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.
18 to a remote system.
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
32 data into this area. Like this, the low 1M can be reused as system RAM
44 All of the necessary information about the system kernel's core image is
47 passed to the dump-capture kernel through the elfcorehdr= boot
51 With the dump-capture kernel, you can access the memory image through
52 /proc/vmcore. This exports the dump as an ELF-format file that you can
55 options, e.g with '-d 31' it will only write out kernel data. Further,
63 Install kexec-tools
64 -------------------
68 2) Download the kexec-tools user-space package from the following URL:
70 http://kernel.org/pub/linux/utils/kernel/kexec/kexec-tools.tar.gz
74 The latest kexec-tools git tree is available at:
76 - git://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
77 - http://www.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
80 http://www.kernel.org/git/?p=utils/kernel/kexec/kexec-tools.git
82 More information about kexec-tools can be found at
87 	tar xvpzf kexec-tools.tar.gz
89 4) Change to the kexec-tools directory, as follows::
91 	cd kexec-tools-VERSION
106 Build the system and dump-capture kernels
107 -----------------------------------------
110 1) Build a separate custom dump-capture kernel for capturing the
113 2) Or use the system kernel binary itself as dump-capture kernel and there is
114    no need to build a separate dump-capture kernel. This is possible
124 Following are the configuration setting required for system and
125 dump-capture kernels for enabling kdump support.
127 System kernel config options
128 ----------------------------
130 1) Enable "kexec system call" or "kexec file based system call" in
139 2) Enable "sysfs file system support" in "Filesystem" -> "Pseudo
144    Note that "sysfs file system support" might not appear in the "Pseudo
159 Dump-capture kernel config options (Arch Independent)
160 -----------------------------------------------------
171 2) Enable "/proc/vmcore support" under "Filesystems" -> "Pseudo filesystems"::
177 Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
178 --------------------------------------------------------------------
186    command line when loading the dump-capture kernel because one
208    kexec boot loader will load it in memory region reserved for dump-capture
213    start of memory region reserved for dump-capture kernel.
220 Dump-capture kernel config options (Arch Dependent, ppc64)
221 ----------------------------------------------------------
233 Dump-capture kernel config options (Arch Dependent, arm)
234 ----------------------------------------------------------
236 -   To use a relocatable kernel,
241 Dump-capture kernel config options (Arch Dependent, arm64)
242 ----------------------------------------------------------
244 - Please note that kvm of the dump-capture kernel will not be enabled
245   on non-VHE systems even if it is configured. This is because the CPU
252    Here 'size' specifies how much memory to reserve for the dump-capture kernel
254    "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
255    starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
257    The crashkernel region can be automatically placed by the system
276    on the value of System RAM -- that's mostly for distributors that pre-setup
277    the kernel command line to avoid a unbootable system after some memory has
283        range=start-[end]
287        crashkernel=512M-2G:64M,2G-:128M
300    so could be above 4G if system has more than 4G RAM installed. Otherwise,
314 Boot into System Kernel
315 -----------------------
319 2) Boot the system kernel with the boot parameter "crashkernel=Y@X".
328    on the memory consumption of the kdump system. In general this is not
329    dependent on the memory size of the production system.
338 Load the Dump-capture Kernel
341 After booting to the system kernel, dump-capture kernel needs to be
346 of dump-capture kernel. Following is the summary.
350 	- Use bzImage/vmlinuz if kernel is relocatable.
351 	- Use vmlinux if kernel is not relocatable.
355 	- Use vmlinux
359 	- Use image or bzImage
363 	- Use zImage
367 	- Use vmlinux or Image
370 to load dump-capture kernel::
372    kexec -p <dump-capture-kernel-vmlinux-image> \
373    --initrd=<initrd-for-dump-capture-kernel> --args-linux \
374    --append="root=<root-dev> <arch-specific-options>"
377 to load dump-capture kernel::
379    kexec -p <dump-capture-kernel-bzImage> \
380    --initrd=<initrd-for-dump-capture-kernel> \
381    --append="root=<root-dev> <arch-specific-options>"
384 to load dump-capture kernel::
386    kexec --type zImage -p <dump-capture-kernel-bzImage> \
387    --initrd=<initrd-for-dump-capture-kernel> \
388    --dtb=<dtb-for-dump-capture-kernel> \
389    --append="root=<root-dev> <arch-specific-options>"
392 to load dump-capture kernel::
394    kexec -p <dump-capture-kernel-Image> \
395    --initrd=<initrd-for-dump-capture-kernel> \
396    --append="root=<root-dev> <arch-specific-options>"
399 loading dump-capture kernel.
421 Notes on loading the dump-capture kernel:
426   So, on non-PAE systems, ELF32 is always used.
428   The --elf32-core-headers option can be used to force the generation of ELF32
430   with ELF64 headers on 32-bit systems.
433   due to shared interrupts in the dump-capture kernel.
435 * You must specify <root-dev> in the format corresponding to the root
438 * Boot parameter "1" boots the dump-capture kernel into single-user
442   dump. Hence generally it is useful either to build a UP dump-capture
443   kernel or specify maxcpus=1 option while loading dump-capture kernel.
447 * You should enable multi-cpu support in dump-capture kernel if you intend
448   to use multi-thread programs with it, such as parallel dump feature of
449   makedumpfile. Otherwise, the multi-thread program may have a great
450   performance degradation. To enable multi-cpu support, you should bring up an
451   SMP dump-capture kernel and specify maxcpus/nr_cpus options while loading it.
470 After successfully loading the dump-capture kernel as previously
471 described, the system will reboot into the dump-capture kernel if a
472 system crash is triggered.  Trigger points are located in panic(),
473 die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).
477 If a hard lockup is detected and "NMI watchdog" is configured, the system
478 will boot into the dump-capture kernel ( die_nmi() ).
482 the system will boot into the dump-capture kernel.
484 On powerpc systems when a soft-reset is generated, die() is called by all cpus
485 and the system will boot into the dump-capture kernel.
487 For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
488 "echo c > /proc/sysrq-trigger" or write a module to force the panic.
493 After the dump-capture kernel is booted, write out the dump file with
496    cp /proc/vmcore <dump-file>
500    scp /proc/vmcore remote_username@remote_ip:<dump-file>
505    makedumpfile -l --message-level 1 -d 31 /proc/vmcore <dump-file>
513 /proc/vmcore. Use the debug vmlinux built with -g and run the following
516    gdb vmlinux <dump-file>
523 ELF32-format headers using the --elf32-core-headers kernel option on the
529    https://github.com/crash-utility/crash
532    https://crash-utility.github.io/
548 This will cause a kdump to occur at the add_taint()->panic() call.
553 - kexec@lists.infradead.org