admin-guide/kdump/kdump.rst

2 Documentation for Kdump - The kexec-based Crash Dumping Solution
11 Kdump uses kexec to quickly boot to a dump-capture kernel whenever a
12 dump of the system kernel's memory needs to be taken (for example, when
13 the system panics). The system kernel's memory image is preserved across
14 the reboot and is accessible to the dump-capture kernel.
23 When the system kernel boots, it reserves a small section of memory for
24 the dump-capture kernel. This ensures that ongoing Direct Memory Access
25 (DMA) from the system kernel does not corrupt the dump-capture kernel.
26 The kexec -p command loads the dump-capture kernel into this reserved
30 regardless of where the kernel loads. Therefore, kexec backs up this
31 region just before rebooting into the dump-capture kernel.
34 booting regardless of where the kernel is loaded and to support 64K page
38 with the region [0, crashkernel region size] and then the kdump kernel
39 runs in [0, crashkernel region size]. Therefore no relocatable kernel is
42 All of the necessary information about the system kernel's core image is
43 encoded in the ELF format, and stored in a reserved area of memory
45 passed to the dump-capture kernel through the elfcorehdr= boot
50 With the dump-capture kernel, you can access the memory image through
51 /proc/vmcore. This exports the dump as an ELF-format file that you can
61 Install kexec-tools
62 -------------------
66 2) Download the kexec-tools user-space package from the following URL:
68 http://kernel.org/pub/linux/utils/kernel/kexec/kexec-tools.tar.gz
72 The latest kexec-tools git tree is available at:
74 - git://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
75 - http://www.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
78 http://www.kernel.org/git/?p=utils/kernel/kexec/kexec-tools.git
80 More information about kexec-tools can be found at
85 	tar xvpzf kexec-tools.tar.gz
87 4) Change to the kexec-tools directory, as follows::
89 	cd kexec-tools-VERSION
104 Build the system and dump-capture kernels
105 -----------------------------------------
108 1) Build a separate custom dump-capture kernel for capturing the
109    kernel core dump.
111 2) Or use the system kernel binary itself as dump-capture kernel and there is
112    no need to build a separate dump-capture kernel. This is possible
113    only with the architectures which support a relocatable kernel. As
115    relocatable kernel.
117 Building a relocatable kernel is advantageous from the point of view that
118 one does not have to build a second kernel for capturing the dump. But
119 at the same time one might want to build a custom dump capture kernel
123 dump-capture kernels for enabling kdump support.
125 System kernel config options
126 ----------------------------
128 1) Enable "kexec system call" in "Processor type and features."::
132 2) Enable "sysfs file system support" in "Filesystem" -> "Pseudo
137    Note that "sysfs file system support" might not appear in the "Pseudo
138    filesystems" menu if "Configure standard kernel features (for small
139    systems)" is not enabled in "General Setup." In this case, check the
144 3) Enable "Compile the kernel with debug info" in "Kernel hacking."::
148    This causes the kernel to be built with debug symbols. The dump
149    analysis tools require a vmlinux with debug symbols in order to read
152 Dump-capture kernel config options (Arch Independent)
153 -----------------------------------------------------
155 1) Enable "kernel crash dumps" support under "Processor type and
160 2) Enable "/proc/vmcore support" under "Filesystems" -> "Pseudo filesystems"::
166 Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
167 --------------------------------------------------------------------
178 2) On i386 and x86_64, disable symmetric multi-processing support
183    (If CONFIG_SMP=y, then specify maxcpus=1 on the kernel command line
184    when loading the dump-capture kernel, see section "Load the Dump-capture
185    Kernel".)
187 3) If one wants to build and use a relocatable kernel,
188    Enable "Build a relocatable kernel" support under "Processor type and
193 4) Use a suitable value for "Physical address where the kernel is
195    "kernel crash dumps" is enabled. A suitable value depends upon
196    whether kernel is relocatable or not.
198    If you are using a relocatable kernel use CONFIG_PHYSICAL_START=0x100000
199    This will compile the kernel for physical address 1MB, but given the fact
200    kernel is relocatable, it can be run from any physical address hence
201    kexec boot loader will load it in memory region reserved for dump-capture
202    kernel.
205    second kernel using boot parameter "crashkernel=Y@X". Here X is
206    start of memory region reserved for dump-capture kernel.
210 5) Make and install the kernel and its modules. DO NOT add this kernel
213 Dump-capture kernel config options (Arch Dependent, ppc64)
214 ----------------------------------------------------------
216 1) Enable "Build a kdump crash kernel" support under "Kernel" options::
220 2)   Enable "Build a relocatable kernel" support::
224    Make and install the kernel and its modules.
226 Dump-capture kernel config options (Arch Dependent, ia64)
227 ----------------------------------------------------------
229 - No specific options are required to create a dump-capture kernel
230   for ia64, other than those specified in the arch independent section
231   above. This means that it is possible to use the system kernel
232   as a dump-capture kernel if desired.
235   kernel at run time. This is done by specifying the base address as 0,
245   kernel will be aligned to 64Mb, so if the start address is not then
248 Dump-capture kernel config options (Arch Dependent, arm)
249 ----------------------------------------------------------
251 -   To use a relocatable kernel,
256 Dump-capture kernel config options (Arch Dependent, arm64)
257 ----------------------------------------------------------
259 - Please note that kvm of the dump-capture kernel will not be enabled
260   on non-VHE systems even if it is configured. This is because the CPU
268 on the value of System RAM -- that's mostly for distributors that pre-setup
269 the kernel command line to avoid a unbootable system after some memory has
275     range=start-[end]
279     crashkernel=512M-2G:64M,2G-:128M
290 Boot into System Kernel
296 2) Boot the system kernel with the boot parameter "crashkernel=Y@X",
297    where Y specifies how much memory to reserve for the dump-capture kernel
299    "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
300    starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
308    dump-capture kernel config option notes above.
312    on the memory consumption of the kdump system. In general this is not
316    kernel will automatically locate the crash kernel image within the
320    the kernel, X if explicitly specified, must be aligned to 2MiB (0x200000).
322 Load the Dump-capture Kernel
325 After booting to the system kernel, dump-capture kernel needs to be
330 of dump-capture kernel. Following is the summary.
334 	- Use vmlinux if kernel is not relocatable.
335 	- Use bzImage/vmlinuz if kernel is relocatable.
339 	- Use vmlinux
343 	- Use vmlinux or vmlinuz.gz
347 	- Use image or bzImage
351 	- Use zImage
355 	- Use vmlinux or Image
358 to load dump-capture kernel::
360    kexec -p <dump-capture-kernel-vmlinux-image> \
361    --initrd=<initrd-for-dump-capture-kernel> --args-linux \
362    --append="root=<root-dev> <arch-specific-options>"
365 to load dump-capture kernel::
367    kexec -p <dump-capture-kernel-bzImage> \
368    --initrd=<initrd-for-dump-capture-kernel> \
369    --append="root=<root-dev> <arch-specific-options>"
372 to load dump-capture kernel::
374    kexec --type zImage -p <dump-capture-kernel-bzImage> \
375    --initrd=<initrd-for-dump-capture-kernel> \
376    --dtb=<dtb-for-dump-capture-kernel> \
377    --append="root=<root-dev> <arch-specific-options>"
380 to load dump-capture kernel::
382    kexec -p <dump-capture-kernel-Image> \
383    --initrd=<initrd-for-dump-capture-kernel> \
384    --append="root=<root-dev> <arch-specific-options>"
386 Please note, that --args-linux does not need to be specified for ia64.
387 It is planned to make this a no-op on that architecture, but for now
391 loading dump-capture kernel.
413 Notes on loading the dump-capture kernel:
415 * By default, the ELF headers are stored in ELF64 format to support
418   So, on non-PAE systems, ELF32 is always used.
420   The --elf32-core-headers option can be used to force the generation of ELF32
422   with ELF64 headers on 32-bit systems.
425   due to shared interrupts in the dump-capture kernel.
427 * You must specify <root-dev> in the format corresponding to the root
428   device name in the output of mount command.
430 * Boot parameter "1" boots the dump-capture kernel into single-user
433 * We generally don't have to bring up a SMP kernel just to capture the
434   dump. Hence generally it is useful either to build a UP dump-capture
435   kernel or specify maxcpus=1 option while loading dump-capture kernel.
439 * You should enable multi-cpu support in dump-capture kernel if you intend
440   to use multi-thread programs with it, such as parallel dump feature of
441   makedumpfile. Otherwise, the multi-thread program may have a great
442   performance degradation. To enable multi-cpu support, you should bring up an
443   SMP dump-capture kernel and specify maxcpus/nr_cpus, disable_cpu_apicid=[X]
447   the elfcorehdr= kernel parameter, it is used by the kdump kernel as it
448   is done on all other architectures. If no elfcorehdr= kernel parameter is
449   specified, the s390x kdump kernel dynamically creates the header. The
453 * For s390x systems with many attached devices the "cio_ignore" kernel
454   parameter should be used for the kdump kernel in order to prevent allocation
455   of kernel memory for devices that are not relevant for kdump. The same
456   applies to systems that use SCSI/FCP devices. In that case the
460 Kernel Panic
463 After successfully loading the dump-capture kernel as previously
464 described, the system will reboot into the dump-capture kernel if a
465 system crash is triggered.  Trigger points are located in panic(),
466 die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).
471 will boot into the dump-capture kernel ( die_nmi() ).
475 the system will boot into the dump-capture kernel.
477 On powerpc systems when a soft-reset is generated, die() is called by all cpus
478 and the system will boot into the dump-capture kernel.
480 For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
481 "echo c > /proc/sysrq-trigger" or write a module to force the panic.
486 After the dump-capture kernel is booted, write out the dump file with
489    cp /proc/vmcore <dump-file>
495 Before analyzing the dump image, you should reboot into a stable kernel.
498 /proc/vmcore. Use the debug vmlinux built with -g and run the following
501    gdb vmlinux <dump-file>
506 Note: GDB cannot analyze core files generated in ELF64 format for x86.
508 ELF32-format headers using the --elf32-core-headers kernel option on the
509 dump kernel.
511 You can also use the Crash utility to analyze dump files in Kdump
514    https://github.com/crash-utility/crash
517    https://crash-utility.github.io/
522 The kernel parameter, panic_on_warn, calls panic() in all WARN() paths.  This
523 will cause a kdump to occur at the panic() call.  In cases where a user wants
524 to specify this during runtime, /proc/sys/kernel/panic_on_warn can be set to 1
530 The kernel parameter panic_on_taint facilitates a conditional call to panic()
531 from within add_taint() whenever the value set in this bitmask matches with the
533 This will cause a kdump to occur at the add_taint()->panic() call.
538 - Vivek Goyal (vgoyal@redhat.com)
539 - Maneesh Soni (maneesh@in.ibm.com)