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 "blk" 814 815 The block tracer. The tracer used by the blktrace user 816 application. 817 818 "hwlat" 819 820 The Hardware Latency tracer is used to detect if the hardware 821 produces any latency. See "Hardware Latency Detector" section 822 below. 823 824 "irqsoff" 825 826 Traces the areas that disable interrupts and saves 827 the trace with the longest max latency. 828 See tracing_max_latency. When a new max is recorded, 829 it replaces the old trace. It is best to view this 830 trace with the latency-format option enabled, which 831 happens automatically when the tracer is selected. 832 833 "preemptoff" 834 835 Similar to irqsoff but traces and records the amount of 836 time for which preemption is disabled. 837 838 "preemptirqsoff" 839 840 Similar to irqsoff and preemptoff, but traces and 841 records the largest time for which irqs and/or preemption 842 is disabled. 843 844 "wakeup" 845 846 Traces and records the max latency that it takes for 847 the highest priority task to get scheduled after 848 it has been woken up. 849 Traces all tasks as an average developer would expect. 850 851 "wakeup_rt" 852 853 Traces and records the max latency that it takes for just 854 RT tasks (as the current "wakeup" does). This is useful 855 for those interested in wake up timings of RT tasks. 856 857 "wakeup_dl" 858 859 Traces and records the max latency that it takes for 860 a SCHED_DEADLINE task to be woken (as the "wakeup" and 861 "wakeup_rt" does). 862 863 "mmiotrace" 864 865 A special tracer that is used to trace binary module. 866 It will trace all the calls that a module makes to the 867 hardware. Everything it writes and reads from the I/O 868 as well. 869 870 "branch" 871 872 This tracer can be configured when tracing likely/unlikely 873 calls within the kernel. It will trace when a likely and 874 unlikely branch is hit and if it was correct in its prediction 875 of being correct. 876 877 "nop" 878 879 This is the "trace nothing" tracer. To remove all 880 tracers from tracing simply echo "nop" into 881 current_tracer. 882 883Error conditions 884---------------- 885 886 For most ftrace commands, failure modes are obvious and communicated 887 using standard return codes. 888 889 For other more involved commands, extended error information may be 890 available via the tracing/error_log file. For the commands that 891 support it, reading the tracing/error_log file after an error will 892 display more detailed information about what went wrong, if 893 information is available. The tracing/error_log file is a circular 894 error log displaying a small number (currently, 8) of ftrace errors 895 for the last (8) failed commands. 896 897 The extended error information and usage takes the form shown in 898 this example:: 899 900 # echo xxx > /sys/kernel/tracing/events/sched/sched_wakeup/trigger 901 echo: write error: Invalid argument 902 903 # cat /sys/kernel/tracing/error_log 904 [ 5348.887237] location: error: Couldn't yyy: zzz 905 Command: xxx 906 ^ 907 [ 7517.023364] location: error: Bad rrr: sss 908 Command: ppp qqq 909 ^ 910 911 To clear the error log, echo the empty string into it:: 912 913 # echo > /sys/kernel/tracing/error_log 914 915Examples of using the tracer 916---------------------------- 917 918Here are typical examples of using the tracers when controlling 919them only with the tracefs interface (without using any 920user-land utilities). 921 922Output format: 923-------------- 924 925Here is an example of the output format of the file "trace":: 926 927 # tracer: function 928 # 929 # entries-in-buffer/entries-written: 140080/250280 #P:4 930 # 931 # _-----=> irqs-off 932 # / _----=> need-resched 933 # | / _---=> hardirq/softirq 934 # || / _--=> preempt-depth 935 # ||| / delay 936 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 937 # | | | |||| | | 938 bash-1977 [000] .... 17284.993652: sys_close <-system_call_fastpath 939 bash-1977 [000] .... 17284.993653: __close_fd <-sys_close 940 bash-1977 [000] .... 17284.993653: _raw_spin_lock <-__close_fd 941 sshd-1974 [003] .... 17284.993653: __srcu_read_unlock <-fsnotify 942 bash-1977 [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock 943 bash-1977 [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd 944 bash-1977 [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock 945 bash-1977 [000] .... 17284.993657: filp_close <-__close_fd 946 bash-1977 [000] .... 17284.993657: dnotify_flush <-filp_close 947 sshd-1974 [003] .... 17284.993658: sys_select <-system_call_fastpath 948 .... 949 950A header is printed with the tracer name that is represented by 951the trace. In this case the tracer is "function". Then it shows the 952number of events in the buffer as well as the total number of entries 953that were written. The difference is the number of entries that were 954lost due to the buffer filling up (250280 - 140080 = 110200 events 955lost). 956 957The header explains the content of the events. Task name "bash", the task 958PID "1977", the CPU that it was running on "000", the latency format 959(explained below), the timestamp in <secs>.<usecs> format, the 960function name that was traced "sys_close" and the parent function that 961called this function "system_call_fastpath". The timestamp is the time 962at which the function was entered. 963 964Latency trace format 965-------------------- 966 967When the latency-format option is enabled or when one of the latency 968tracers is set, the trace file gives somewhat more information to see 969why a latency happened. Here is a typical trace:: 970 971 # tracer: irqsoff 972 # 973 # irqsoff latency trace v1.1.5 on 3.8.0-test+ 974 # -------------------------------------------------------------------- 975 # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 976 # ----------------- 977 # | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0) 978 # ----------------- 979 # => started at: __lock_task_sighand 980 # => ended at: _raw_spin_unlock_irqrestore 981 # 982 # 983 # _------=> CPU# 984 # / _-----=> irqs-off 985 # | / _----=> need-resched 986 # || / _---=> hardirq/softirq 987 # ||| / _--=> preempt-depth 988 # |||| / delay 989 # cmd pid ||||| time | caller 990 # \ / ||||| \ | / 991 ps-6143 2d... 0us!: trace_hardirqs_off <-__lock_task_sighand 992 ps-6143 2d..1 259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore 993 ps-6143 2d..1 263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore 994 ps-6143 2d..1 306us : <stack trace> 995 => trace_hardirqs_on_caller 996 => trace_hardirqs_on 997 => _raw_spin_unlock_irqrestore 998 => do_task_stat 999 => proc_tgid_stat 1000 => proc_single_show 1001 => seq_read 1002 => vfs_read 1003 => sys_read 1004 => system_call_fastpath 1005 1006 1007This shows that the current tracer is "irqsoff" tracing the time 1008for which interrupts were disabled. It gives the trace version (which 1009never changes) and the version of the kernel upon which this was executed on 1010(3.8). Then it displays the max latency in microseconds (259 us). The number 1011of trace entries displayed and the total number (both are four: #4/4). 1012VP, KP, SP, and HP are always zero and are reserved for later use. 1013#P is the number of online CPUs (#P:4). 1014 1015The task is the process that was running when the latency 1016occurred. (ps pid: 6143). 1017 1018The start and stop (the functions in which the interrupts were 1019disabled and enabled respectively) that caused the latencies: 1020 1021 - __lock_task_sighand is where the interrupts were disabled. 1022 - _raw_spin_unlock_irqrestore is where they were enabled again. 1023 1024The next lines after the header are the trace itself. The header 1025explains which is which. 1026 1027 cmd: The name of the process in the trace. 1028 1029 pid: The PID of that process. 1030 1031 CPU#: The CPU which the process was running on. 1032 1033 irqs-off: 'd' interrupts are disabled. '.' otherwise. 1034 .. caution:: If the architecture does not support a way to 1035 read the irq flags variable, an 'X' will always 1036 be printed here. 1037 1038 need-resched: 1039 - 'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set, 1040 - 'n' only TIF_NEED_RESCHED is set, 1041 - 'p' only PREEMPT_NEED_RESCHED is set, 1042 - '.' otherwise. 1043 1044 hardirq/softirq: 1045 - 'Z' - NMI occurred inside a hardirq 1046 - 'z' - NMI is running 1047 - 'H' - hard irq occurred inside a softirq. 1048 - 'h' - hard irq is running 1049 - 's' - soft irq is running 1050 - '.' - normal context. 1051 1052 preempt-depth: The level of preempt_disabled 1053 1054The above is mostly meaningful for kernel developers. 1055 1056 time: 1057 When the latency-format option is enabled, the trace file 1058 output includes a timestamp relative to the start of the 1059 trace. This differs from the output when latency-format 1060 is disabled, which includes an absolute timestamp. 1061 1062 delay: 1063 This is just to help catch your eye a bit better. And 1064 needs to be fixed to be only relative to the same CPU. 1065 The marks are determined by the difference between this 1066 current trace and the next trace. 1067 1068 - '$' - greater than 1 second 1069 - '@' - greater than 100 millisecond 1070 - '*' - greater than 10 millisecond 1071 - '#' - greater than 1000 microsecond 1072 - '!' - greater than 100 microsecond 1073 - '+' - greater than 10 microsecond 1074 - ' ' - less than or equal to 10 microsecond. 1075 1076 The rest is the same as the 'trace' file. 1077 1078 Note, the latency tracers will usually end with a back trace 1079 to easily find where the latency occurred. 1080 1081trace_options 1082------------- 1083 1084The trace_options file (or the options directory) is used to control 1085what gets printed in the trace output, or manipulate the tracers. 1086To see what is available, simply cat the file:: 1087 1088 cat trace_options 1089 print-parent 1090 nosym-offset 1091 nosym-addr 1092 noverbose 1093 noraw 1094 nohex 1095 nobin 1096 noblock 1097 nofields 1098 trace_printk 1099 annotate 1100 nouserstacktrace 1101 nosym-userobj 1102 noprintk-msg-only 1103 context-info 1104 nolatency-format 1105 record-cmd 1106 norecord-tgid 1107 overwrite 1108 nodisable_on_free 1109 irq-info 1110 markers 1111 noevent-fork 1112 function-trace 1113 nofunction-fork 1114 nodisplay-graph 1115 nostacktrace 1116 nobranch 1117 1118To disable one of the options, echo in the option prepended with 1119"no":: 1120 1121 echo noprint-parent > trace_options 1122 1123To enable an option, leave off the "no":: 1124 1125 echo sym-offset > trace_options 1126 1127Here are the available options: 1128 1129 print-parent 1130 On function traces, display the calling (parent) 1131 function as well as the function being traced. 1132 :: 1133 1134 print-parent: 1135 bash-4000 [01] 1477.606694: simple_strtoul <-kstrtoul 1136 1137 noprint-parent: 1138 bash-4000 [01] 1477.606694: simple_strtoul 1139 1140 1141 sym-offset 1142 Display not only the function name, but also the 1143 offset in the function. For example, instead of 1144 seeing just "ktime_get", you will see 1145 "ktime_get+0xb/0x20". 1146 :: 1147 1148 sym-offset: 1149 bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0 1150 1151 sym-addr 1152 This will also display the function address as well 1153 as the function name. 1154 :: 1155 1156 sym-addr: 1157 bash-4000 [01] 1477.606694: simple_strtoul <c0339346> 1158 1159 verbose 1160 This deals with the trace file when the 1161 latency-format option is enabled. 1162 :: 1163 1164 bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \ 1165 (+0.000ms): simple_strtoul (kstrtoul) 1166 1167 raw 1168 This will display raw numbers. This option is best for 1169 use with user applications that can translate the raw 1170 numbers better than having it done in the kernel. 1171 1172 hex 1173 Similar to raw, but the numbers will be in a hexadecimal format. 1174 1175 bin 1176 This will print out the formats in raw binary. 1177 1178 block 1179 When set, reading trace_pipe will not block when polled. 1180 1181 fields 1182 Print the fields as described by their types. This is a better 1183 option than using hex, bin or raw, as it gives a better parsing 1184 of the content of the event. 1185 1186 trace_printk 1187 Can disable trace_printk() from writing into the buffer. 1188 1189 annotate 1190 It is sometimes confusing when the CPU buffers are full 1191 and one CPU buffer had a lot of events recently, thus 1192 a shorter time frame, were another CPU may have only had 1193 a few events, which lets it have older events. When 1194 the trace is reported, it shows the oldest events first, 1195 and it may look like only one CPU ran (the one with the 1196 oldest events). When the annotate option is set, it will 1197 display when a new CPU buffer started:: 1198 1199 <idle>-0 [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on 1200 <idle>-0 [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on 1201 <idle>-0 [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore 1202 ##### CPU 2 buffer started #### 1203 <idle>-0 [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle 1204 <idle>-0 [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog 1205 <idle>-0 [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock 1206 1207 userstacktrace 1208 This option changes the trace. It records a 1209 stacktrace of the current user space thread after 1210 each trace event. 1211 1212 sym-userobj 1213 when user stacktrace are enabled, look up which 1214 object the address belongs to, and print a 1215 relative address. This is especially useful when 1216 ASLR is on, otherwise you don't get a chance to 1217 resolve the address to object/file/line after 1218 the app is no longer running 1219 1220 The lookup is performed when you read 1221 trace,trace_pipe. Example:: 1222 1223 a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0 1224 x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6] 1225 1226 1227 printk-msg-only 1228 When set, trace_printk()s will only show the format 1229 and not their parameters (if trace_bprintk() or 1230 trace_bputs() was used to save the trace_printk()). 1231 1232 context-info 1233 Show only the event data. Hides the comm, PID, 1234 timestamp, CPU, and other useful data. 1235 1236 latency-format 1237 This option changes the trace output. When it is enabled, 1238 the trace displays additional information about the 1239 latency, as described in "Latency trace format". 1240 1241 pause-on-trace 1242 When set, opening the trace file for read, will pause 1243 writing to the ring buffer (as if tracing_on was set to zero). 1244 This simulates the original behavior of the trace file. 1245 When the file is closed, tracing will be enabled again. 1246 1247 hash-ptr 1248 When set, "%p" in the event printk format displays the 1249 hashed pointer value instead of real address. 1250 This will be useful if you want to find out which hashed 1251 value is corresponding to the real value in trace log. 1252 1253 record-cmd 1254 When any event or tracer is enabled, a hook is enabled 1255 in the sched_switch trace point to fill comm cache 1256 with mapped pids and comms. But this may cause some 1257 overhead, and if you only care about pids, and not the 1258 name of the task, disabling this option can lower the 1259 impact of tracing. See "saved_cmdlines". 1260 1261 record-tgid 1262 When any event or tracer is enabled, a hook is enabled 1263 in the sched_switch trace point to fill the cache of 1264 mapped Thread Group IDs (TGID) mapping to pids. See 1265 "saved_tgids". 1266 1267 overwrite 1268 This controls what happens when the trace buffer is 1269 full. If "1" (default), the oldest events are 1270 discarded and overwritten. If "0", then the newest 1271 events are discarded. 1272 (see per_cpu/cpu0/stats for overrun and dropped) 1273 1274 disable_on_free 1275 When the free_buffer is closed, tracing will 1276 stop (tracing_on set to 0). 1277 1278 irq-info 1279 Shows the interrupt, preempt count, need resched data. 1280 When disabled, the trace looks like:: 1281 1282 # tracer: function 1283 # 1284 # entries-in-buffer/entries-written: 144405/9452052 #P:4 1285 # 1286 # TASK-PID CPU# TIMESTAMP FUNCTION 1287 # | | | | | 1288 <idle>-0 [002] 23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up 1289 <idle>-0 [002] 23636.756054: activate_task <-ttwu_do_activate.constprop.89 1290 <idle>-0 [002] 23636.756055: enqueue_task <-activate_task 1291 1292 1293 markers 1294 When set, the trace_marker is writable (only by root). 1295 When disabled, the trace_marker will error with EINVAL 1296 on write. 1297 1298 event-fork 1299 When set, tasks with PIDs listed in set_event_pid will have 1300 the PIDs of their children added to set_event_pid when those 1301 tasks fork. Also, when tasks with PIDs in set_event_pid exit, 1302 their PIDs will be removed from the file. 1303 1304 This affects PIDs listed in set_event_notrace_pid as well. 1305 1306 function-trace 1307 The latency tracers will enable function tracing 1308 if this option is enabled (default it is). When 1309 it is disabled, the latency tracers do not trace 1310 functions. This keeps the overhead of the tracer down 1311 when performing latency tests. 1312 1313 function-fork 1314 When set, tasks with PIDs listed in set_ftrace_pid will 1315 have the PIDs of their children added to set_ftrace_pid 1316 when those tasks fork. Also, when tasks with PIDs in 1317 set_ftrace_pid exit, their PIDs will be removed from the 1318 file. 1319 1320 This affects PIDs in set_ftrace_notrace_pid as well. 1321 1322 display-graph 1323 When set, the latency tracers (irqsoff, wakeup, etc) will 1324 use function graph tracing instead of function tracing. 1325 1326 stacktrace 1327 When set, a stack trace is recorded after any trace event 1328 is recorded. 1329 1330 branch 1331 Enable branch tracing with the tracer. This enables branch 1332 tracer along with the currently set tracer. Enabling this 1333 with the "nop" tracer is the same as just enabling the 1334 "branch" tracer. 1335 1336.. tip:: Some tracers have their own options. They only appear in this 1337 file when the tracer is active. They always appear in the 1338 options directory. 1339 1340 1341Here are the per tracer options: 1342 1343Options for function tracer: 1344 1345 func_stack_trace 1346 When set, a stack trace is recorded after every 1347 function that is recorded. NOTE! Limit the functions 1348 that are recorded before enabling this, with 1349 "set_ftrace_filter" otherwise the system performance 1350 will be critically degraded. Remember to disable 1351 this option before clearing the function filter. 1352 1353Options for function_graph tracer: 1354 1355 Since the function_graph tracer has a slightly different output 1356 it has its own options to control what is displayed. 1357 1358 funcgraph-overrun 1359 When set, the "overrun" of the graph stack is 1360 displayed after each function traced. The 1361 overrun, is when the stack depth of the calls 1362 is greater than what is reserved for each task. 1363 Each task has a fixed array of functions to 1364 trace in the call graph. If the depth of the 1365 calls exceeds that, the function is not traced. 1366 The overrun is the number of functions missed 1367 due to exceeding this array. 1368 1369 funcgraph-cpu 1370 When set, the CPU number of the CPU where the trace 1371 occurred is displayed. 1372 1373 funcgraph-overhead 1374 When set, if the function takes longer than 1375 A certain amount, then a delay marker is 1376 displayed. See "delay" above, under the 1377 header description. 1378 1379 funcgraph-proc 1380 Unlike other tracers, the process' command line 1381 is not displayed by default, but instead only 1382 when a task is traced in and out during a context 1383 switch. Enabling this options has the command 1384 of each process displayed at every line. 1385 1386 funcgraph-duration 1387 At the end of each function (the return) 1388 the duration of the amount of time in the 1389 function is displayed in microseconds. 1390 1391 funcgraph-abstime 1392 When set, the timestamp is displayed at each line. 1393 1394 funcgraph-irqs 1395 When disabled, functions that happen inside an 1396 interrupt will not be traced. 1397 1398 funcgraph-tail 1399 When set, the return event will include the function 1400 that it represents. By default this is off, and 1401 only a closing curly bracket "}" is displayed for 1402 the return of a function. 1403 1404 funcgraph-retval 1405 When set, the return value of each traced function 1406 will be printed after an equal sign "=". By default 1407 this is off. 1408 1409 funcgraph-retval-hex 1410 When set, the return value will always be printed 1411 in hexadecimal format. If the option is not set and 1412 the return value is an error code, it will be printed 1413 in signed decimal format; otherwise it will also be 1414 printed in hexadecimal format. By default, this option 1415 is off. 1416 1417 sleep-time 1418 When running function graph tracer, to include 1419 the time a task schedules out in its function. 1420 When enabled, it will account time the task has been 1421 scheduled out as part of the function call. 1422 1423 graph-time 1424 When running function profiler with function graph tracer, 1425 to include the time to call nested functions. When this is 1426 not set, the time reported for the function will only 1427 include the time the function itself executed for, not the 1428 time for functions that it called. 1429 1430Options for blk tracer: 1431 1432 blk_classic 1433 Shows a more minimalistic output. 1434 1435 1436irqsoff 1437------- 1438 1439When interrupts are disabled, the CPU can not react to any other 1440external event (besides NMIs and SMIs). This prevents the timer 1441interrupt from triggering or the mouse interrupt from letting 1442the kernel know of a new mouse event. The result is a latency 1443with the reaction time. 1444 1445The irqsoff tracer tracks the time for which interrupts are 1446disabled. When a new maximum latency is hit, the tracer saves 1447the trace leading up to that latency point so that every time a 1448new maximum is reached, the old saved trace is discarded and the 1449new trace is saved. 1450 1451To reset the maximum, echo 0 into tracing_max_latency. Here is 1452an example:: 1453 1454 # echo 0 > options/function-trace 1455 # echo irqsoff > current_tracer 1456 # echo 1 > tracing_on 1457 # echo 0 > tracing_max_latency 1458 # ls -ltr 1459 [...] 1460 # echo 0 > tracing_on 1461 # cat trace 1462 # tracer: irqsoff 1463 # 1464 # irqsoff latency trace v1.1.5 on 3.8.0-test+ 1465 # -------------------------------------------------------------------- 1466 # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1467 # ----------------- 1468 # | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0) 1469 # ----------------- 1470 # => started at: run_timer_softirq 1471 # => ended at: run_timer_softirq 1472 # 1473 # 1474 # _------=> CPU# 1475 # / _-----=> irqs-off 1476 # | / _----=> need-resched 1477 # || / _---=> hardirq/softirq 1478 # ||| / _--=> preempt-depth 1479 # |||| / delay 1480 # cmd pid ||||| time | caller 1481 # \ / ||||| \ | / 1482 <idle>-0 0d.s2 0us+: _raw_spin_lock_irq <-run_timer_softirq 1483 <idle>-0 0dNs3 17us : _raw_spin_unlock_irq <-run_timer_softirq 1484 <idle>-0 0dNs3 17us+: trace_hardirqs_on <-run_timer_softirq 1485 <idle>-0 0dNs3 25us : <stack trace> 1486 => _raw_spin_unlock_irq 1487 => run_timer_softirq 1488 => __do_softirq 1489 => call_softirq 1490 => do_softirq 1491 => irq_exit 1492 => smp_apic_timer_interrupt 1493 => apic_timer_interrupt 1494 => rcu_idle_exit 1495 => cpu_idle 1496 => rest_init 1497 => start_kernel 1498 => x86_64_start_reservations 1499 => x86_64_start_kernel 1500 1501Here we see that we had a latency of 16 microseconds (which is 1502very good). The _raw_spin_lock_irq in run_timer_softirq disabled 1503interrupts. The difference between the 16 and the displayed 1504timestamp 25us occurred because the clock was incremented 1505between the time of recording the max latency and the time of 1506recording the function that had that latency. 1507 1508Note the above example had function-trace not set. If we set 1509function-trace, we get a much larger output:: 1510 1511 with echo 1 > options/function-trace 1512 1513 # tracer: irqsoff 1514 # 1515 # irqsoff latency trace v1.1.5 on 3.8.0-test+ 1516 # -------------------------------------------------------------------- 1517 # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1518 # ----------------- 1519 # | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0) 1520 # ----------------- 1521 # => started at: ata_scsi_queuecmd 1522 # => ended at: ata_scsi_queuecmd 1523 # 1524 # 1525 # _------=> CPU# 1526 # / _-----=> irqs-off 1527 # | / _----=> need-resched 1528 # || / _---=> hardirq/softirq 1529 # ||| / _--=> preempt-depth 1530 # |||| / delay 1531 # cmd pid ||||| time | caller 1532 # \ / ||||| \ | / 1533 bash-2042 3d... 0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd 1534 bash-2042 3d... 0us : add_preempt_count <-_raw_spin_lock_irqsave 1535 bash-2042 3d..1 1us : ata_scsi_find_dev <-ata_scsi_queuecmd 1536 bash-2042 3d..1 1us : __ata_scsi_find_dev <-ata_scsi_find_dev 1537 bash-2042 3d..1 2us : ata_find_dev.part.14 <-__ata_scsi_find_dev 1538 bash-2042 3d..1 2us : ata_qc_new_init <-__ata_scsi_queuecmd 1539 bash-2042 3d..1 3us : ata_sg_init <-__ata_scsi_queuecmd 1540 bash-2042 3d..1 4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd 1541 bash-2042 3d..1 4us : ata_build_rw_tf <-ata_scsi_rw_xlat 1542 [...] 1543 bash-2042 3d..1 67us : delay_tsc <-__delay 1544 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc 1545 bash-2042 3d..2 67us : sub_preempt_count <-delay_tsc 1546 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc 1547 bash-2042 3d..2 68us : sub_preempt_count <-delay_tsc 1548 bash-2042 3d..1 68us+: ata_bmdma_start <-ata_bmdma_qc_issue 1549 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd 1550 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd 1551 bash-2042 3d..1 72us+: trace_hardirqs_on <-ata_scsi_queuecmd 1552 bash-2042 3d..1 120us : <stack trace> 1553 => _raw_spin_unlock_irqrestore 1554 => ata_scsi_queuecmd 1555 => scsi_dispatch_cmd 1556 => scsi_request_fn 1557 => __blk_run_queue_uncond 1558 => __blk_run_queue 1559 => blk_queue_bio 1560 => submit_bio_noacct 1561 => submit_bio 1562 => submit_bh 1563 => __ext3_get_inode_loc 1564 => ext3_iget 1565 => ext3_lookup 1566 => lookup_real 1567 => __lookup_hash 1568 => walk_component 1569 => lookup_last 1570 => path_lookupat 1571 => filename_lookup 1572 => user_path_at_empty 1573 => user_path_at 1574 => vfs_fstatat 1575 => vfs_stat 1576 => sys_newstat 1577 => system_call_fastpath 1578 1579 1580Here we traced a 71 microsecond latency. But we also see all the 1581functions that were called during that time. Note that by 1582enabling function tracing, we incur an added overhead. This 1583overhead may extend the latency times. But nevertheless, this 1584trace has provided some very helpful debugging information. 1585 1586If we prefer function graph output instead of function, we can set 1587display-graph option:: 1588 1589 with echo 1 > options/display-graph 1590 1591 # tracer: irqsoff 1592 # 1593 # irqsoff latency trace v1.1.5 on 4.20.0-rc6+ 1594 # -------------------------------------------------------------------- 1595 # latency: 3751 us, #274/274, CPU#0 | (M:desktop VP:0, KP:0, SP:0 HP:0 #P:4) 1596 # ----------------- 1597 # | task: bash-1507 (uid:0 nice:0 policy:0 rt_prio:0) 1598 # ----------------- 1599 # => started at: free_debug_processing 1600 # => ended at: return_to_handler 1601 # 1602 # 1603 # _-----=> irqs-off 1604 # / _----=> need-resched 1605 # | / _---=> hardirq/softirq 1606 # || / _--=> preempt-depth 1607 # ||| / 1608 # REL TIME CPU TASK/PID |||| DURATION FUNCTION CALLS 1609 # | | | | |||| | | | | | | 1610 0 us | 0) bash-1507 | d... | 0.000 us | _raw_spin_lock_irqsave(); 1611 0 us | 0) bash-1507 | d..1 | 0.378 us | do_raw_spin_trylock(); 1612 1 us | 0) bash-1507 | d..2 | | set_track() { 1613 2 us | 0) bash-1507 | d..2 | | save_stack_trace() { 1614 2 us | 0) bash-1507 | d..2 | | __save_stack_trace() { 1615 3 us | 0) bash-1507 | d..2 | | __unwind_start() { 1616 3 us | 0) bash-1507 | d..2 | | get_stack_info() { 1617 3 us | 0) bash-1507 | d..2 | 0.351 us | in_task_stack(); 1618 4 us | 0) bash-1507 | d..2 | 1.107 us | } 1619 [...] 1620 3750 us | 0) bash-1507 | d..1 | 0.516 us | do_raw_spin_unlock(); 1621 3750 us | 0) bash-1507 | d..1 | 0.000 us | _raw_spin_unlock_irqrestore(); 1622 3764 us | 0) bash-1507 | d..1 | 0.000 us | tracer_hardirqs_on(); 1623 bash-1507 0d..1 3792us : <stack trace> 1624 => free_debug_processing 1625 => __slab_free 1626 => kmem_cache_free 1627 => vm_area_free 1628 => remove_vma 1629 => exit_mmap 1630 => mmput 1631 => begin_new_exec 1632 => load_elf_binary 1633 => search_binary_handler 1634 => __do_execve_file.isra.32 1635 => __x64_sys_execve 1636 => do_syscall_64 1637 => entry_SYSCALL_64_after_hwframe 1638 1639preemptoff 1640---------- 1641 1642When preemption is disabled, we may be able to receive 1643interrupts but the task cannot be preempted and a higher 1644priority task must wait for preemption to be enabled again 1645before it can preempt a lower priority task. 1646 1647The preemptoff tracer traces the places that disable preemption. 1648Like the irqsoff tracer, it records the maximum latency for 1649which preemption was disabled. The control of preemptoff tracer 1650is much like the irqsoff tracer. 1651:: 1652 1653 # echo 0 > options/function-trace 1654 # echo preemptoff > current_tracer 1655 # echo 1 > tracing_on 1656 # echo 0 > tracing_max_latency 1657 # ls -ltr 1658 [...] 1659 # echo 0 > tracing_on 1660 # cat trace 1661 # tracer: preemptoff 1662 # 1663 # preemptoff latency trace v1.1.5 on 3.8.0-test+ 1664 # -------------------------------------------------------------------- 1665 # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1666 # ----------------- 1667 # | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0) 1668 # ----------------- 1669 # => started at: do_IRQ 1670 # => ended at: do_IRQ 1671 # 1672 # 1673 # _------=> CPU# 1674 # / _-----=> irqs-off 1675 # | / _----=> need-resched 1676 # || / _---=> hardirq/softirq 1677 # ||| / _--=> preempt-depth 1678 # |||| / delay 1679 # cmd pid ||||| time | caller 1680 # \ / ||||| \ | / 1681 sshd-1991 1d.h. 0us+: irq_enter <-do_IRQ 1682 sshd-1991 1d..1 46us : irq_exit <-do_IRQ 1683 sshd-1991 1d..1 47us+: trace_preempt_on <-do_IRQ 1684 sshd-1991 1d..1 52us : <stack trace> 1685 => sub_preempt_count 1686 => irq_exit 1687 => do_IRQ 1688 => ret_from_intr 1689 1690 1691This has some more changes. Preemption was disabled when an 1692interrupt came in (notice the 'h'), and was enabled on exit. 1693But we also see that interrupts have been disabled when entering 1694the preempt off section and leaving it (the 'd'). We do not know if 1695interrupts were enabled in the mean time or shortly after this 1696was over. 1697:: 1698 1699 # tracer: preemptoff 1700 # 1701 # preemptoff latency trace v1.1.5 on 3.8.0-test+ 1702 # -------------------------------------------------------------------- 1703 # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1704 # ----------------- 1705 # | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0) 1706 # ----------------- 1707 # => started at: wake_up_new_task 1708 # => ended at: task_rq_unlock 1709 # 1710 # 1711 # _------=> CPU# 1712 # / _-----=> irqs-off 1713 # | / _----=> need-resched 1714 # || / _---=> hardirq/softirq 1715 # ||| / _--=> preempt-depth 1716 # |||| / delay 1717 # cmd pid ||||| time | caller 1718 # \ / ||||| \ | / 1719 bash-1994 1d..1 0us : _raw_spin_lock_irqsave <-wake_up_new_task 1720 bash-1994 1d..1 0us : select_task_rq_fair <-select_task_rq 1721 bash-1994 1d..1 1us : __rcu_read_lock <-select_task_rq_fair 1722 bash-1994 1d..1 1us : source_load <-select_task_rq_fair 1723 bash-1994 1d..1 1us : source_load <-select_task_rq_fair 1724 [...] 1725 bash-1994 1d..1 12us : irq_enter <-smp_apic_timer_interrupt 1726 bash-1994 1d..1 12us : rcu_irq_enter <-irq_enter 1727 bash-1994 1d..1 13us : add_preempt_count <-irq_enter 1728 bash-1994 1d.h1 13us : exit_idle <-smp_apic_timer_interrupt 1729 bash-1994 1d.h1 13us : hrtimer_interrupt <-smp_apic_timer_interrupt 1730 bash-1994 1d.h1 13us : _raw_spin_lock <-hrtimer_interrupt 1731 bash-1994 1d.h1 14us : add_preempt_count <-_raw_spin_lock 1732 bash-1994 1d.h2 14us : ktime_get_update_offsets <-hrtimer_interrupt 1733 [...] 1734 bash-1994 1d.h1 35us : lapic_next_event <-clockevents_program_event 1735 bash-1994 1d.h1 35us : irq_exit <-smp_apic_timer_interrupt 1736 bash-1994 1d.h1 36us : sub_preempt_count <-irq_exit 1737 bash-1994 1d..2 36us : do_softirq <-irq_exit 1738 bash-1994 1d..2 36us : __do_softirq <-call_softirq 1739 bash-1994 1d..2 36us : __local_bh_disable <-__do_softirq 1740 bash-1994 1d.s2 37us : add_preempt_count <-_raw_spin_lock_irq 1741 bash-1994 1d.s3 38us : _raw_spin_unlock <-run_timer_softirq 1742 bash-1994 1d.s3 39us : sub_preempt_count <-_raw_spin_unlock 1743 bash-1994 1d.s2 39us : call_timer_fn <-run_timer_softirq 1744 [...] 1745 bash-1994 1dNs2 81us : cpu_needs_another_gp <-rcu_process_callbacks 1746 bash-1994 1dNs2 82us : __local_bh_enable <-__do_softirq 1747 bash-1994 1dNs2 82us : sub_preempt_count <-__local_bh_enable 1748 bash-1994 1dN.2 82us : idle_cpu <-irq_exit 1749 bash-1994 1dN.2 83us : rcu_irq_exit <-irq_exit 1750 bash-1994 1dN.2 83us : sub_preempt_count <-irq_exit 1751 bash-1994 1.N.1 84us : _raw_spin_unlock_irqrestore <-task_rq_unlock 1752 bash-1994 1.N.1 84us+: trace_preempt_on <-task_rq_unlock 1753 bash-1994 1.N.1 104us : <stack trace> 1754 => sub_preempt_count 1755 => _raw_spin_unlock_irqrestore 1756 => task_rq_unlock 1757 => wake_up_new_task 1758 => do_fork 1759 => sys_clone 1760 => stub_clone 1761 1762 1763The above is an example of the preemptoff trace with 1764function-trace set. Here we see that interrupts were not disabled 1765the entire time. The irq_enter code lets us know that we entered 1766an interrupt 'h'. Before that, the functions being traced still 1767show that it is not in an interrupt, but we can see from the 1768functions themselves that this is not the case. 1769 1770preemptirqsoff 1771-------------- 1772 1773Knowing the locations that have interrupts disabled or 1774preemption disabled for the longest times is helpful. But 1775sometimes we would like to know when either preemption and/or 1776interrupts are disabled. 1777 1778Consider the following code:: 1779 1780 local_irq_disable(); 1781 call_function_with_irqs_off(); 1782 preempt_disable(); 1783 call_function_with_irqs_and_preemption_off(); 1784 local_irq_enable(); 1785 call_function_with_preemption_off(); 1786 preempt_enable(); 1787 1788The irqsoff tracer will record the total length of 1789call_function_with_irqs_off() and 1790call_function_with_irqs_and_preemption_off(). 1791 1792The preemptoff tracer will record the total length of 1793call_function_with_irqs_and_preemption_off() and 1794call_function_with_preemption_off(). 1795 1796But neither will trace the time that interrupts and/or 1797preemption is disabled. This total time is the time that we can 1798not schedule. To record this time, use the preemptirqsoff 1799tracer. 1800 1801Again, using this trace is much like the irqsoff and preemptoff 1802tracers. 1803:: 1804 1805 # echo 0 > options/function-trace 1806 # echo preemptirqsoff > current_tracer 1807 # echo 1 > tracing_on 1808 # echo 0 > tracing_max_latency 1809 # ls -ltr 1810 [...] 1811 # echo 0 > tracing_on 1812 # cat trace 1813 # tracer: preemptirqsoff 1814 # 1815 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+ 1816 # -------------------------------------------------------------------- 1817 # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1818 # ----------------- 1819 # | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0) 1820 # ----------------- 1821 # => started at: ata_scsi_queuecmd 1822 # => ended at: ata_scsi_queuecmd 1823 # 1824 # 1825 # _------=> CPU# 1826 # / _-----=> irqs-off 1827 # | / _----=> need-resched 1828 # || / _---=> hardirq/softirq 1829 # ||| / _--=> preempt-depth 1830 # |||| / delay 1831 # cmd pid ||||| time | caller 1832 # \ / ||||| \ | / 1833 ls-2230 3d... 0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd 1834 ls-2230 3...1 100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd 1835 ls-2230 3...1 101us+: trace_preempt_on <-ata_scsi_queuecmd 1836 ls-2230 3...1 111us : <stack trace> 1837 => sub_preempt_count 1838 => _raw_spin_unlock_irqrestore 1839 => ata_scsi_queuecmd 1840 => scsi_dispatch_cmd 1841 => scsi_request_fn 1842 => __blk_run_queue_uncond 1843 => __blk_run_queue 1844 => blk_queue_bio 1845 => submit_bio_noacct 1846 => submit_bio 1847 => submit_bh 1848 => ext3_bread 1849 => ext3_dir_bread 1850 => htree_dirblock_to_tree 1851 => ext3_htree_fill_tree 1852 => ext3_readdir 1853 => vfs_readdir 1854 => sys_getdents 1855 => system_call_fastpath 1856 1857 1858The trace_hardirqs_off_thunk is called from assembly on x86 when 1859interrupts are disabled in the assembly code. Without the 1860function tracing, we do not know if interrupts were enabled 1861within the preemption points. We do see that it started with 1862preemption enabled. 1863 1864Here is a trace with function-trace set:: 1865 1866 # tracer: preemptirqsoff 1867 # 1868 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+ 1869 # -------------------------------------------------------------------- 1870 # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1871 # ----------------- 1872 # | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0) 1873 # ----------------- 1874 # => started at: schedule 1875 # => ended at: mutex_unlock 1876 # 1877 # 1878 # _------=> CPU# 1879 # / _-----=> irqs-off 1880 # | / _----=> need-resched 1881 # || / _---=> hardirq/softirq 1882 # ||| / _--=> preempt-depth 1883 # |||| / delay 1884 # cmd pid ||||| time | caller 1885 # \ / ||||| \ | / 1886 kworker/-59 3...1 0us : __schedule <-schedule 1887 kworker/-59 3d..1 0us : rcu_preempt_qs <-rcu_note_context_switch 1888 kworker/-59 3d..1 1us : add_preempt_count <-_raw_spin_lock_irq 1889 kworker/-59 3d..2 1us : deactivate_task <-__schedule 1890 kworker/-59 3d..2 1us : dequeue_task <-deactivate_task 1891 kworker/-59 3d..2 2us : update_rq_clock <-dequeue_task 1892 kworker/-59 3d..2 2us : dequeue_task_fair <-dequeue_task 1893 kworker/-59 3d..2 2us : update_curr <-dequeue_task_fair 1894 kworker/-59 3d..2 2us : update_min_vruntime <-update_curr 1895 kworker/-59 3d..2 3us : cpuacct_charge <-update_curr 1896 kworker/-59 3d..2 3us : __rcu_read_lock <-cpuacct_charge 1897 kworker/-59 3d..2 3us : __rcu_read_unlock <-cpuacct_charge 1898 kworker/-59 3d..2 3us : update_cfs_rq_blocked_load <-dequeue_task_fair 1899 kworker/-59 3d..2 4us : clear_buddies <-dequeue_task_fair 1900 kworker/-59 3d..2 4us : account_entity_dequeue <-dequeue_task_fair 1901 kworker/-59 3d..2 4us : update_min_vruntime <-dequeue_task_fair 1902 kworker/-59 3d..2 4us : update_cfs_shares <-dequeue_task_fair 1903 kworker/-59 3d..2 5us : hrtick_update <-dequeue_task_fair 1904 kworker/-59 3d..2 5us : wq_worker_sleeping <-__schedule 1905 kworker/-59 3d..2 5us : kthread_data <-wq_worker_sleeping 1906 kworker/-59 3d..2 5us : put_prev_task_fair <-__schedule 1907 kworker/-59 3d..2 6us : pick_next_task_fair <-pick_next_task 1908 kworker/-59 3d..2 6us : clear_buddies <-pick_next_task_fair 1909 kworker/-59 3d..2 6us : set_next_entity <-pick_next_task_fair 1910 kworker/-59 3d..2 6us : update_stats_wait_end <-set_next_entity 1911 ls-2269 3d..2 7us : finish_task_switch <-__schedule 1912 ls-2269 3d..2 7us : _raw_spin_unlock_irq <-finish_task_switch 1913 ls-2269 3d..2 8us : do_IRQ <-ret_from_intr 1914 ls-2269 3d..2 8us : irq_enter <-do_IRQ 1915 ls-2269 3d..2 8us : rcu_irq_enter <-irq_enter 1916 ls-2269 3d..2 9us : add_preempt_count <-irq_enter 1917 ls-2269 3d.h2 9us : exit_idle <-do_IRQ 1918 [...] 1919 ls-2269 3d.h3 20us : sub_preempt_count <-_raw_spin_unlock 1920 ls-2269 3d.h2 20us : irq_exit <-do_IRQ 1921 ls-2269 3d.h2 21us : sub_preempt_count <-irq_exit 1922 ls-2269 3d..3 21us : do_softirq <-irq_exit 1923 ls-2269 3d..3 21us : __do_softirq <-call_softirq 1924 ls-2269 3d..3 21us+: __local_bh_disable <-__do_softirq 1925 ls-2269 3d.s4 29us : sub_preempt_count <-_local_bh_enable_ip 1926 ls-2269 3d.s5 29us : sub_preempt_count <-_local_bh_enable_ip 1927 ls-2269 3d.s5 31us : do_IRQ <-ret_from_intr 1928 ls-2269 3d.s5 31us : irq_enter <-do_IRQ 1929 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter 1930 [...] 1931 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter 1932 ls-2269 3d.s5 32us : add_preempt_count <-irq_enter 1933 ls-2269 3d.H5 32us : exit_idle <-do_IRQ 1934 ls-2269 3d.H5 32us : handle_irq <-do_IRQ 1935 ls-2269 3d.H5 32us : irq_to_desc <-handle_irq 1936 ls-2269 3d.H5 33us : handle_fasteoi_irq <-handle_irq 1937 [...] 1938 ls-2269 3d.s5 158us : _raw_spin_unlock_irqrestore <-rtl8139_poll 1939 ls-2269 3d.s3 158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action 1940 ls-2269 3d.s3 159us : __local_bh_enable <-__do_softirq 1941 ls-2269 3d.s3 159us : sub_preempt_count <-__local_bh_enable 1942 ls-2269 3d..3 159us : idle_cpu <-irq_exit 1943 ls-2269 3d..3 159us : rcu_irq_exit <-irq_exit 1944 ls-2269 3d..3 160us : sub_preempt_count <-irq_exit 1945 ls-2269 3d... 161us : __mutex_unlock_slowpath <-mutex_unlock 1946 ls-2269 3d... 162us+: trace_hardirqs_on <-mutex_unlock 1947 ls-2269 3d... 186us : <stack trace> 1948 => __mutex_unlock_slowpath 1949 => mutex_unlock 1950 => process_output 1951 => n_tty_write 1952 => tty_write 1953 => vfs_write 1954 => sys_write 1955 => system_call_fastpath 1956 1957This is an interesting trace. It started with kworker running and 1958scheduling out and ls taking over. But as soon as ls released the 1959rq lock and enabled interrupts (but not preemption) an interrupt 1960triggered. When the interrupt finished, it started running softirqs. 1961But while the softirq was running, another interrupt triggered. 1962When an interrupt is running inside a softirq, the annotation is 'H'. 1963 1964 1965wakeup 1966------ 1967 1968One common case that people are interested in tracing is the 1969time it takes for a task that is woken to actually wake up. 1970Now for non Real-Time tasks, this can be arbitrary. But tracing 1971it none the less can be interesting. 1972 1973Without function tracing:: 1974 1975 # echo 0 > options/function-trace 1976 # echo wakeup > current_tracer 1977 # echo 1 > tracing_on 1978 # echo 0 > tracing_max_latency 1979 # chrt -f 5 sleep 1 1980 # echo 0 > tracing_on 1981 # cat trace 1982 # tracer: wakeup 1983 # 1984 # wakeup latency trace v1.1.5 on 3.8.0-test+ 1985 # -------------------------------------------------------------------- 1986 # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1987 # ----------------- 1988 # | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0) 1989 # ----------------- 1990 # 1991 # _------=> CPU# 1992 # / _-----=> irqs-off 1993 # | / _----=> need-resched 1994 # || / _---=> hardirq/softirq 1995 # ||| / _--=> preempt-depth 1996 # |||| / delay 1997 # cmd pid ||||| time | caller 1998 # \ / ||||| \ | / 1999 <idle>-0 3dNs7 0us : 0:120:R + [003] 312:100:R kworker/3:1H 2000 <idle>-0 3dNs7 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up 2001 <idle>-0 3d..3 15us : __schedule <-schedule 2002 <idle>-0 3d..3 15us : 0:120:R ==> [003] 312:100:R kworker/3:1H 2003 2004The tracer only traces the highest priority task in the system 2005to avoid tracing the normal circumstances. Here we see that 2006the kworker with a nice priority of -20 (not very nice), took 2007just 15 microseconds from the time it woke up, to the time it 2008ran. 2009 2010Non Real-Time tasks are not that interesting. A more interesting 2011trace is to concentrate only on Real-Time tasks. 2012 2013wakeup_rt 2014--------- 2015 2016In a Real-Time environment it is very important to know the 2017wakeup time it takes for the highest priority task that is woken 2018up to the time that it executes. This is also known as "schedule 2019latency". I stress the point that this is about RT tasks. It is 2020also important to know the scheduling latency of non-RT tasks, 2021but the average schedule latency is better for non-RT tasks. 2022Tools like LatencyTop are more appropriate for such 2023measurements. 2024 2025Real-Time environments are interested in the worst case latency. 2026That is the longest latency it takes for something to happen, 2027and not the average. We can have a very fast scheduler that may 2028only have a large latency once in a while, but that would not 2029work well with Real-Time tasks. The wakeup_rt tracer was designed 2030to record the worst case wakeups of RT tasks. Non-RT tasks are 2031not recorded because the tracer only records one worst case and 2032tracing non-RT tasks that are unpredictable will overwrite the 2033worst case latency of RT tasks (just run the normal wakeup 2034tracer for a while to see that effect). 2035 2036Since this tracer only deals with RT tasks, we will run this 2037slightly differently than we did with the previous tracers. 2038Instead of performing an 'ls', we will run 'sleep 1' under 2039'chrt' which changes the priority of the task. 2040:: 2041 2042 # echo 0 > options/function-trace 2043 # echo wakeup_rt > current_tracer 2044 # echo 1 > tracing_on 2045 # echo 0 > tracing_max_latency 2046 # chrt -f 5 sleep 1 2047 # echo 0 > tracing_on 2048 # cat trace 2049 # tracer: wakeup 2050 # 2051 # tracer: wakeup_rt 2052 # 2053 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ 2054 # -------------------------------------------------------------------- 2055 # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 2056 # ----------------- 2057 # | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5) 2058 # ----------------- 2059 # 2060 # _------=> CPU# 2061 # / _-----=> irqs-off 2062 # | / _----=> need-resched 2063 # || / _---=> hardirq/softirq 2064 # ||| / _--=> preempt-depth 2065 # |||| / delay 2066 # cmd pid ||||| time | caller 2067 # \ / ||||| \ | / 2068 <idle>-0 3d.h4 0us : 0:120:R + [003] 2389: 94:R sleep 2069 <idle>-0 3d.h4 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up 2070 <idle>-0 3d..3 5us : __schedule <-schedule 2071 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep 2072 2073 2074Running this on an idle system, we see that it only took 5 microseconds 2075to perform the task switch. Note, since the trace point in the schedule 2076is before the actual "switch", we stop the tracing when the recorded task 2077is about to schedule in. This may change if we add a new marker at the 2078end of the scheduler. 2079 2080Notice that the recorded task is 'sleep' with the PID of 2389 2081and it has an rt_prio of 5. This priority is user-space priority 2082and not the internal kernel priority. The policy is 1 for 2083SCHED_FIFO and 2 for SCHED_RR. 2084 2085Note, that the trace data shows the internal priority (99 - rtprio). 2086:: 2087 2088 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep 2089 2090The 0:120:R means idle was running with a nice priority of 0 (120 - 120) 2091and in the running state 'R'. The sleep task was scheduled in with 20922389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94) 2093and it too is in the running state. 2094 2095Doing the same with chrt -r 5 and function-trace set. 2096:: 2097 2098 echo 1 > options/function-trace 2099 2100 # tracer: wakeup_rt 2101 # 2102 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ 2103 # -------------------------------------------------------------------- 2104 # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 2105 # ----------------- 2106 # | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5) 2107 # ----------------- 2108 # 2109 # _------=> CPU# 2110 # / _-----=> irqs-off 2111 # | / _----=> need-resched 2112 # || / _---=> hardirq/softirq 2113 # ||| / _--=> preempt-depth 2114 # |||| / delay 2115 # cmd pid ||||| time | caller 2116 # \ / ||||| \ | / 2117 <idle>-0 3d.h4 1us+: 0:120:R + [003] 2448: 94:R sleep 2118 <idle>-0 3d.h4 2us : ttwu_do_activate.constprop.87 <-try_to_wake_up 2119 <idle>-0 3d.h3 3us : check_preempt_curr <-ttwu_do_wakeup 2120 <idle>-0 3d.h3 3us : resched_curr <-check_preempt_curr 2121 <idle>-0 3dNh3 4us : task_woken_rt <-ttwu_do_wakeup 2122 <idle>-0 3dNh3 4us : _raw_spin_unlock <-try_to_wake_up 2123 <idle>-0 3dNh3 4us : sub_preempt_count <-_raw_spin_unlock 2124 <idle>-0 3dNh2 5us : ttwu_stat <-try_to_wake_up 2125 <idle>-0 3dNh2 5us : _raw_spin_unlock_irqrestore <-try_to_wake_up 2126 <idle>-0 3dNh2 6us : sub_preempt_count <-_raw_spin_unlock_irqrestore 2127 <idle>-0 3dNh1 6us : _raw_spin_lock <-__run_hrtimer 2128 <idle>-0 3dNh1 6us : add_preempt_count <-_raw_spin_lock 2129 <idle>-0 3dNh2 7us : _raw_spin_unlock <-hrtimer_interrupt 2130 <idle>-0 3dNh2 7us : sub_preempt_count <-_raw_spin_unlock 2131 <idle>-0 3dNh1 7us : tick_program_event <-hrtimer_interrupt 2132 <idle>-0 3dNh1 7us : clockevents_program_event <-tick_program_event 2133 <idle>-0 3dNh1 8us : ktime_get <-clockevents_program_event 2134 <idle>-0 3dNh1 8us : lapic_next_event <-clockevents_program_event 2135 <idle>-0 3dNh1 8us : irq_exit <-smp_apic_timer_interrupt 2136 <idle>-0 3dNh1 9us : sub_preempt_count <-irq_exit 2137 <idle>-0 3dN.2 9us : idle_cpu <-irq_exit 2138 <idle>-0 3dN.2 9us : rcu_irq_exit <-irq_exit 2139 <idle>-0 3dN.2 10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit 2140 <idle>-0 3dN.2 10us : sub_preempt_count <-irq_exit 2141 <idle>-0 3.N.1 11us : rcu_idle_exit <-cpu_idle 2142 <idle>-0 3dN.1 11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit 2143 <idle>-0 3.N.1 11us : tick_nohz_idle_exit <-cpu_idle 2144 <idle>-0 3dN.1 12us : menu_hrtimer_cancel <-tick_nohz_idle_exit 2145 <idle>-0 3dN.1 12us : ktime_get <-tick_nohz_idle_exit 2146 <idle>-0 3dN.1 12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit 2147 <idle>-0 3dN.1 13us : cpu_load_update_nohz <-tick_nohz_idle_exit 2148 <idle>-0 3dN.1 13us : _raw_spin_lock <-cpu_load_update_nohz 2149 <idle>-0 3dN.1 13us : add_preempt_count <-_raw_spin_lock 2150 <idle>-0 3dN.2 13us : __cpu_load_update <-cpu_load_update_nohz 2151 <idle>-0 3dN.2 14us : sched_avg_update <-__cpu_load_update 2152 <idle>-0 3dN.2 14us : _raw_spin_unlock <-cpu_load_update_nohz 2153 <idle>-0 3dN.2 14us : sub_preempt_count <-_raw_spin_unlock 2154 <idle>-0 3dN.1 15us : calc_load_nohz_stop <-tick_nohz_idle_exit 2155 <idle>-0 3dN.1 15us : touch_softlockup_watchdog <-tick_nohz_idle_exit 2156 <idle>-0 3dN.1 15us : hrtimer_cancel <-tick_nohz_idle_exit 2157 <idle>-0 3dN.1 15us : hrtimer_try_to_cancel <-hrtimer_cancel 2158 <idle>-0 3dN.1 16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel 2159 <idle>-0 3dN.1 16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18 2160 <idle>-0 3dN.1 16us : add_preempt_count <-_raw_spin_lock_irqsave 2161 <idle>-0 3dN.2 17us : __remove_hrtimer <-remove_hrtimer.part.16 2162 <idle>-0 3dN.2 17us : hrtimer_force_reprogram <-__remove_hrtimer 2163 <idle>-0 3dN.2 17us : tick_program_event <-hrtimer_force_reprogram 2164 <idle>-0 3dN.2 18us : clockevents_program_event <-tick_program_event 2165 <idle>-0 3dN.2 18us : ktime_get <-clockevents_program_event 2166 <idle>-0 3dN.2 18us : lapic_next_event <-clockevents_program_event 2167 <idle>-0 3dN.2 19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel 2168 <idle>-0 3dN.2 19us : sub_preempt_count <-_raw_spin_unlock_irqrestore 2169 <idle>-0 3dN.1 19us : hrtimer_forward <-tick_nohz_idle_exit 2170 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward 2171 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward 2172 <idle>-0 3dN.1 20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11 2173 <idle>-0 3dN.1 20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns 2174 <idle>-0 3dN.1 21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns 2175 <idle>-0 3dN.1 21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18 2176 <idle>-0 3dN.1 21us : add_preempt_count <-_raw_spin_lock_irqsave 2177 <idle>-0 3dN.2 22us : ktime_add_safe <-__hrtimer_start_range_ns 2178 <idle>-0 3dN.2 22us : enqueue_hrtimer <-__hrtimer_start_range_ns 2179 <idle>-0 3dN.2 22us : tick_program_event <-__hrtimer_start_range_ns 2180 <idle>-0 3dN.2 23us : clockevents_program_event <-tick_program_event 2181 <idle>-0 3dN.2 23us : ktime_get <-clockevents_program_event 2182 <idle>-0 3dN.2 23us : lapic_next_event <-clockevents_program_event 2183 <idle>-0 3dN.2 24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns 2184 <idle>-0 3dN.2 24us : sub_preempt_count <-_raw_spin_unlock_irqrestore 2185 <idle>-0 3dN.1 24us : account_idle_ticks <-tick_nohz_idle_exit 2186 <idle>-0 3dN.1 24us : account_idle_time <-account_idle_ticks 2187 <idle>-0 3.N.1 25us : sub_preempt_count <-cpu_idle 2188 <idle>-0 3.N.. 25us : schedule <-cpu_idle 2189 <idle>-0 3.N.. 25us : __schedule <-preempt_schedule 2190 <idle>-0 3.N.. 26us : add_preempt_count <-__schedule 2191 <idle>-0 3.N.1 26us : rcu_note_context_switch <-__schedule 2192 <idle>-0 3.N.1 26us : rcu_sched_qs <-rcu_note_context_switch 2193 <idle>-0 3dN.1 27us : rcu_preempt_qs <-rcu_note_context_switch 2194 <idle>-0 3.N.1 27us : _raw_spin_lock_irq <-__schedule 2195 <idle>-0 3dN.1 27us : add_preempt_count <-_raw_spin_lock_irq 2196 <idle>-0 3dN.2 28us : put_prev_task_idle <-__schedule 2197 <idle>-0 3dN.2 28us : pick_next_task_stop <-pick_next_task 2198 <idle>-0 3dN.2 28us : pick_next_task_rt <-pick_next_task 2199 <idle>-0 3dN.2 29us : dequeue_pushable_task <-pick_next_task_rt 2200 <idle>-0 3d..3 29us : __schedule <-preempt_schedule 2201 <idle>-0 3d..3 30us : 0:120:R ==> [003] 2448: 94:R sleep 2202 2203This isn't that big of a trace, even with function tracing enabled, 2204so I included the entire trace. 2205 2206The interrupt went off while when the system was idle. Somewhere 2207before task_woken_rt() was called, the NEED_RESCHED flag was set, 2208this is indicated by the first occurrence of the 'N' flag. 2209 2210Latency tracing and events 2211-------------------------- 2212As function tracing can induce a much larger latency, but without 2213seeing what happens within the latency it is hard to know what 2214caused it. There is a middle ground, and that is with enabling 2215events. 2216:: 2217 2218 # echo 0 > options/function-trace 2219 # echo wakeup_rt > current_tracer 2220 # echo 1 > events/enable 2221 # echo 1 > tracing_on 2222 # echo 0 > tracing_max_latency 2223 # chrt -f 5 sleep 1 2224 # echo 0 > tracing_on 2225 # cat trace 2226 # tracer: wakeup_rt 2227 # 2228 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ 2229 # -------------------------------------------------------------------- 2230 # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 2231 # ----------------- 2232 # | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5) 2233 # ----------------- 2234 # 2235 # _------=> CPU# 2236 # / _-----=> irqs-off 2237 # | / _----=> need-resched 2238 # || / _---=> hardirq/softirq 2239 # ||| / _--=> preempt-depth 2240 # |||| / delay 2241 # cmd pid ||||| time | caller 2242 # \ / ||||| \ | / 2243 <idle>-0 2d.h4 0us : 0:120:R + [002] 5882: 94:R sleep 2244 <idle>-0 2d.h4 0us : ttwu_do_activate.constprop.87 <-try_to_wake_up 2245 <idle>-0 2d.h4 1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002 2246 <idle>-0 2dNh2 1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8 2247 <idle>-0 2.N.2 2us : power_end: cpu_id=2 2248 <idle>-0 2.N.2 3us : cpu_idle: state=4294967295 cpu_id=2 2249 <idle>-0 2dN.3 4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0 2250 <idle>-0 2dN.3 4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000 2251 <idle>-0 2.N.2 5us : rcu_utilization: Start context switch 2252 <idle>-0 2.N.2 5us : rcu_utilization: End context switch 2253 <idle>-0 2d..3 6us : __schedule <-schedule 2254 <idle>-0 2d..3 6us : 0:120:R ==> [002] 5882: 94:R sleep 2255 2256 2257Hardware Latency Detector 2258------------------------- 2259 2260The hardware latency detector is executed by enabling the "hwlat" tracer. 2261 2262NOTE, this tracer will affect the performance of the system as it will 2263periodically make a CPU constantly busy with interrupts disabled. 2264:: 2265 2266 # echo hwlat > current_tracer 2267 # sleep 100 2268 # cat trace 2269 # tracer: hwlat 2270 # 2271 # entries-in-buffer/entries-written: 13/13 #P:8 2272 # 2273 # _-----=> irqs-off 2274 # / _----=> need-resched 2275 # | / _---=> hardirq/softirq 2276 # || / _--=> preempt-depth 2277 # ||| / delay 2278 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 2279 # | | | |||| | | 2280 <...>-1729 [001] d... 678.473449: #1 inner/outer(us): 11/12 ts:1581527483.343962693 count:6 2281 <...>-1729 [004] d... 689.556542: #2 inner/outer(us): 16/9 ts:1581527494.889008092 count:1 2282 <...>-1729 [005] d... 714.756290: #3 inner/outer(us): 16/16 ts:1581527519.678961629 count:5 2283 <...>-1729 [001] d... 718.788247: #4 inner/outer(us): 9/17 ts:1581527523.889012713 count:1 2284 <...>-1729 [002] d... 719.796341: #5 inner/outer(us): 13/9 ts:1581527524.912872606 count:1 2285 <...>-1729 [006] d... 844.787091: #6 inner/outer(us): 9/12 ts:1581527649.889048502 count:2 2286 <...>-1729 [003] d... 849.827033: #7 inner/outer(us): 18/9 ts:1581527654.889013793 count:1 2287 <...>-1729 [007] d... 853.859002: #8 inner/outer(us): 9/12 ts:1581527658.889065736 count:1 2288 <...>-1729 [001] d... 855.874978: #9 inner/outer(us): 9/11 ts:1581527660.861991877 count:1 2289 <...>-1729 [001] d... 863.938932: #10 inner/outer(us): 9/11 ts:1581527668.970010500 count:1 nmi-total:7 nmi-count:1 2290 <...>-1729 [007] d... 878.050780: #11 inner/outer(us): 9/12 ts:1581527683.385002600 count:1 nmi-total:5 nmi-count:1 2291 <...>-1729 [007] d... 886.114702: #12 inner/outer(us): 9/12 ts:1581527691.385001600 count:1 2292 2293 2294The above output is somewhat the same in the header. All events will have 2295interrupts disabled 'd'. Under the FUNCTION title there is: 2296 2297 #1 2298 This is the count of events recorded that were greater than the 2299 tracing_threshold (See below). 2300 2301 inner/outer(us): 11/11 2302 2303 This shows two numbers as "inner latency" and "outer latency". The test 2304 runs in a loop checking a timestamp twice. The latency detected within 2305 the two timestamps is the "inner latency" and the latency detected 2306 after the previous timestamp and the next timestamp in the loop is 2307 the "outer latency". 2308 2309 ts:1581527483.343962693 2310 2311 The absolute timestamp that the first latency was recorded in the window. 2312 2313 count:6 2314 2315 The number of times a latency was detected during the window. 2316 2317 nmi-total:7 nmi-count:1 2318 2319 On architectures that support it, if an NMI comes in during the 2320 test, the time spent in NMI is reported in "nmi-total" (in 2321 microseconds). 2322 2323 All architectures that have NMIs will show the "nmi-count" if an 2324 NMI comes in during the test. 2325 2326hwlat files: 2327 2328 tracing_threshold 2329 This gets automatically set to "10" to represent 10 2330 microseconds. This is the threshold of latency that 2331 needs to be detected before the trace will be recorded. 2332 2333 Note, when hwlat tracer is finished (another tracer is 2334 written into "current_tracer"), the original value for 2335 tracing_threshold is placed back into this file. 2336 2337 hwlat_detector/width 2338 The length of time the test runs with interrupts disabled. 2339 2340 hwlat_detector/window 2341 The length of time of the window which the test 2342 runs. That is, the test will run for "width" 2343 microseconds per "window" microseconds 2344 2345 tracing_cpumask 2346 When the test is started. A kernel thread is created that 2347 runs the test. This thread will alternate between CPUs 2348 listed in the tracing_cpumask between each period 2349 (one "window"). To limit the test to specific CPUs 2350 set the mask in this file to only the CPUs that the test 2351 should run on. 2352 2353function 2354-------- 2355 2356This tracer is the function tracer. Enabling the function tracer 2357can be done from the debug file system. Make sure the 2358ftrace_enabled is set; otherwise this tracer is a nop. 2359See the "ftrace_enabled" section below. 2360:: 2361 2362 # sysctl kernel.ftrace_enabled=1 2363 # echo function > current_tracer 2364 # echo 1 > tracing_on 2365 # usleep 1 2366 # echo 0 > tracing_on 2367 # cat trace 2368 # tracer: function 2369 # 2370 # entries-in-buffer/entries-written: 24799/24799 #P:4 2371 # 2372 # _-----=> irqs-off 2373 # / _----=> need-resched 2374 # | / _---=> hardirq/softirq 2375 # || / _--=> preempt-depth 2376 # ||| / delay 2377 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 2378 # | | | |||| | | 2379 bash-1994 [002] .... 3082.063030: mutex_unlock <-rb_simple_write 2380 bash-1994 [002] .... 3082.063031: __mutex_unlock_slowpath <-mutex_unlock 2381 bash-1994 [002] .... 3082.063031: __fsnotify_parent <-fsnotify_modify 2382 bash-1994 [002] .... 3082.063032: fsnotify <-fsnotify_modify 2383 bash-1994 [002] .... 3082.063032: __srcu_read_lock <-fsnotify 2384 bash-1994 [002] .... 3082.063032: add_preempt_count <-__srcu_read_lock 2385 bash-1994 [002] ...1 3082.063032: sub_preempt_count <-__srcu_read_lock 2386 bash-1994 [002] .... 3082.063033: __srcu_read_unlock <-fsnotify 2387 [...] 2388 2389 2390Note: function tracer uses ring buffers to store the above 2391entries. The newest data may overwrite the oldest data. 2392Sometimes using echo to stop the trace is not sufficient because 2393the tracing could have overwritten the data that you wanted to 2394record. For this reason, it is sometimes better to disable 2395tracing directly from a program. This allows you to stop the 2396tracing at the point that you hit the part that you are 2397interested in. To disable the tracing directly from a C program, 2398something like following code snippet can be used:: 2399 2400 int trace_fd; 2401 [...] 2402 int main(int argc, char *argv[]) { 2403 [...] 2404 trace_fd = open(tracing_file("tracing_on"), O_WRONLY); 2405 [...] 2406 if (condition_hit()) { 2407 write(trace_fd, "0", 1); 2408 } 2409 [...] 2410 } 2411 2412 2413Single thread tracing 2414--------------------- 2415 2416By writing into set_ftrace_pid you can trace a 2417single thread. For example:: 2418 2419 # cat set_ftrace_pid 2420 no pid 2421 # echo 3111 > set_ftrace_pid 2422 # cat set_ftrace_pid 2423 3111 2424 # echo function > current_tracer 2425 # cat trace | head 2426 # tracer: function 2427 # 2428 # TASK-PID CPU# TIMESTAMP FUNCTION 2429 # | | | | | 2430 yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return 2431 yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range 2432 yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel 2433 yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel 2434 yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll 2435 yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll 2436 # echo > set_ftrace_pid 2437 # cat trace |head 2438 # tracer: function 2439 # 2440 # TASK-PID CPU# TIMESTAMP FUNCTION 2441 # | | | | | 2442 ##### CPU 3 buffer started #### 2443 yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait 2444 yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry 2445 yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry 2446 yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit 2447 yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit 2448 2449If you want to trace a function when executing, you could use 2450something like this simple program. 2451:: 2452 2453 #include <stdio.h> 2454 #include <stdlib.h> 2455 #include <sys/types.h> 2456 #include <sys/stat.h> 2457 #include <fcntl.h> 2458 #include <unistd.h> 2459 #include <string.h> 2460 2461 #define _STR(x) #x 2462 #define STR(x) _STR(x) 2463 #define MAX_PATH 256 2464 2465 const char *find_tracefs(void) 2466 { 2467 static char tracefs[MAX_PATH+1]; 2468 static int tracefs_found; 2469 char type[100]; 2470 FILE *fp; 2471 2472 if (tracefs_found) 2473 return tracefs; 2474 2475 if ((fp = fopen("/proc/mounts","r")) == NULL) { 2476 perror("/proc/mounts"); 2477 return NULL; 2478 } 2479 2480 while (fscanf(fp, "%*s %" 2481 STR(MAX_PATH) 2482 "s %99s %*s %*d %*d\n", 2483 tracefs, type) == 2) { 2484 if (strcmp(type, "tracefs") == 0) 2485 break; 2486 } 2487 fclose(fp); 2488 2489 if (strcmp(type, "tracefs") != 0) { 2490 fprintf(stderr, "tracefs not mounted"); 2491 return NULL; 2492 } 2493 2494 strcat(tracefs, "/tracing/"); 2495 tracefs_found = 1; 2496 2497 return tracefs; 2498 } 2499 2500 const char *tracing_file(const char *file_name) 2501 { 2502 static char trace_file[MAX_PATH+1]; 2503 snprintf(trace_file, MAX_PATH, "%s/%s", find_tracefs(), file_name); 2504 return trace_file; 2505 } 2506 2507 int main (int argc, char **argv) 2508 { 2509 if (argc < 1) 2510 exit(-1); 2511 2512 if (fork() > 0) { 2513 int fd, ffd; 2514 char line[64]; 2515 int s; 2516 2517 ffd = open(tracing_file("current_tracer"), O_WRONLY); 2518 if (ffd < 0) 2519 exit(-1); 2520 write(ffd, "nop", 3); 2521 2522 fd = open(tracing_file("set_ftrace_pid"), O_WRONLY); 2523 s = sprintf(line, "%d\n", getpid()); 2524 write(fd, line, s); 2525 2526 write(ffd, "function", 8); 2527 2528 close(fd); 2529 close(ffd); 2530 2531 execvp(argv[1], argv+1); 2532 } 2533 2534 return 0; 2535 } 2536 2537Or this simple script! 2538:: 2539 2540 #!/bin/bash 2541 2542 tracefs=`sed -ne 's/^tracefs \(.*\) tracefs.*/\1/p' /proc/mounts` 2543 echo 0 > $tracefs/tracing_on 2544 echo $$ > $tracefs/set_ftrace_pid 2545 echo function > $tracefs/current_tracer 2546 echo 1 > $tracefs/tracing_on 2547 exec "$@" 2548 2549 2550function graph tracer 2551--------------------------- 2552 2553This tracer is similar to the function tracer except that it 2554probes a function on its entry and its exit. This is done by 2555using a dynamically allocated stack of return addresses in each 2556task_struct. On function entry the tracer overwrites the return 2557address of each function traced to set a custom probe. Thus the 2558original return address is stored on the stack of return address 2559in the task_struct. 2560 2561Probing on both ends of a function leads to special features 2562such as: 2563 2564- measure of a function's time execution 2565- having a reliable call stack to draw function calls graph 2566 2567This tracer is useful in several situations: 2568 2569- you want to find the reason of a strange kernel behavior and 2570 need to see what happens in detail on any areas (or specific 2571 ones). 2572 2573- you are experiencing weird latencies but it's difficult to 2574 find its origin. 2575 2576- you want to find quickly which path is taken by a specific 2577 function 2578 2579- you just want to peek inside a working kernel and want to see 2580 what happens there. 2581 2582:: 2583 2584 # tracer: function_graph 2585 # 2586 # CPU DURATION FUNCTION CALLS 2587 # | | | | | | | 2588 2589 0) | sys_open() { 2590 0) | do_sys_open() { 2591 0) | getname() { 2592 0) | kmem_cache_alloc() { 2593 0) 1.382 us | __might_sleep(); 2594 0) 2.478 us | } 2595 0) | strncpy_from_user() { 2596 0) | might_fault() { 2597 0) 1.389 us | __might_sleep(); 2598 0) 2.553 us | } 2599 0) 3.807 us | } 2600 0) 7.876 us | } 2601 0) | alloc_fd() { 2602 0) 0.668 us | _spin_lock(); 2603 0) 0.570 us | expand_files(); 2604 0) 0.586 us | _spin_unlock(); 2605 2606 2607There are several columns that can be dynamically 2608enabled/disabled. You can use every combination of options you 2609want, depending on your needs. 2610 2611- The cpu number on which the function executed is default 2612 enabled. It is sometimes better to only trace one cpu (see 2613 tracing_cpumask file) or you might sometimes see unordered 2614 function calls while cpu tracing switch. 2615 2616 - hide: echo nofuncgraph-cpu > trace_options 2617 - show: echo funcgraph-cpu > trace_options 2618 2619- The duration (function's time of execution) is displayed on 2620 the closing bracket line of a function or on the same line 2621 than the current function in case of a leaf one. It is default 2622 enabled. 2623 2624 - hide: echo nofuncgraph-duration > trace_options 2625 - show: echo funcgraph-duration > trace_options 2626 2627- The overhead field precedes the duration field in case of 2628 reached duration thresholds. 2629 2630 - hide: echo nofuncgraph-overhead > trace_options 2631 - show: echo funcgraph-overhead > trace_options 2632 - depends on: funcgraph-duration 2633 2634 ie:: 2635 2636 3) # 1837.709 us | } /* __switch_to */ 2637 3) | finish_task_switch() { 2638 3) 0.313 us | _raw_spin_unlock_irq(); 2639 3) 3.177 us | } 2640 3) # 1889.063 us | } /* __schedule */ 2641 3) ! 140.417 us | } /* __schedule */ 2642 3) # 2034.948 us | } /* schedule */ 2643 3) * 33998.59 us | } /* schedule_preempt_disabled */ 2644 2645 [...] 2646 2647 1) 0.260 us | msecs_to_jiffies(); 2648 1) 0.313 us | __rcu_read_unlock(); 2649 1) + 61.770 us | } 2650 1) + 64.479 us | } 2651 1) 0.313 us | rcu_bh_qs(); 2652 1) 0.313 us | __local_bh_enable(); 2653 1) ! 217.240 us | } 2654 1) 0.365 us | idle_cpu(); 2655 1) | rcu_irq_exit() { 2656 1) 0.417 us | rcu_eqs_enter_common.isra.47(); 2657 1) 3.125 us | } 2658 1) ! 227.812 us | } 2659 1) ! 457.395 us | } 2660 1) @ 119760.2 us | } 2661 2662 [...] 2663 2664 2) | handle_IPI() { 2665 1) 6.979 us | } 2666 2) 0.417 us | scheduler_ipi(); 2667 1) 9.791 us | } 2668 1) + 12.917 us | } 2669 2) 3.490 us | } 2670 1) + 15.729 us | } 2671 1) + 18.542 us | } 2672 2) $ 3594274 us | } 2673 2674Flags:: 2675 2676 + means that the function exceeded 10 usecs. 2677 ! means that the function exceeded 100 usecs. 2678 # means that the function exceeded 1000 usecs. 2679 * means that the function exceeded 10 msecs. 2680 @ means that the function exceeded 100 msecs. 2681 $ means that the function exceeded 1 sec. 2682 2683 2684- The task/pid field displays the thread cmdline and pid which 2685 executed the function. It is default disabled. 2686 2687 - hide: echo nofuncgraph-proc > trace_options 2688 - show: echo funcgraph-proc > trace_options 2689 2690 ie:: 2691 2692 # tracer: function_graph 2693 # 2694 # CPU TASK/PID DURATION FUNCTION CALLS 2695 # | | | | | | | | | 2696 0) sh-4802 | | d_free() { 2697 0) sh-4802 | | call_rcu() { 2698 0) sh-4802 | | __call_rcu() { 2699 0) sh-4802 | 0.616 us | rcu_process_gp_end(); 2700 0) sh-4802 | 0.586 us | check_for_new_grace_period(); 2701 0) sh-4802 | 2.899 us | } 2702 0) sh-4802 | 4.040 us | } 2703 0) sh-4802 | 5.151 us | } 2704 0) sh-4802 | + 49.370 us | } 2705 2706 2707- The absolute time field is an absolute timestamp given by the 2708 system clock since it started. A snapshot of this time is 2709 given on each entry/exit of functions 2710 2711 - hide: echo nofuncgraph-abstime > trace_options 2712 - show: echo funcgraph-abstime > trace_options 2713 2714 ie:: 2715 2716 # 2717 # TIME CPU DURATION FUNCTION CALLS 2718 # | | | | | | | | 2719 360.774522 | 1) 0.541 us | } 2720 360.774522 | 1) 4.663 us | } 2721 360.774523 | 1) 0.541 us | __wake_up_bit(); 2722 360.774524 | 1) 6.796 us | } 2723 360.774524 | 1) 7.952 us | } 2724 360.774525 | 1) 9.063 us | } 2725 360.774525 | 1) 0.615 us | journal_mark_dirty(); 2726 360.774527 | 1) 0.578 us | __brelse(); 2727 360.774528 | 1) | reiserfs_prepare_for_journal() { 2728 360.774528 | 1) | unlock_buffer() { 2729 360.774529 | 1) | wake_up_bit() { 2730 360.774529 | 1) | bit_waitqueue() { 2731 360.774530 | 1) 0.594 us | __phys_addr(); 2732 2733 2734The function name is always displayed after the closing bracket 2735for a function if the start of that function is not in the 2736trace buffer. 2737 2738Display of the function name after the closing bracket may be 2739enabled for functions whose start is in the trace buffer, 2740allowing easier searching with grep for function durations. 2741It is default disabled. 2742 2743 - hide: echo nofuncgraph-tail > trace_options 2744 - show: echo funcgraph-tail > trace_options 2745 2746 Example with nofuncgraph-tail (default):: 2747 2748 0) | putname() { 2749 0) | kmem_cache_free() { 2750 0) 0.518 us | __phys_addr(); 2751 0) 1.757 us | } 2752 0) 2.861 us | } 2753 2754 Example with funcgraph-tail:: 2755 2756 0) | putname() { 2757 0) | kmem_cache_free() { 2758 0) 0.518 us | __phys_addr(); 2759 0) 1.757 us | } /* kmem_cache_free() */ 2760 0) 2.861 us | } /* putname() */ 2761 2762The return value of each traced function can be displayed after 2763an equal sign "=". When encountering system call failures, it 2764can be very helpful to quickly locate the function that first 2765returns an error code. 2766 2767 - hide: echo nofuncgraph-retval > trace_options 2768 - show: echo funcgraph-retval > trace_options 2769 2770 Example with funcgraph-retval:: 2771 2772 1) | cgroup_migrate() { 2773 1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */ 2774 1) | cgroup_migrate_execute() { 2775 1) | cpu_cgroup_can_attach() { 2776 1) | cgroup_taskset_first() { 2777 1) 0.732 us | cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */ 2778 1) 1.232 us | } /* cgroup_taskset_first = 0xffff93fc8fb20000 */ 2779 1) 0.380 us | sched_rt_can_attach(); /* = 0x0 */ 2780 1) 2.335 us | } /* cpu_cgroup_can_attach = -22 */ 2781 1) 4.369 us | } /* cgroup_migrate_execute = -22 */ 2782 1) 7.143 us | } /* cgroup_migrate = -22 */ 2783 2784The above example shows that the function cpu_cgroup_can_attach 2785returned the error code -22 firstly, then we can read the code 2786of this function to get the root cause. 2787 2788When the option funcgraph-retval-hex is not set, the return value can 2789be displayed in a smart way. Specifically, if it is an error code, 2790it will be printed in signed decimal format, otherwise it will 2791printed in hexadecimal format. 2792 2793 - smart: echo nofuncgraph-retval-hex > trace_options 2794 - hexadecimal: echo funcgraph-retval-hex > trace_options 2795 2796 Example with funcgraph-retval-hex:: 2797 2798 1) | cgroup_migrate() { 2799 1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */ 2800 1) | cgroup_migrate_execute() { 2801 1) | cpu_cgroup_can_attach() { 2802 1) | cgroup_taskset_first() { 2803 1) 0.732 us | cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */ 2804 1) 1.232 us | } /* cgroup_taskset_first = 0xffff93fc8fb20000 */ 2805 1) 0.380 us | sched_rt_can_attach(); /* = 0x0 */ 2806 1) 2.335 us | } /* cpu_cgroup_can_attach = 0xffffffea */ 2807 1) 4.369 us | } /* cgroup_migrate_execute = 0xffffffea */ 2808 1) 7.143 us | } /* cgroup_migrate = 0xffffffea */ 2809 2810At present, there are some limitations when using the funcgraph-retval 2811option, and these limitations will be eliminated in the future: 2812 2813- Even if the function return type is void, a return value will still 2814 be printed, and you can just ignore it. 2815 2816- Even if return values are stored in multiple registers, only the 2817 value contained in the first register will be recorded and printed. 2818 To illustrate, in the x86 architecture, eax and edx are used to store 2819 a 64-bit return value, with the lower 32 bits saved in eax and the 2820 upper 32 bits saved in edx. However, only the value stored in eax 2821 will be recorded and printed. 2822 2823- In certain procedure call standards, such as arm64's AAPCS64, when a 2824 type is smaller than a GPR, it is the responsibility of the consumer 2825 to perform the narrowing, and the upper bits may contain UNKNOWN values. 2826 Therefore, it is advisable to check the code for such cases. For instance, 2827 when using a u8 in a 64-bit GPR, bits [63:8] may contain arbitrary values, 2828 especially when larger types are truncated, whether explicitly or implicitly. 2829 Here are some specific cases to illustrate this point: 2830 2831 **Case One**: 2832 2833 The function narrow_to_u8 is defined as follows:: 2834 2835 u8 narrow_to_u8(u64 val) 2836 { 2837 // implicitly truncated 2838 return val; 2839 } 2840 2841 It may be compiled to:: 2842 2843 narrow_to_u8: 2844 < ... ftrace instrumentation ... > 2845 RET 2846 2847 If you pass 0x123456789abcdef to this function and want to narrow it, 2848 it may be recorded as 0x123456789abcdef instead of 0xef. 2849 2850 **Case Two**: 2851 2852 The function error_if_not_4g_aligned is defined as follows:: 2853 2854 int error_if_not_4g_aligned(u64 val) 2855 { 2856 if (val & GENMASK(31, 0)) 2857 return -EINVAL; 2858 2859 return 0; 2860 } 2861 2862 It could be compiled to:: 2863 2864 error_if_not_4g_aligned: 2865 CBNZ w0, .Lnot_aligned 2866 RET // bits [31:0] are zero, bits 2867 // [63:32] are UNKNOWN 2868 .Lnot_aligned: 2869 MOV x0, #-EINVAL 2870 RET 2871 2872 When passing 0x2_0000_0000 to it, the return value may be recorded as 2873 0x2_0000_0000 instead of 0. 2874 2875You can put some comments on specific functions by using 2876trace_printk() For example, if you want to put a comment inside 2877the __might_sleep() function, you just have to include 2878<linux/ftrace.h> and call trace_printk() inside __might_sleep():: 2879 2880 trace_printk("I'm a comment!\n") 2881 2882will produce:: 2883 2884 1) | __might_sleep() { 2885 1) | /* I'm a comment! */ 2886 1) 1.449 us | } 2887 2888 2889You might find other useful features for this tracer in the 2890following "dynamic ftrace" section such as tracing only specific 2891functions or tasks. 2892 2893dynamic ftrace 2894-------------- 2895 2896If CONFIG_DYNAMIC_FTRACE is set, the system will run with 2897virtually no overhead when function tracing is disabled. The way 2898this works is the mcount function call (placed at the start of 2899every kernel function, produced by the -pg switch in gcc), 2900starts of pointing to a simple return. (Enabling FTRACE will 2901include the -pg switch in the compiling of the kernel.) 2902 2903At compile time every C file object is run through the 2904recordmcount program (located in the scripts directory). This 2905program will parse the ELF headers in the C object to find all 2906the locations in the .text section that call mcount. Starting 2907with gcc version 4.6, the -mfentry has been added for x86, which 2908calls "__fentry__" instead of "mcount". Which is called before 2909the creation of the stack frame. 2910 2911Note, not all sections are traced. They may be prevented by either 2912a notrace, or blocked another way and all inline functions are not 2913traced. Check the "available_filter_functions" file to see what functions 2914can be traced. 2915 2916A section called "__mcount_loc" is created that holds 2917references to all the mcount/fentry call sites in the .text section. 2918The recordmcount program re-links this section back into the 2919original object. The final linking stage of the kernel will add all these 2920references into a single table. 2921 2922On boot up, before SMP is initialized, the dynamic ftrace code 2923scans this table and updates all the locations into nops. It 2924also records the locations, which are added to the 2925available_filter_functions list. Modules are processed as they 2926are loaded and before they are executed. When a module is 2927unloaded, it also removes its functions from the ftrace function 2928list. This is automatic in the module unload code, and the 2929module author does not need to worry about it. 2930 2931When tracing is enabled, the process of modifying the function 2932tracepoints is dependent on architecture. The old method is to use 2933kstop_machine to prevent races with the CPUs executing code being 2934modified (which can cause the CPU to do undesirable things, especially 2935if the modified code crosses cache (or page) boundaries), and the nops are 2936patched back to calls. But this time, they do not call mcount 2937(which is just a function stub). They now call into the ftrace 2938infrastructure. 2939 2940The new method of modifying the function tracepoints is to place 2941a breakpoint at the location to be modified, sync all CPUs, modify 2942the rest of the instruction not covered by the breakpoint. Sync 2943all CPUs again, and then remove the breakpoint with the finished 2944version to the ftrace call site. 2945 2946Some archs do not even need to monkey around with the synchronization, 2947and can just slap the new code on top of the old without any 2948problems with other CPUs executing it at the same time. 2949 2950One special side-effect to the recording of the functions being 2951traced is that we can now selectively choose which functions we 2952wish to trace and which ones we want the mcount calls to remain 2953as nops. 2954 2955Two files are used, one for enabling and one for disabling the 2956tracing of specified functions. They are: 2957 2958 set_ftrace_filter 2959 2960and 2961 2962 set_ftrace_notrace 2963 2964A list of available functions that you can add to these files is 2965listed in: 2966 2967 available_filter_functions 2968 2969:: 2970 2971 # cat available_filter_functions 2972 put_prev_task_idle 2973 kmem_cache_create 2974 pick_next_task_rt 2975 cpus_read_lock 2976 pick_next_task_fair 2977 mutex_lock 2978 [...] 2979 2980If I am only interested in sys_nanosleep and hrtimer_interrupt:: 2981 2982 # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter 2983 # echo function > current_tracer 2984 # echo 1 > tracing_on 2985 # usleep 1 2986 # echo 0 > tracing_on 2987 # cat trace 2988 # tracer: function 2989 # 2990 # entries-in-buffer/entries-written: 5/5 #P:4 2991 # 2992 # _-----=> irqs-off 2993 # / _----=> need-resched 2994 # | / _---=> hardirq/softirq 2995 # || / _--=> preempt-depth 2996 # ||| / delay 2997 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 2998 # | | | |||| | | 2999 usleep-2665 [001] .... 4186.475355: sys_nanosleep <-system_call_fastpath 3000 <idle>-0 [001] d.h1 4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt 3001 usleep-2665 [001] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt 3002 <idle>-0 [003] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt 3003 <idle>-0 [002] d.h1 4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt 3004 3005To see which functions are being traced, you can cat the file: 3006:: 3007 3008 # cat set_ftrace_filter 3009 hrtimer_interrupt 3010 sys_nanosleep 3011 3012 3013Perhaps this is not enough. The filters also allow glob(7) matching. 3014 3015 ``<match>*`` 3016 will match functions that begin with <match> 3017 ``*<match>`` 3018 will match functions that end with <match> 3019 ``*<match>*`` 3020 will match functions that have <match> in it 3021 ``<match1>*<match2>`` 3022 will match functions that begin with <match1> and end with <match2> 3023 3024.. note:: 3025 It is better to use quotes to enclose the wild cards, 3026 otherwise the shell may expand the parameters into names 3027 of files in the local directory. 3028 3029:: 3030 3031 # echo 'hrtimer_*' > set_ftrace_filter 3032 3033Produces:: 3034 3035 # tracer: function 3036 # 3037 # entries-in-buffer/entries-written: 897/897 #P:4 3038 # 3039 # _-----=> irqs-off 3040 # / _----=> need-resched 3041 # | / _---=> hardirq/softirq 3042 # || / _--=> preempt-depth 3043 # ||| / delay 3044 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3045 # | | | |||| | | 3046 <idle>-0 [003] dN.1 4228.547803: hrtimer_cancel <-tick_nohz_idle_exit 3047 <idle>-0 [003] dN.1 4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel 3048 <idle>-0 [003] dN.2 4228.547805: hrtimer_force_reprogram <-__remove_hrtimer 3049 <idle>-0 [003] dN.1 4228.547805: hrtimer_forward <-tick_nohz_idle_exit 3050 <idle>-0 [003] dN.1 4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11 3051 <idle>-0 [003] d..1 4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt 3052 <idle>-0 [003] d..1 4228.547859: hrtimer_start <-__tick_nohz_idle_enter 3053 <idle>-0 [003] d..2 4228.547860: hrtimer_force_reprogram <-__rem 3054 3055Notice that we lost the sys_nanosleep. 3056:: 3057 3058 # cat set_ftrace_filter 3059 hrtimer_run_queues 3060 hrtimer_run_pending 3061 hrtimer_init 3062 hrtimer_cancel 3063 hrtimer_try_to_cancel 3064 hrtimer_forward 3065 hrtimer_start 3066 hrtimer_reprogram 3067 hrtimer_force_reprogram 3068 hrtimer_get_next_event 3069 hrtimer_interrupt 3070 hrtimer_nanosleep 3071 hrtimer_wakeup 3072 hrtimer_get_remaining 3073 hrtimer_get_res 3074 hrtimer_init_sleeper 3075 3076 3077This is because the '>' and '>>' act just like they do in bash. 3078To rewrite the filters, use '>' 3079To append to the filters, use '>>' 3080 3081To clear out a filter so that all functions will be recorded 3082again:: 3083 3084 # echo > set_ftrace_filter 3085 # cat set_ftrace_filter 3086 # 3087 3088Again, now we want to append. 3089 3090:: 3091 3092 # echo sys_nanosleep > set_ftrace_filter 3093 # cat set_ftrace_filter 3094 sys_nanosleep 3095 # echo 'hrtimer_*' >> set_ftrace_filter 3096 # cat set_ftrace_filter 3097 hrtimer_run_queues 3098 hrtimer_run_pending 3099 hrtimer_init 3100 hrtimer_cancel 3101 hrtimer_try_to_cancel 3102 hrtimer_forward 3103 hrtimer_start 3104 hrtimer_reprogram 3105 hrtimer_force_reprogram 3106 hrtimer_get_next_event 3107 hrtimer_interrupt 3108 sys_nanosleep 3109 hrtimer_nanosleep 3110 hrtimer_wakeup 3111 hrtimer_get_remaining 3112 hrtimer_get_res 3113 hrtimer_init_sleeper 3114 3115 3116The set_ftrace_notrace prevents those functions from being 3117traced. 3118:: 3119 3120 # echo '*preempt*' '*lock*' > set_ftrace_notrace 3121 3122Produces:: 3123 3124 # tracer: function 3125 # 3126 # entries-in-buffer/entries-written: 39608/39608 #P:4 3127 # 3128 # _-----=> irqs-off 3129 # / _----=> need-resched 3130 # | / _---=> hardirq/softirq 3131 # || / _--=> preempt-depth 3132 # ||| / delay 3133 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3134 # | | | |||| | | 3135 bash-1994 [000] .... 4342.324896: file_ra_state_init <-do_dentry_open 3136 bash-1994 [000] .... 4342.324897: open_check_o_direct <-do_last 3137 bash-1994 [000] .... 4342.324897: ima_file_check <-do_last 3138 bash-1994 [000] .... 4342.324898: process_measurement <-ima_file_check 3139 bash-1994 [000] .... 4342.324898: ima_get_action <-process_measurement 3140 bash-1994 [000] .... 4342.324898: ima_match_policy <-ima_get_action 3141 bash-1994 [000] .... 4342.324899: do_truncate <-do_last 3142 bash-1994 [000] .... 4342.324899: setattr_should_drop_suidgid <-do_truncate 3143 bash-1994 [000] .... 4342.324899: notify_change <-do_truncate 3144 bash-1994 [000] .... 4342.324900: current_fs_time <-notify_change 3145 bash-1994 [000] .... 4342.324900: current_kernel_time <-current_fs_time 3146 bash-1994 [000] .... 4342.324900: timespec_trunc <-current_fs_time 3147 3148We can see that there's no more lock or preempt tracing. 3149 3150Selecting function filters via index 3151------------------------------------ 3152 3153Because processing of strings is expensive (the address of the function 3154needs to be looked up before comparing to the string being passed in), 3155an index can be used as well to enable functions. This is useful in the 3156case of setting thousands of specific functions at a time. By passing 3157in a list of numbers, no string processing will occur. Instead, the function 3158at the specific location in the internal array (which corresponds to the 3159functions in the "available_filter_functions" file), is selected. 3160 3161:: 3162 3163 # echo 1 > set_ftrace_filter 3164 3165Will select the first function listed in "available_filter_functions" 3166 3167:: 3168 3169 # head -1 available_filter_functions 3170 trace_initcall_finish_cb 3171 3172 # cat set_ftrace_filter 3173 trace_initcall_finish_cb 3174 3175 # head -50 available_filter_functions | tail -1 3176 x86_pmu_commit_txn 3177 3178 # echo 1 50 > set_ftrace_filter 3179 # cat set_ftrace_filter 3180 trace_initcall_finish_cb 3181 x86_pmu_commit_txn 3182 3183Dynamic ftrace with the function graph tracer 3184--------------------------------------------- 3185 3186Although what has been explained above concerns both the 3187function tracer and the function-graph-tracer, there are some 3188special features only available in the function-graph tracer. 3189 3190If you want to trace only one function and all of its children, 3191you just have to echo its name into set_graph_function:: 3192 3193 echo __do_fault > set_graph_function 3194 3195will produce the following "expanded" trace of the __do_fault() 3196function:: 3197 3198 0) | __do_fault() { 3199 0) | filemap_fault() { 3200 0) | find_lock_page() { 3201 0) 0.804 us | find_get_page(); 3202 0) | __might_sleep() { 3203 0) 1.329 us | } 3204 0) 3.904 us | } 3205 0) 4.979 us | } 3206 0) 0.653 us | _spin_lock(); 3207 0) 0.578 us | page_add_file_rmap(); 3208 0) 0.525 us | native_set_pte_at(); 3209 0) 0.585 us | _spin_unlock(); 3210 0) | unlock_page() { 3211 0) 0.541 us | page_waitqueue(); 3212 0) 0.639 us | __wake_up_bit(); 3213 0) 2.786 us | } 3214 0) + 14.237 us | } 3215 0) | __do_fault() { 3216 0) | filemap_fault() { 3217 0) | find_lock_page() { 3218 0) 0.698 us | find_get_page(); 3219 0) | __might_sleep() { 3220 0) 1.412 us | } 3221 0) 3.950 us | } 3222 0) 5.098 us | } 3223 0) 0.631 us | _spin_lock(); 3224 0) 0.571 us | page_add_file_rmap(); 3225 0) 0.526 us | native_set_pte_at(); 3226 0) 0.586 us | _spin_unlock(); 3227 0) | unlock_page() { 3228 0) 0.533 us | page_waitqueue(); 3229 0) 0.638 us | __wake_up_bit(); 3230 0) 2.793 us | } 3231 0) + 14.012 us | } 3232 3233You can also expand several functions at once:: 3234 3235 echo sys_open > set_graph_function 3236 echo sys_close >> set_graph_function 3237 3238Now if you want to go back to trace all functions you can clear 3239this special filter via:: 3240 3241 echo > set_graph_function 3242 3243 3244ftrace_enabled 3245-------------- 3246 3247Note, the proc sysctl ftrace_enable is a big on/off switch for the 3248function tracer. By default it is enabled (when function tracing is 3249enabled in the kernel). If it is disabled, all function tracing is 3250disabled. This includes not only the function tracers for ftrace, but 3251also for any other uses (perf, kprobes, stack tracing, profiling, etc). It 3252cannot be disabled if there is a callback with FTRACE_OPS_FL_PERMANENT set 3253registered. 3254 3255Please disable this with care. 3256 3257This can be disable (and enabled) with:: 3258 3259 sysctl kernel.ftrace_enabled=0 3260 sysctl kernel.ftrace_enabled=1 3261 3262 or 3263 3264 echo 0 > /proc/sys/kernel/ftrace_enabled 3265 echo 1 > /proc/sys/kernel/ftrace_enabled 3266 3267 3268Filter commands 3269--------------- 3270 3271A few commands are supported by the set_ftrace_filter interface. 3272Trace commands have the following format:: 3273 3274 <function>:<command>:<parameter> 3275 3276The following commands are supported: 3277 3278- mod: 3279 This command enables function filtering per module. The 3280 parameter defines the module. For example, if only the write* 3281 functions in the ext3 module are desired, run: 3282 3283 echo 'write*:mod:ext3' > set_ftrace_filter 3284 3285 This command interacts with the filter in the same way as 3286 filtering based on function names. Thus, adding more functions 3287 in a different module is accomplished by appending (>>) to the 3288 filter file. Remove specific module functions by prepending 3289 '!':: 3290 3291 echo '!writeback*:mod:ext3' >> set_ftrace_filter 3292 3293 Mod command supports module globbing. Disable tracing for all 3294 functions except a specific module:: 3295 3296 echo '!*:mod:!ext3' >> set_ftrace_filter 3297 3298 Disable tracing for all modules, but still trace kernel:: 3299 3300 echo '!*:mod:*' >> set_ftrace_filter 3301 3302 Enable filter only for kernel:: 3303 3304 echo '*write*:mod:!*' >> set_ftrace_filter 3305 3306 Enable filter for module globbing:: 3307 3308 echo '*write*:mod:*snd*' >> set_ftrace_filter 3309 3310- traceon/traceoff: 3311 These commands turn tracing on and off when the specified 3312 functions are hit. The parameter determines how many times the 3313 tracing system is turned on and off. If unspecified, there is 3314 no limit. For example, to disable tracing when a schedule bug 3315 is hit the first 5 times, run:: 3316 3317 echo '__schedule_bug:traceoff:5' > set_ftrace_filter 3318 3319 To always disable tracing when __schedule_bug is hit:: 3320 3321 echo '__schedule_bug:traceoff' > set_ftrace_filter 3322 3323 These commands are cumulative whether or not they are appended 3324 to set_ftrace_filter. To remove a command, prepend it by '!' 3325 and drop the parameter:: 3326 3327 echo '!__schedule_bug:traceoff:0' > set_ftrace_filter 3328 3329 The above removes the traceoff command for __schedule_bug 3330 that have a counter. To remove commands without counters:: 3331 3332 echo '!__schedule_bug:traceoff' > set_ftrace_filter 3333 3334- snapshot: 3335 Will cause a snapshot to be triggered when the function is hit. 3336 :: 3337 3338 echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter 3339 3340 To only snapshot once: 3341 :: 3342 3343 echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter 3344 3345 To remove the above commands:: 3346 3347 echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter 3348 echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter 3349 3350- enable_event/disable_event: 3351 These commands can enable or disable a trace event. Note, because 3352 function tracing callbacks are very sensitive, when these commands 3353 are registered, the trace point is activated, but disabled in 3354 a "soft" mode. That is, the tracepoint will be called, but 3355 just will not be traced. The event tracepoint stays in this mode 3356 as long as there's a command that triggers it. 3357 :: 3358 3359 echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \ 3360 set_ftrace_filter 3361 3362 The format is:: 3363 3364 <function>:enable_event:<system>:<event>[:count] 3365 <function>:disable_event:<system>:<event>[:count] 3366 3367 To remove the events commands:: 3368 3369 echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \ 3370 set_ftrace_filter 3371 echo '!schedule:disable_event:sched:sched_switch' > \ 3372 set_ftrace_filter 3373 3374- dump: 3375 When the function is hit, it will dump the contents of the ftrace 3376 ring buffer to the console. This is useful if you need to debug 3377 something, and want to dump the trace when a certain function 3378 is hit. Perhaps it's a function that is called before a triple 3379 fault happens and does not allow you to get a regular dump. 3380 3381- cpudump: 3382 When the function is hit, it will dump the contents of the ftrace 3383 ring buffer for the current CPU to the console. Unlike the "dump" 3384 command, it only prints out the contents of the ring buffer for the 3385 CPU that executed the function that triggered the dump. 3386 3387- stacktrace: 3388 When the function is hit, a stack trace is recorded. 3389 3390trace_pipe 3391---------- 3392 3393The trace_pipe outputs the same content as the trace file, but 3394the effect on the tracing is different. Every read from 3395trace_pipe is consumed. This means that subsequent reads will be 3396different. The trace is live. 3397:: 3398 3399 # echo function > current_tracer 3400 # cat trace_pipe > /tmp/trace.out & 3401 [1] 4153 3402 # echo 1 > tracing_on 3403 # usleep 1 3404 # echo 0 > tracing_on 3405 # cat trace 3406 # tracer: function 3407 # 3408 # entries-in-buffer/entries-written: 0/0 #P:4 3409 # 3410 # _-----=> irqs-off 3411 # / _----=> need-resched 3412 # | / _---=> hardirq/softirq 3413 # || / _--=> preempt-depth 3414 # ||| / delay 3415 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3416 # | | | |||| | | 3417 3418 # 3419 # cat /tmp/trace.out 3420 bash-1994 [000] .... 5281.568961: mutex_unlock <-rb_simple_write 3421 bash-1994 [000] .... 5281.568963: __mutex_unlock_slowpath <-mutex_unlock 3422 bash-1994 [000] .... 5281.568963: __fsnotify_parent <-fsnotify_modify 3423 bash-1994 [000] .... 5281.568964: fsnotify <-fsnotify_modify 3424 bash-1994 [000] .... 5281.568964: __srcu_read_lock <-fsnotify 3425 bash-1994 [000] .... 5281.568964: add_preempt_count <-__srcu_read_lock 3426 bash-1994 [000] ...1 5281.568965: sub_preempt_count <-__srcu_read_lock 3427 bash-1994 [000] .... 5281.568965: __srcu_read_unlock <-fsnotify 3428 bash-1994 [000] .... 5281.568967: sys_dup2 <-system_call_fastpath 3429 3430 3431Note, reading the trace_pipe file will block until more input is 3432added. This is contrary to the trace file. If any process opened 3433the trace file for reading, it will actually disable tracing and 3434prevent new entries from being added. The trace_pipe file does 3435not have this limitation. 3436 3437trace entries 3438------------- 3439 3440Having too much or not enough data can be troublesome in 3441diagnosing an issue in the kernel. The file buffer_size_kb is 3442used to modify the size of the internal trace buffers. The 3443number listed is the number of entries that can be recorded per 3444CPU. To know the full size, multiply the number of possible CPUs 3445with the number of entries. 3446:: 3447 3448 # cat buffer_size_kb 3449 1408 (units kilobytes) 3450 3451Or simply read buffer_total_size_kb 3452:: 3453 3454 # cat buffer_total_size_kb 3455 5632 3456 3457To modify the buffer, simple echo in a number (in 1024 byte segments). 3458:: 3459 3460 # echo 10000 > buffer_size_kb 3461 # cat buffer_size_kb 3462 10000 (units kilobytes) 3463 3464It will try to allocate as much as possible. If you allocate too 3465much, it can cause Out-Of-Memory to trigger. 3466:: 3467 3468 # echo 1000000000000 > buffer_size_kb 3469 -bash: echo: write error: Cannot allocate memory 3470 # cat buffer_size_kb 3471 85 3472 3473The per_cpu buffers can be changed individually as well: 3474:: 3475 3476 # echo 10000 > per_cpu/cpu0/buffer_size_kb 3477 # echo 100 > per_cpu/cpu1/buffer_size_kb 3478 3479When the per_cpu buffers are not the same, the buffer_size_kb 3480at the top level will just show an X 3481:: 3482 3483 # cat buffer_size_kb 3484 X 3485 3486This is where the buffer_total_size_kb is useful: 3487:: 3488 3489 # cat buffer_total_size_kb 3490 12916 3491 3492Writing to the top level buffer_size_kb will reset all the buffers 3493to be the same again. 3494 3495Snapshot 3496-------- 3497CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature 3498available to all non latency tracers. (Latency tracers which 3499record max latency, such as "irqsoff" or "wakeup", can't use 3500this feature, since those are already using the snapshot 3501mechanism internally.) 3502 3503Snapshot preserves a current trace buffer at a particular point 3504in time without stopping tracing. Ftrace swaps the current 3505buffer with a spare buffer, and tracing continues in the new 3506current (=previous spare) buffer. 3507 3508The following tracefs files in "tracing" are related to this 3509feature: 3510 3511 snapshot: 3512 3513 This is used to take a snapshot and to read the output 3514 of the snapshot. Echo 1 into this file to allocate a 3515 spare buffer and to take a snapshot (swap), then read 3516 the snapshot from this file in the same format as 3517 "trace" (described above in the section "The File 3518 System"). Both reads snapshot and tracing are executable 3519 in parallel. When the spare buffer is allocated, echoing 3520 0 frees it, and echoing else (positive) values clear the 3521 snapshot contents. 3522 More details are shown in the table below. 3523 3524 +--------------+------------+------------+------------+ 3525 |status\\input | 0 | 1 | else | 3526 +==============+============+============+============+ 3527 |not allocated |(do nothing)| alloc+swap |(do nothing)| 3528 +--------------+------------+------------+------------+ 3529 |allocated | free | swap | clear | 3530 +--------------+------------+------------+------------+ 3531 3532Here is an example of using the snapshot feature. 3533:: 3534 3535 # echo 1 > events/sched/enable 3536 # echo 1 > snapshot 3537 # cat snapshot 3538 # tracer: nop 3539 # 3540 # entries-in-buffer/entries-written: 71/71 #P:8 3541 # 3542 # _-----=> irqs-off 3543 # / _----=> need-resched 3544 # | / _---=> hardirq/softirq 3545 # || / _--=> preempt-depth 3546 # ||| / delay 3547 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3548 # | | | |||| | | 3549 <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 3550 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 3551 [...] 3552 <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 3553 3554 # cat trace 3555 # tracer: nop 3556 # 3557 # entries-in-buffer/entries-written: 77/77 #P:8 3558 # 3559 # _-----=> irqs-off 3560 # / _----=> need-resched 3561 # | / _---=> hardirq/softirq 3562 # || / _--=> preempt-depth 3563 # ||| / delay 3564 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3565 # | | | |||| | | 3566 <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 3567 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 3568 [...] 3569 3570 3571If you try to use this snapshot feature when current tracer is 3572one of the latency tracers, you will get the following results. 3573:: 3574 3575 # echo wakeup > current_tracer 3576 # echo 1 > snapshot 3577 bash: echo: write error: Device or resource busy 3578 # cat snapshot 3579 cat: snapshot: Device or resource busy 3580 3581 3582Instances 3583--------- 3584In the tracefs tracing directory, there is a directory called "instances". 3585This directory can have new directories created inside of it using 3586mkdir, and removing directories with rmdir. The directory created 3587with mkdir in this directory will already contain files and other 3588directories after it is created. 3589:: 3590 3591 # mkdir instances/foo 3592 # ls instances/foo 3593 buffer_size_kb buffer_total_size_kb events free_buffer per_cpu 3594 set_event snapshot trace trace_clock trace_marker trace_options 3595 trace_pipe tracing_on 3596 3597As you can see, the new directory looks similar to the tracing directory 3598itself. In fact, it is very similar, except that the buffer and 3599events are agnostic from the main directory, or from any other 3600instances that are created. 3601 3602The files in the new directory work just like the files with the 3603same name in the tracing directory except the buffer that is used 3604is a separate and new buffer. The files affect that buffer but do not 3605affect the main buffer with the exception of trace_options. Currently, 3606the trace_options affect all instances and the top level buffer 3607the same, but this may change in future releases. That is, options 3608may become specific to the instance they reside in. 3609 3610Notice that none of the function tracer files are there, nor is 3611current_tracer and available_tracers. This is because the buffers 3612can currently only have events enabled for them. 3613:: 3614 3615 # mkdir instances/foo 3616 # mkdir instances/bar 3617 # mkdir instances/zoot 3618 # echo 100000 > buffer_size_kb 3619 # echo 1000 > instances/foo/buffer_size_kb 3620 # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb 3621 # echo function > current_trace 3622 # echo 1 > instances/foo/events/sched/sched_wakeup/enable 3623 # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable 3624 # echo 1 > instances/foo/events/sched/sched_switch/enable 3625 # echo 1 > instances/bar/events/irq/enable 3626 # echo 1 > instances/zoot/events/syscalls/enable 3627 # cat trace_pipe 3628 CPU:2 [LOST 11745 EVENTS] 3629 bash-2044 [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist 3630 bash-2044 [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave 3631 bash-2044 [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist 3632 bash-2044 [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist 3633 bash-2044 [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock 3634 bash-2044 [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype 3635 bash-2044 [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist 3636 bash-2044 [002] d... 10594.481034: zone_statistics <-get_page_from_freelist 3637 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics 3638 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics 3639 bash-2044 [002] .... 10594.481035: arch_dup_task_struct <-copy_process 3640 [...] 3641 3642 # cat instances/foo/trace_pipe 3643 bash-1998 [000] d..4 136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000 3644 bash-1998 [000] dN.4 136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000 3645 <idle>-0 [003] d.h3 136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003 3646 <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 3647 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 3648 bash-1998 [000] d..4 136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000 3649 bash-1998 [000] dN.4 136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000 3650 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 3651 kworker/0:1-59 [000] d..4 136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001 3652 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 3653 [...] 3654 3655 # cat instances/bar/trace_pipe 3656 migration/1-14 [001] d.h3 138.732674: softirq_raise: vec=3 [action=NET_RX] 3657 <idle>-0 [001] dNh3 138.732725: softirq_raise: vec=3 [action=NET_RX] 3658 bash-1998 [000] d.h1 138.733101: softirq_raise: vec=1 [action=TIMER] 3659 bash-1998 [000] d.h1 138.733102: softirq_raise: vec=9 [action=RCU] 3660 bash-1998 [000] ..s2 138.733105: softirq_entry: vec=1 [action=TIMER] 3661 bash-1998 [000] ..s2 138.733106: softirq_exit: vec=1 [action=TIMER] 3662 bash-1998 [000] ..s2 138.733106: softirq_entry: vec=9 [action=RCU] 3663 bash-1998 [000] ..s2 138.733109: softirq_exit: vec=9 [action=RCU] 3664 sshd-1995 [001] d.h1 138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4 3665 sshd-1995 [001] d.h1 138.733280: irq_handler_exit: irq=21 ret=unhandled 3666 sshd-1995 [001] d.h1 138.733281: irq_handler_entry: irq=21 name=eth0 3667 sshd-1995 [001] d.h1 138.733283: irq_handler_exit: irq=21 ret=handled 3668 [...] 3669 3670 # cat instances/zoot/trace 3671 # tracer: nop 3672 # 3673 # entries-in-buffer/entries-written: 18996/18996 #P:4 3674 # 3675 # _-----=> irqs-off 3676 # / _----=> need-resched 3677 # | / _---=> hardirq/softirq 3678 # || / _--=> preempt-depth 3679 # ||| / delay 3680 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3681 # | | | |||| | | 3682 bash-1998 [000] d... 140.733501: sys_write -> 0x2 3683 bash-1998 [000] d... 140.733504: sys_dup2(oldfd: a, newfd: 1) 3684 bash-1998 [000] d... 140.733506: sys_dup2 -> 0x1 3685 bash-1998 [000] d... 140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0) 3686 bash-1998 [000] d... 140.733509: sys_fcntl -> 0x1 3687 bash-1998 [000] d... 140.733510: sys_close(fd: a) 3688 bash-1998 [000] d... 140.733510: sys_close -> 0x0 3689 bash-1998 [000] d... 140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8) 3690 bash-1998 [000] d... 140.733515: sys_rt_sigprocmask -> 0x0 3691 bash-1998 [000] d... 140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8) 3692 bash-1998 [000] d... 140.733516: sys_rt_sigaction -> 0x0 3693 3694You can see that the trace of the top most trace buffer shows only 3695the function tracing. The foo instance displays wakeups and task 3696switches. 3697 3698To remove the instances, simply delete their directories: 3699:: 3700 3701 # rmdir instances/foo 3702 # rmdir instances/bar 3703 # rmdir instances/zoot 3704 3705Note, if a process has a trace file open in one of the instance 3706directories, the rmdir will fail with EBUSY. 3707 3708 3709Stack trace 3710----------- 3711Since the kernel has a fixed sized stack, it is important not to 3712waste it in functions. A kernel developer must be conscious of 3713what they allocate on the stack. If they add too much, the system 3714can be in danger of a stack overflow, and corruption will occur, 3715usually leading to a system panic. 3716 3717There are some tools that check this, usually with interrupts 3718periodically checking usage. But if you can perform a check 3719at every function call that will become very useful. As ftrace provides 3720a function tracer, it makes it convenient to check the stack size 3721at every function call. This is enabled via the stack tracer. 3722 3723CONFIG_STACK_TRACER enables the ftrace stack tracing functionality. 3724To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled. 3725:: 3726 3727 # echo 1 > /proc/sys/kernel/stack_tracer_enabled 3728 3729You can also enable it from the kernel command line to trace 3730the stack size of the kernel during boot up, by adding "stacktrace" 3731to the kernel command line parameter. 3732 3733After running it for a few minutes, the output looks like: 3734:: 3735 3736 # cat stack_max_size 3737 2928 3738 3739 # cat stack_trace 3740 Depth Size Location (18 entries) 3741 ----- ---- -------- 3742 0) 2928 224 update_sd_lb_stats+0xbc/0x4ac 3743 1) 2704 160 find_busiest_group+0x31/0x1f1 3744 2) 2544 256 load_balance+0xd9/0x662 3745 3) 2288 80 idle_balance+0xbb/0x130 3746 4) 2208 128 __schedule+0x26e/0x5b9 3747 5) 2080 16 schedule+0x64/0x66 3748 6) 2064 128 schedule_timeout+0x34/0xe0 3749 7) 1936 112 wait_for_common+0x97/0xf1 3750 8) 1824 16 wait_for_completion+0x1d/0x1f 3751 9) 1808 128 flush_work+0xfe/0x119 3752 10) 1680 16 tty_flush_to_ldisc+0x1e/0x20 3753 11) 1664 48 input_available_p+0x1d/0x5c 3754 12) 1616 48 n_tty_poll+0x6d/0x134 3755 13) 1568 64 tty_poll+0x64/0x7f 3756 14) 1504 880 do_select+0x31e/0x511 3757 15) 624 400 core_sys_select+0x177/0x216 3758 16) 224 96 sys_select+0x91/0xb9 3759 17) 128 128 system_call_fastpath+0x16/0x1b 3760 3761Note, if -mfentry is being used by gcc, functions get traced before 3762they set up the stack frame. This means that leaf level functions 3763are not tested by the stack tracer when -mfentry is used. 3764 3765Currently, -mfentry is used by gcc 4.6.0 and above on x86 only. 3766 3767More 3768---- 3769More details can be found in the source code, in the `kernel/trace/*.c` files. 3770