit. Perhaps it's a function that is called before a triple 3398 fault happens and does not allow you to get a regular dump. 3399 3400- cpudump: 3401 When the function is hit, it will dump the contents of the ftrace 3402 ring buffer for the current CPU to the console. Unlike the "dump" 3403 command, it only prints out the contents of the ring buffer for the 3404 CPU that executed the function that triggered the dump. 3405 3406- stacktrace: 3407 When the function is hit, a stack trace is recorded. 3408 3409trace_pipe 3410---------- 3411 3412The trace_pipe outputs the same content as the trace file, but 3413the effect on the tracing is different. Every read from 3414trace_pipe is consumed. This means that subsequent reads will be 3415different. The trace is live. 3416:: 3417 3418 # echo function > current_tracer 3419 # cat trace_pipe > /tmp/trace.out & 3420 [1] 4153 3421 # echo 1 > tracing_on 3422 # usleep 1 3423 # echo 0 > tracing_on 3424 # cat trace 3425 # tracer: function 3426 # 3427 # entries-in-buffer/entries-written: 0/0 #P:4 3428 # 3429 # _-----=> irqs-off 3430 # / _----=> need-resched 3431 # | / _---=> hardirq/softirq 3432 # || / _--=> preempt-depth 3433 # ||| / delay 3434 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3435 # | | | |||| | | 3436 3437 # 3438 # cat /tmp/trace.out 3439 bash-1994 [000] .... 5281.568961: mutex_unlock <-rb_simple_write 3440 bash-1994 [000] .... 5281.568963: __mutex_unlock_slowpath <-mutex_unlock 3441 bash-1994 [000] .... 5281.568963: __fsnotify_parent <-fsnotify_modify 3442 bash-1994 [000] .... 5281.568964: fsnotify <-fsnotify_modify 3443 bash-1994 [000] .... 5281.568964: __srcu_read_lock <-fsnotify 3444 bash-1994 [000] .... 5281.568964: add_preempt_count <-__srcu_read_lock 3445 bash-1994 [000] ...1 5281.568965: sub_preempt_count <-__srcu_read_lock 3446 bash-1994 [000] .... 5281.568965: __srcu_read_unlock <-fsnotify 3447 bash-1994 [000] .... 5281.568967: sys_dup2 <-system_call_fastpath 3448 3449 3450Note, reading the trace_pipe file will block until more input is 3451added. This is contrary to the trace file. If any process opened 3452the trace file for reading, it will actually disable tracing and 3453prevent new entries from being added. The trace_pipe file does 3454not have this limitation. 3455 3456trace entries 3457------------- 3458 3459Having too much or not enough data can be troublesome in 3460diagnosing an issue in the kernel. The file buffer_size_kb is 3461used to modify the size of the internal trace buffers. The 3462number listed is the number of entries that can be recorded per 3463CPU. To know the full size, multiply the number of possible CPUs 3464with the number of entries. 3465:: 3466 3467 # cat buffer_size_kb 3468 1408 (units kilobytes) 3469 3470Or simply read buffer_total_size_kb 3471:: 3472 3473 # cat buffer_total_size_kb 3474 5632 3475 3476To modify the buffer, simple echo in a number (in 1024 byte segments). 3477:: 3478 3479 # echo 10000 > buffer_size_kb 3480 # cat buffer_size_kb 3481 10000 (units kilobytes) 3482 3483It will try to allocate as much as possible. If you allocate too 3484much, it can cause Out-Of-Memory to trigger. 3485:: 3486 3487 # echo 1000000000000 > buffer_size_kb 3488 -bash: echo: write error: Cannot allocate memory 3489 # cat buffer_size_kb 3490 85 3491 3492The per_cpu buffers can be changed individually as well: 3493:: 3494 3495 # echo 10000 > per_cpu/cpu0/buffer_size_kb 3496 # echo 100 > per_cpu/cpu1/buffer_size_kb 3497 3498When the per_cpu buffers are not the same, the buffer_size_kb 3499at the top level will just show an X 3500:: 3501 3502 # cat buffer_size_kb 3503 X 3504 3505This is where the buffer_total_size_kb is useful: 3506:: 3507 3508 # cat buffer_total_size_kb 3509 12916 3510 3511Writing to the top level buffer_size_kb will reset all the buffers 3512to be the same again. 3513 3514Snapshot 3515-------- 3516CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature 3517available to all non latency tracers. (Latency tracers which 3518record max latency, such as "irqsoff" or "wakeup", can't use 3519this feature, since those are already using the snapshot 3520mechanism internally.) 3521 3522Snapshot preserves a current trace buffer at a particular point 3523in time without stopping tracing. Ftrace swaps the current 3524buffer with a spare buffer, and tracing continues in the new 3525current (=previous spare) buffer. 3526 3527The following tracefs files in "tracing" are related to this 3528feature: 3529 3530 snapshot: 3531 3532 This is used to take a snapshot and to read the output 3533 of the snapshot. Echo 1 into this file to allocate a 3534 spare buffer and to take a snapshot (swap), then read 3535 the snapshot from this file in the same format as 3536 "trace" (described above in the section "The File 3537 System"). Both reads snapshot and tracing are executable 3538 in parallel. When the spare buffer is allocated, echoing 3539 0 frees it, and echoing else (positive) values clear the 3540 snapshot contents. 3541 More details are shown in the table below. 3542 3543 +--------------+------------+------------+------------+ 3544 |status\\input | 0 | 1 | else | 3545 +==============+============+============+============+ 3546 |not allocated |(do nothing)| alloc+swap |(do nothing)| 3547 +--------------+------------+------------+------------+ 3548 |allocated | free | swap | clear | 3549 +--------------+------------+------------+------------+ 3550 3551Here is an example of using the snapshot feature. 3552:: 3553 3554 # echo 1 > events/sched/enable 3555 # echo 1 > snapshot 3556 # cat snapshot 3557 # tracer: nop 3558 # 3559 # entries-in-buffer/entries-written: 71/71 #P:8 3560 # 3561 # _-----=> irqs-off 3562 # / _----=> need-resched 3563 # | / _---=> hardirq/softirq 3564 # || / _--=> preempt-depth 3565 # ||| / delay 3566 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3567 # | | | |||| | | 3568 <idle>-0 [005] d... 2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120 3569 sleep-2242 [005] d... 2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120 prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120 3570 [...] 3571 <idle>-0 [002] d... 2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120 3572 3573 # cat trace 3574 # tracer: nop 3575 # 3576 # entries-in-buffer/entries-written: 77/77 #P:8 3577 # 3578 # _-----=> irqs-off 3579 # / _----=> need-resched 3580 # | / _---=> hardirq/softirq 3581 # || / _--=> preempt-depth 3582 # ||| / delay 3583 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3584 # | | | |||| | | 3585 <idle>-0 [007] d... 2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120 3586 snapshot-test-2-2229 [002] d... 2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120 3587 [...] 3588 3589 3590If you try to use this snapshot feature when current tracer is 3591one of the latency tracers, you will get the following results. 3592:: 3593 3594 # echo wakeup > current_tracer 3595 # echo 1 > snapshot 3596 bash: echo: write error: Device or resource busy 3597 # cat snapshot 3598 cat: snapshot: Device or resource busy 3599 3600 3601Instances 3602--------- 3603In the tracefs tracing directory, there is a directory called "instances". 3604This directory can have new directories created inside of it using 3605mkdir, and removing directories with rmdir. The directory created 3606with mkdir in this directory will already contain files and other 3607directories after it is created. 3608:: 3609 3610 # mkdir instances/foo 3611 # ls instances/foo 3612 buffer_size_kb buffer_total_size_kb events free_buffer per_cpu 3613 set_event snapshot trace trace_clock trace_marker trace_options 3614 trace_pipe tracing_on 3615 3616As you can see, the new directory looks similar to the tracing directory 3617itself. In fact, it is very similar, except that the buffer and 3618events are agnostic from the main directory, or from any other 3619instances that are created. 3620 3621The files in the new directory work just like the files with the 3622same name in the tracing directory except the buffer that is used 3623is a separate and new buffer. The files affect that buffer but do not 3624affect the main buffer with the exception of trace_options. Currently, 3625the trace_options affect all instances and the top level buffer 3626the same, but this may change in future releases. That is, options 3627may become specific to the instance they reside in. 3628 3629Notice that none of the function tracer files are there, nor is 3630current_tracer and available_tracers. This is because the buffers 3631can currently only have events enabled for them. 3632:: 3633 3634 # mkdir instances/foo 3635 # mkdir instances/bar 3636 # mkdir instances/zoot 3637 # echo 100000 > buffer_size_kb 3638 # echo 1000 > instances/foo/buffer_size_kb 3639 # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb 3640 # echo function > current_trace 3641 # echo 1 > instances/foo/events/sched/sched_wakeup/enable 3642 # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable 3643 # echo 1 > instances/foo/events/sched/sched_switch/enable 3644 # echo 1 > instances/bar/events/irq/enable 3645 # echo 1 > instances/zoot/events/syscalls/enable 3646 # cat trace_pipe 3647 CPU:2 [LOST 11745 EVENTS] 3648 bash-2044 [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist 3649 bash-2044 [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave 3650 bash-2044 [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist 3651 bash-2044 [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist 3652 bash-2044 [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock 3653 bash-2044 [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype 3654 bash-2044 [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist 3655 bash-2044 [002] d... 10594.481034: zone_statistics <-get_page_from_freelist 3656 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics 3657 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics 3658 bash-2044 [002] .... 10594.481035: arch_dup_task_struct <-copy_process 3659 [...] 3660 3661 # cat instances/foo/trace_pipe 3662 bash-1998 [000] d..4 136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000 3663 bash-1998 [000] dN.4 136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000 3664 <idle>-0 [003] d.h3 136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003 3665 <idle>-0 [003] d..3 136.676909: sched_switch: prev_comm=swapper/3 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_preempt next_pid=9 next_prio=120 3666 rcu_preempt-9 [003] d..3 136.676916: sched_switch: prev_comm=rcu_preempt prev_pid=9 prev_prio=120 prev_state=S ==> next_comm=swapper/3 next_pid=0 next_prio=120 3667 bash-1998 [000] d..4 136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000 3668 bash-1998 [000] dN.4 136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000 3669 bash-1998 [000] d..3 136.677018: sched_switch: prev_comm=bash prev_pid=1998 prev_prio=120 prev_state=R+ ==> next_comm=kworker/0:1 next_pid=59 next_prio=120 3670 kworker/0:1-59 [000] d..4 136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001 3671 kworker/0:1-59 [000] d..3 136.677025: sched_switch: prev_comm=kworker/0:1 prev_pid=59 prev_prio=120 prev_state=S ==> next_comm=bash next_pid=1998 next_prio=120 3672 [...] 3673 3674 # cat instances/bar/trace_pipe 3675 migration/1-14 [001] d.h3 138.732674: softirq_raise: vec=3 [action=NET_RX] 3676 <idle>-0 [001] dNh3 138.732725: softirq_raise: vec=3 [action=NET_RX] 3677 bash-1998 [000] d.h1 138.733101: softirq_raise: vec=1 [action=TIMER] 3678 bash-1998 [000] d.h1 138.733102: softirq_raise: vec=9 [action=RCU] 3679 bash-1998 [000] ..s2 138.733105: softirq_entry: vec=1 [action=TIMER] 3680 bash-1998 [000] ..s2 138.733106: softirq_exit: vec=1 [action=TIMER] 3681 bash-1998 [000] ..s2 138.733106: softirq_entry: vec=9 [action=RCU] 3682 bash-1998 [000] ..s2 138.733109: softirq_exit: vec=9 [action=RCU] 3683 sshd-1995 [001] d.h1 138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4 3684 sshd-1995 [001] d.h1 138.733280: irq_handler_exit: irq=21 ret=unhandled 3685 sshd-1995 [001] d.h1 138.733281: irq_handler_entry: irq=21 name=eth0 3686 sshd-1995 [001] d.h1 138.733283: irq_handler_exit: irq=21 ret=handled 3687 [...] 3688 3689 # cat instances/zoot/trace 3690 # tracer: nop 3691 # 3692 # entries-in-buffer/entries-written: 18996/18996 #P:4 3693 # 3694 # _-----=> irqs-off 3695 # / _----=> need-resched 3696 # | / _---=> hardirq/softirq 3697 # || / _--=> preempt-depth 3698 # ||| / delay 3699 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3700 # | | | |||| | | 3701 bash-1998 [000] d... 140.733501: sys_write -> 0x2 3702 bash-1998 [000] d... 140.733504: sys_dup2(oldfd: a, newfd: 1) 3703 bash-1998 [000] d... 140.733506: sys_dup2 -> 0x1 3704 bash-1998 [000] d... 140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0) 3705 bash-1998 [000] d... 140.733509: sys_fcntl -> 0x1 3706 bash-1998 [000] d... 140.733510: sys_close(fd: a) 3707 bash-1998 [000] d... 140.733510: sys_close -> 0x0 3708 bash-1998 [000] d... 140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8) 3709 bash-1998 [000] d... 140.733515: sys_rt_sigprocmask -> 0x0 3710 bash-1998 [000] d... 140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8) 3711 bash-1998 [000] d... 140.733516: sys_rt_sigaction -> 0x0 3712 3713You can see that the trace of the top most trace buffer shows only 3714the function tracing. The foo instance displays wakeups and task 3715switches. 3716 3717To remove the instances, simply delete their directories: 3718:: 3719 3720 # rmdir instances/foo 3721 # rmdir instances/bar 3722 # rmdir instances/zoot 3723 3724Note, if a process has a trace file open in one of the instance 3725directories, the rmdir will fail with EBUSY. 3726 3727 3728Stack trace 3729----------- 3730Since the kernel has a fixed sized stack, it is important not to 3731waste it in functions. A kernel developer must be conscious of 3732what they allocate on the stack. If they add too much, the system 3733can be in danger of a stack overflow, and corruption will occur, 3734usually leading to a system panic. 3735 3736There are some tools that check this, usually with interrupts 3737periodically checking usage. But if you can perform a check 3738at every function call that will become very useful. As ftrace provides 3739a function tracer, it makes it convenient to check the stack size 3740at every function call. This is enabled via the stack tracer. 3741 3742CONFIG_STACK_TRACER enables the ftrace stack tracing functionality. 3743To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled. 3744:: 3745 3746 # echo 1 > /proc/sys/kernel/stack_tracer_enabled 3747 3748You can also enable it from the kernel command line to trace 3749the stack size of the kernel during boot up, by adding "stacktrace" 3750to the kernel command line parameter. 3751 3752After running it for a few minutes, the output looks like: 3753:: 3754 3755 # cat stack_max_size 3756 2928 3757 3758 # cat stack_trace 3759 Depth Size Location (18 entries) 3760 ----- ---- -------- 3761 0) 2928 224 update_sd_lb_stats+0xbc/0x4ac 3762 1) 2704 160 find_busiest_group+0x31/0x1f1 3763 2) 2544 256 load_balance+0xd9/0x662 3764 3) 2288 80 idle_balance+0xbb/0x130 3765 4) 2208 128 __schedule+0x26e/0x5b9 3766 5) 2080 16 schedule+0x64/0x66 3767 6) 2064 128 schedule_timeout+0x34/0xe0 3768 7) 1936 112 wait_for_common+0x97/0xf1 3769 8) 1824 16 wait_for_completion+0x1d/0x1f 3770 9) 1808 128 flush_work+0xfe/0x119 3771 10) 1680 16 tty_flush_to_ldisc+0x1e/0x20 3772 11) 1664 48 input_available_p+0x1d/0x5c 3773 12) 1616 48 n_tty_poll+0x6d/0x134 3774 13) 1568 64 tty_poll+0x64/0x7f 3775 14) 1504 880 do_select+0x31e/0x511 3776 15) 624 400 core_sys_select+0x177/0x216 3777 16) 224 96 sys_select+0x91/0xb9 3778 17) 128 128 system_call_fastpath+0x16/0x1b 3779 3780Note, if -mfentry is being used by gcc, functions get traced before 3781they set up the stack frame. This means that leaf level functions 3782are not tested by the stack tracer when -mfentry is used. 3783 3784Currently, -mfentry is used by gcc 4.6.0 and above on x86 only. 3785 3786More 3787---- 3788More details can be found in the source code, in the `kernel/trace/*.c` files. 3789