1 ================================================================
2 Documentation for Kdump - The kexec-based Crash Dumping Solution
3 ================================================================
5 This document includes overview, setup, installation, and analysis
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.
16 You can use common commands, such as cp, scp or makedumpfile to copy
17 the memory image to a dump file on the local disk, or across the network
20 Kdump and kexec are currently supported on the x86, x86_64, ppc64,
21 s390x, arm and arm64 architectures.
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
29 On x86 machines, the first 640 KB of physical memory is needed for boot,
30 regardless of where the kernel loads. For simpler handling, the whole
31 low 1M is reserved to avoid any later kernel or device driver writing
32 data into this area. Like this, the low 1M can be reused as system RAM
33 by kdump kernel without extra handling.
35 On PPC64 machines first 32KB of physical memory is needed for booting
36 regardless of where the kernel is loaded and to support 64K page size
37 kexec backs up the first 64KB memory.
39 For s390x, when kdump is triggered, the crashkernel region is exchanged
40 with the region [0, crashkernel region size] and then the kdump kernel
41 runs in [0, crashkernel region size]. Therefore no relocatable kernel is
44 All of the necessary information about the system kernel's core image is
45 encoded in the ELF format, and stored in a reserved area of memory
46 before a crash. The physical address of the start of the ELF header is
47 passed to the dump-capture kernel through the elfcorehdr= boot
48 parameter. Optionally the size of the ELF header can also be passed
49 when using the elfcorehdr=[size[KMG]@]offset[KMG] syntax.
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
53 write out using file copy commands such as cp or scp. You can also use
54 makedumpfile utility to analyze and write out filtered contents with
55 options, e.g with '-d 31' it will only write out kernel data. Further,
56 you can use analysis tools such as the GNU Debugger (GDB) and the Crash
57 tool to debug the dump file. This method ensures that the dump pages are
60 Setup and Installation
61 ======================
66 1) Login as the root user.
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
72 This is a symlink to the latest version.
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
79 There is also a gitweb interface available at
80 http://www.kernel.org/git/?p=utils/kernel/kexec/kexec-tools.git
82 More information about kexec-tools can be found at
83 http://horms.net/projects/kexec/
85 3) Unpack the tarball with the tar command, as follows::
87 tar xvpzf kexec-tools.tar.gz
89 4) Change to the kexec-tools directory, as follows::
91 cd kexec-tools-VERSION
93 5) Configure the package, as follows::
97 6) Compile the package, as follows::
101 7) Install the package, as follows::
106 Build the system and dump-capture kernels
107 -----------------------------------------
108 There are two possible methods of using Kdump.
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
115 only with the architectures which support a relocatable kernel. As
116 of today, i386, x86_64, ppc64, arm and arm64 architectures support
119 Building a relocatable kernel is advantageous from the point of view that
120 one does not have to build a second kernel for capturing the dump. But
121 at the same time one might want to build a custom dump capture kernel
122 suitable to his needs.
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
131 "Processor type and features."::
133 CONFIG_KEXEC=y or CONFIG_KEXEC_FILE=y
135 And both of them will select KEXEC_CORE::
139 Subsequently, CRASH_CORE is selected by KEXEC_CORE::
143 2) Enable "sysfs file system support" in "Filesystem" -> "Pseudo
144 filesystems." This is usually enabled by default::
148 Note that "sysfs file system support" might not appear in the "Pseudo
149 filesystems" menu if "Configure standard kernel features (expert users)"
150 is not enabled in "General Setup." In this case, check the .config file
151 itself to ensure that sysfs is turned on, as follows::
153 grep 'CONFIG_SYSFS' .config
155 3) Enable "Compile the kernel with debug info" in "Kernel hacking."::
159 This causes the kernel to be built with debug symbols. The dump
160 analysis tools require a vmlinux with debug symbols in order to read
161 and analyze a dump file.
163 Dump-capture kernel config options (Arch Independent)
164 -----------------------------------------------------
166 1) Enable "kernel crash dumps" support under "Processor type and
171 2) Enable "/proc/vmcore support" under "Filesystems" -> "Pseudo filesystems"::
175 (CONFIG_PROC_VMCORE is set by default when CONFIG_CRASH_DUMP is selected.)
177 Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
178 --------------------------------------------------------------------
180 1) On i386, enable high memory support under "Processor type and
189 2) With CONFIG_SMP=y, usually nr_cpus=1 need specified on the kernel
190 command line when loading the dump-capture kernel because one
191 CPU is enough for kdump kernel to dump vmcore on most of systems.
193 However, you can also specify nr_cpus=X to enable multiple processors
194 in kdump kernel. In this case, "disable_cpu_apicid=" is needed to
195 tell kdump kernel which cpu is 1st kernel's BSP. Please refer to
196 admin-guide/kernel-parameters.txt for more details.
198 With CONFIG_SMP=n, the above things are not related.
200 3) A relocatable kernel is suggested to be built by default. If not yet,
201 enable "Build a relocatable kernel" support under "Processor type and
206 4) Use a suitable value for "Physical address where the kernel is
207 loaded" (under "Processor type and features"). This only appears when
208 "kernel crash dumps" is enabled. A suitable value depends upon
209 whether kernel is relocatable or not.
211 If you are using a relocatable kernel use CONFIG_PHYSICAL_START=0x100000
212 This will compile the kernel for physical address 1MB, but given the fact
213 kernel is relocatable, it can be run from any physical address hence
214 kexec boot loader will load it in memory region reserved for dump-capture
217 Otherwise it should be the start of memory region reserved for
218 second kernel using boot parameter "crashkernel=Y@X". Here X is
219 start of memory region reserved for dump-capture kernel.
220 Generally X is 16MB (0x1000000). So you can set
221 CONFIG_PHYSICAL_START=0x1000000
223 5) Make and install the kernel and its modules. DO NOT add this kernel
224 to the boot loader configuration files.
226 Dump-capture kernel config options (Arch Dependent, ppc64)
227 ----------------------------------------------------------
229 1) Enable "Build a kdump crash kernel" support under "Kernel" options::
233 2) Enable "Build a relocatable kernel" support::
237 Make and install the kernel and its modules.
239 Dump-capture kernel config options (Arch Dependent, arm)
240 ----------------------------------------------------------
242 - To use a relocatable kernel,
243 Enable "AUTO_ZRELADDR" support under "Boot" options::
247 Dump-capture kernel config options (Arch Dependent, arm64)
248 ----------------------------------------------------------
250 - Please note that kvm of the dump-capture kernel will not be enabled
251 on non-VHE systems even if it is configured. This is because the CPU
252 will not be reset to EL2 on panic.
255 ===========================
256 1) crashkernel=size@offset
258 Here 'size' specifies how much memory to reserve for the dump-capture kernel
259 and 'offset' specifies the beginning of this reserved memory. For example,
260 "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
261 starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
263 The crashkernel region can be automatically placed by the system
264 kernel at run time. This is done by specifying the base address as 0,
265 or omitting it all together::
273 If the start address is specified, note that the start address of the
274 kernel will be aligned to a value (which is Arch dependent), so if the
275 start address is not then any space below the alignment point will be
278 2) range1:size1[,range2:size2,...][@offset]
280 While the "crashkernel=size[@offset]" syntax is sufficient for most
281 configurations, sometimes it's handy to have the reserved memory dependent
282 on the value of System RAM -- that's mostly for distributors that pre-setup
283 the kernel command line to avoid a unbootable system after some memory has
284 been removed from the machine.
288 crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
293 crashkernel=512M-2G:64M,2G-:128M
297 1) if the RAM is smaller than 512M, then don't reserve anything
298 (this is the "rescue" case)
299 2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
300 3) if the RAM size is larger than 2G, then reserve 128M
302 3) crashkernel=size,high and crashkernel=size,low
304 If memory above 4G is preferred, crashkernel=size,high can be used to
305 fulfill that. With it, physical memory is allowed to be allocated from top,
306 so could be above 4G if system has more than 4G RAM installed. Otherwise,
307 memory region will be allocated below 4G if available.
309 When crashkernel=X,high is passed, kernel could allocate physical memory
310 region above 4G, low memory under 4G is needed in this case. There are
311 three ways to get low memory:
313 1) Kernel will allocate at least 256M memory below 4G automatically
314 if crashkernel=Y,low is not specified.
315 2) Let user specify low memory size instead.
316 3) Specified value 0 will disable low memory allocation::
320 Boot into System Kernel
321 -----------------------
322 1) Update the boot loader (such as grub, yaboot, or lilo) configuration
325 2) Boot the system kernel with the boot parameter "crashkernel=Y@X".
327 On x86 and x86_64, use "crashkernel=Y[@X]". Most of the time, the
328 start address 'X' is not necessary, kernel will search a suitable
329 area. Unless an explicit start address is expected.
331 On ppc64, use "crashkernel=128M@32M".
333 On s390x, typically use "crashkernel=xxM". The value of xx is dependent
334 on the memory consumption of the kdump system. In general this is not
335 dependent on the memory size of the production system.
337 On arm, the use of "crashkernel=Y@X" is no longer necessary; the
338 kernel will automatically locate the crash kernel image within the
339 first 512MB of RAM if X is not given.
341 On arm64, use "crashkernel=Y[@X]". Note that the start address of
342 the kernel, X if explicitly specified, must be aligned to 2MiB (0x200000).
344 Load the Dump-capture Kernel
345 ============================
347 After booting to the system kernel, dump-capture kernel needs to be
350 Based on the architecture and type of image (relocatable or not), one
351 can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz
352 of dump-capture kernel. Following is the summary.
356 - Use bzImage/vmlinuz if kernel is relocatable.
357 - Use vmlinux if kernel is not relocatable.
365 - Use image or bzImage
373 - Use vmlinux or Image
375 If you are using an uncompressed vmlinux image then use following command
376 to load dump-capture kernel::
378 kexec -p <dump-capture-kernel-vmlinux-image> \
379 --initrd=<initrd-for-dump-capture-kernel> --args-linux \
380 --append="root=<root-dev> <arch-specific-options>"
382 If you are using a compressed bzImage/vmlinuz, then use following command
383 to load dump-capture kernel::
385 kexec -p <dump-capture-kernel-bzImage> \
386 --initrd=<initrd-for-dump-capture-kernel> \
387 --append="root=<root-dev> <arch-specific-options>"
389 If you are using a compressed zImage, then use following command
390 to load dump-capture kernel::
392 kexec --type zImage -p <dump-capture-kernel-bzImage> \
393 --initrd=<initrd-for-dump-capture-kernel> \
394 --dtb=<dtb-for-dump-capture-kernel> \
395 --append="root=<root-dev> <arch-specific-options>"
397 If you are using an uncompressed Image, then use following command
398 to load dump-capture kernel::
400 kexec -p <dump-capture-kernel-Image> \
401 --initrd=<initrd-for-dump-capture-kernel> \
402 --append="root=<root-dev> <arch-specific-options>"
404 Following are the arch specific command line options to be used while
405 loading dump-capture kernel.
409 "1 irqpoll nr_cpus=1 reset_devices"
413 "1 maxcpus=1 noirqdistrib reset_devices"
417 "1 nr_cpus=1 cgroup_disable=memory"
421 "1 maxcpus=1 reset_devices"
425 "1 nr_cpus=1 reset_devices"
427 Notes on loading the dump-capture kernel:
429 * By default, the ELF headers are stored in ELF64 format to support
430 systems with more than 4GB memory. On i386, kexec automatically checks if
431 the physical RAM size exceeds the 4 GB limit and if not, uses ELF32.
432 So, on non-PAE systems, ELF32 is always used.
434 The --elf32-core-headers option can be used to force the generation of ELF32
435 headers. This is necessary because GDB currently cannot open vmcore files
436 with ELF64 headers on 32-bit systems.
438 * The "irqpoll" boot parameter reduces driver initialization failures
439 due to shared interrupts in the dump-capture kernel.
441 * You must specify <root-dev> in the format corresponding to the root
442 device name in the output of mount command.
444 * Boot parameter "1" boots the dump-capture kernel into single-user
445 mode without networking. If you want networking, use "3".
447 * We generally don't have to bring up a SMP kernel just to capture the
448 dump. Hence generally it is useful either to build a UP dump-capture
449 kernel or specify maxcpus=1 option while loading dump-capture kernel.
450 Note, though maxcpus always works, you had better replace it with
451 nr_cpus to save memory if supported by the current ARCH, such as x86.
453 * You should enable multi-cpu support in dump-capture kernel if you intend
454 to use multi-thread programs with it, such as parallel dump feature of
455 makedumpfile. Otherwise, the multi-thread program may have a great
456 performance degradation. To enable multi-cpu support, you should bring up an
457 SMP dump-capture kernel and specify maxcpus/nr_cpus, disable_cpu_apicid=[X]
458 options while loading it.
460 * For s390x there are two kdump modes: If a ELF header is specified with
461 the elfcorehdr= kernel parameter, it is used by the kdump kernel as it
462 is done on all other architectures. If no elfcorehdr= kernel parameter is
463 specified, the s390x kdump kernel dynamically creates the header. The
464 second mode has the advantage that for CPU and memory hotplug, kdump has
465 not to be reloaded with kexec_load().
467 * For s390x systems with many attached devices the "cio_ignore" kernel
468 parameter should be used for the kdump kernel in order to prevent allocation
469 of kernel memory for devices that are not relevant for kdump. The same
470 applies to systems that use SCSI/FCP devices. In that case the
471 "allow_lun_scan" zfcp module parameter should be set to zero before
472 setting FCP devices online.
477 After successfully loading the dump-capture kernel as previously
478 described, the system will reboot into the dump-capture kernel if a
479 system crash is triggered. Trigger points are located in panic(),
480 die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).
482 The following conditions will execute a crash trigger point:
484 If a hard lockup is detected and "NMI watchdog" is configured, the system
485 will boot into the dump-capture kernel ( die_nmi() ).
487 If die() is called, and it happens to be a thread with pid 0 or 1, or die()
488 is called inside interrupt context or die() is called and panic_on_oops is set,
489 the system will boot into the dump-capture kernel.
491 On powerpc systems when a soft-reset is generated, die() is called by all cpus
492 and the system will boot into the dump-capture kernel.
494 For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
495 "echo c > /proc/sysrq-trigger" or write a module to force the panic.
497 Write Out the Dump File
498 =======================
500 After the dump-capture kernel is booted, write out the dump file with
501 the following command::
503 cp /proc/vmcore <dump-file>
505 or use scp to write out the dump file between hosts on a network, e.g::
507 scp /proc/vmcore remote_username@remote_ip:<dump-file>
509 You can also use makedumpfile utility to write out the dump file
510 with specified options to filter out unwanted contents, e.g::
512 makedumpfile -l --message-level 1 -d 31 /proc/vmcore <dump-file>
517 Before analyzing the dump image, you should reboot into a stable kernel.
519 You can do limited analysis using GDB on the dump file copied out of
520 /proc/vmcore. Use the debug vmlinux built with -g and run the following
523 gdb vmlinux <dump-file>
525 Stack trace for the task on processor 0, register display, and memory
528 Note: GDB cannot analyze core files generated in ELF64 format for x86.
529 On systems with a maximum of 4GB of memory, you can generate
530 ELF32-format headers using the --elf32-core-headers kernel option on the
533 You can also use the Crash utility to analyze dump files in Kdump
534 format. Crash is available at the following URL:
536 https://github.com/crash-utility/crash
538 Crash document can be found at:
539 https://crash-utility.github.io/
541 Trigger Kdump on WARN()
542 =======================
544 The kernel parameter, panic_on_warn, calls panic() in all WARN() paths. This
545 will cause a kdump to occur at the panic() call. In cases where a user wants
546 to specify this during runtime, /proc/sys/kernel/panic_on_warn can be set to 1
547 to achieve the same behaviour.
549 Trigger Kdump on add_taint()
550 ============================
552 The kernel parameter panic_on_taint facilitates a conditional call to panic()
553 from within add_taint() whenever the value set in this bitmask matches with the
554 bit flag being set by add_taint().
555 This will cause a kdump to occur at the add_taint()->panic() call.
560 - kexec@lists.infradead.org
565 .. include:: gdbmacros.txt