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1================================================================2Documentation for Kdump - The kexec-based Crash Dumping Solution3================================================================4 5This document includes overview, setup, installation, and analysis6information.7 8Overview9========10 11Kdump uses kexec to quickly boot to a dump-capture kernel whenever a12dump of the system kernel's memory needs to be taken (for example, when13the system panics). The system kernel's memory image is preserved across14the reboot and is accessible to the dump-capture kernel.15 16You can use common commands, such as cp, scp or makedumpfile to copy17the memory image to a dump file on the local disk, or across the network18to a remote system.19 20Kdump and kexec are currently supported on the x86, x86_64, ppc64,21s390x, arm and arm64 architectures.22 23When the system kernel boots, it reserves a small section of memory for24the dump-capture kernel. This ensures that ongoing Direct Memory Access25(DMA) from the system kernel does not corrupt the dump-capture kernel.26The kexec -p command loads the dump-capture kernel into this reserved27memory.28 29On x86 machines, the first 640 KB of physical memory is needed for boot,30regardless of where the kernel loads. For simpler handling, the whole31low 1M is reserved to avoid any later kernel or device driver writing32data into this area. Like this, the low 1M can be reused as system RAM33by kdump kernel without extra handling.34 35On PPC64 machines first 32KB of physical memory is needed for booting36regardless of where the kernel is loaded and to support 64K page size37kexec backs up the first 64KB memory.38 39For s390x, when kdump is triggered, the crashkernel region is exchanged40with the region [0, crashkernel region size] and then the kdump kernel41runs in [0, crashkernel region size]. Therefore no relocatable kernel is42needed for s390x.43 44All of the necessary information about the system kernel's core image is45encoded in the ELF format, and stored in a reserved area of memory46before a crash. The physical address of the start of the ELF header is47passed to the dump-capture kernel through the elfcorehdr= boot48parameter. Optionally the size of the ELF header can also be passed49when using the elfcorehdr=[size[KMG]@]offset[KMG] syntax.50 51With the dump-capture kernel, you can access the memory image through52/proc/vmcore. This exports the dump as an ELF-format file that you can53write out using file copy commands such as cp or scp. You can also use54makedumpfile utility to analyze and write out filtered contents with55options, e.g with '-d 31' it will only write out kernel data. Further,56you can use analysis tools such as the GNU Debugger (GDB) and the Crash57tool to debug the dump file. This method ensures that the dump pages are58correctly ordered.59 60Setup and Installation61======================62 63Install kexec-tools64-------------------65 661) Login as the root user.67 682) Download the kexec-tools user-space package from the following URL:69 70http://kernel.org/pub/linux/utils/kernel/kexec/kexec-tools.tar.gz71 72This is a symlink to the latest version.73 74The latest kexec-tools git tree is available at:75 76- git://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git77- http://www.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git78 79There is also a gitweb interface available at80http://www.kernel.org/git/?p=utils/kernel/kexec/kexec-tools.git81 82More information about kexec-tools can be found at83http://horms.net/projects/kexec/84 853) Unpack the tarball with the tar command, as follows::86 87 tar xvpzf kexec-tools.tar.gz88 894) Change to the kexec-tools directory, as follows::90 91 cd kexec-tools-VERSION92 935) Configure the package, as follows::94 95 ./configure96 976) Compile the package, as follows::98 99 make100 1017) Install the package, as follows::102 103 make install104 105 106Build the system and dump-capture kernels107-----------------------------------------108There are two possible methods of using Kdump.109 1101) Build a separate custom dump-capture kernel for capturing the111 kernel core dump.112 1132) Or use the system kernel binary itself as dump-capture kernel and there is114 no need to build a separate dump-capture kernel. This is possible115 only with the architectures which support a relocatable kernel. As116 of today, i386, x86_64, ppc64, arm and arm64 architectures support117 relocatable kernel.118 119Building a relocatable kernel is advantageous from the point of view that120one does not have to build a second kernel for capturing the dump. But121at the same time one might want to build a custom dump capture kernel122suitable to his needs.123 124Following are the configuration setting required for system and125dump-capture kernels for enabling kdump support.126 127System kernel config options128----------------------------129 1301) Enable "kexec system call" or "kexec file based system call" in131 "Processor type and features."::132 133 CONFIG_KEXEC=y or CONFIG_KEXEC_FILE=y134 135 And both of them will select KEXEC_CORE::136 137 CONFIG_KEXEC_CORE=y138 1392) Enable "sysfs file system support" in "Filesystem" -> "Pseudo140 filesystems." This is usually enabled by default::141 142 CONFIG_SYSFS=y143 144 Note that "sysfs file system support" might not appear in the "Pseudo145 filesystems" menu if "Configure standard kernel features (expert users)"146 is not enabled in "General Setup." In this case, check the .config file147 itself to ensure that sysfs is turned on, as follows::148 149 grep 'CONFIG_SYSFS' .config150 1513) Enable "Compile the kernel with debug info" in "Kernel hacking."::152 153 CONFIG_DEBUG_INFO=Y154 155 This causes the kernel to be built with debug symbols. The dump156 analysis tools require a vmlinux with debug symbols in order to read157 and analyze a dump file.158 159Dump-capture kernel config options (Arch Independent)160-----------------------------------------------------161 1621) Enable "kernel crash dumps" support under "Processor type and163 features"::164 165 CONFIG_CRASH_DUMP=y166 167 And this will select VMCORE_INFO and CRASH_RESERVE::168 CONFIG_VMCORE_INFO=y169 CONFIG_CRASH_RESERVE=y170 1712) Enable "/proc/vmcore support" under "Filesystems" -> "Pseudo filesystems"::172 173 CONFIG_PROC_VMCORE=y174 175 (CONFIG_PROC_VMCORE is set by default when CONFIG_CRASH_DUMP is selected.)176 177Dump-capture kernel config options (Arch Dependent, i386 and x86_64)178--------------------------------------------------------------------179 1801) On i386, enable high memory support under "Processor type and181 features"::182 183 CONFIG_HIGHMEM64G=y184 185 or::186 187 CONFIG_HIGHMEM4G188 1892) With CONFIG_SMP=y, usually nr_cpus=1 need specified on the kernel190 command line when loading the dump-capture kernel because one191 CPU is enough for kdump kernel to dump vmcore on most of systems.192 193 However, you can also specify nr_cpus=X to enable multiple processors194 in kdump kernel.195 196 With CONFIG_SMP=n, the above things are not related.197 1983) A relocatable kernel is suggested to be built by default. If not yet,199 enable "Build a relocatable kernel" support under "Processor type and200 features"::201 202 CONFIG_RELOCATABLE=y203 2044) Use a suitable value for "Physical address where the kernel is205 loaded" (under "Processor type and features"). This only appears when206 "kernel crash dumps" is enabled. A suitable value depends upon207 whether kernel is relocatable or not.208 209 If you are using a relocatable kernel use CONFIG_PHYSICAL_START=0x100000210 This will compile the kernel for physical address 1MB, but given the fact211 kernel is relocatable, it can be run from any physical address hence212 kexec boot loader will load it in memory region reserved for dump-capture213 kernel.214 215 Otherwise it should be the start of memory region reserved for216 second kernel using boot parameter "crashkernel=Y@X". Here X is217 start of memory region reserved for dump-capture kernel.218 Generally X is 16MB (0x1000000). So you can set219 CONFIG_PHYSICAL_START=0x1000000220 2215) Make and install the kernel and its modules. DO NOT add this kernel222 to the boot loader configuration files.223 224Dump-capture kernel config options (Arch Dependent, ppc64)225----------------------------------------------------------226 2271) Enable "Build a kdump crash kernel" support under "Kernel" options::228 229 CONFIG_CRASH_DUMP=y230 2312) Enable "Build a relocatable kernel" support::232 233 CONFIG_RELOCATABLE=y234 235 Make and install the kernel and its modules.236 237Dump-capture kernel config options (Arch Dependent, arm)238----------------------------------------------------------239 240- To use a relocatable kernel,241 Enable "AUTO_ZRELADDR" support under "Boot" options::242 243 AUTO_ZRELADDR=y244 245Dump-capture kernel config options (Arch Dependent, arm64)246----------------------------------------------------------247 248- Please note that kvm of the dump-capture kernel will not be enabled249 on non-VHE systems even if it is configured. This is because the CPU250 will not be reset to EL2 on panic.251 252crashkernel syntax253===========================2541) crashkernel=size@offset255 256 Here 'size' specifies how much memory to reserve for the dump-capture kernel257 and 'offset' specifies the beginning of this reserved memory. For example,258 "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory259 starting at physical address 0x01000000 (16MB) for the dump-capture kernel.260 261 The crashkernel region can be automatically placed by the system262 kernel at run time. This is done by specifying the base address as 0,263 or omitting it all together::264 265 crashkernel=256M@0266 267 or::268 269 crashkernel=256M270 271 If the start address is specified, note that the start address of the272 kernel will be aligned to a value (which is Arch dependent), so if the273 start address is not then any space below the alignment point will be274 wasted.275 2762) range1:size1[,range2:size2,...][@offset]277 278 While the "crashkernel=size[@offset]" syntax is sufficient for most279 configurations, sometimes it's handy to have the reserved memory dependent280 on the value of System RAM -- that's mostly for distributors that pre-setup281 the kernel command line to avoid a unbootable system after some memory has282 been removed from the machine.283 284 The syntax is::285 286 crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]287 range=start-[end]288 289 For example::290 291 crashkernel=512M-2G:64M,2G-:128M292 293 This would mean:294 295 1) if the RAM is smaller than 512M, then don't reserve anything296 (this is the "rescue" case)297 2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M298 3) if the RAM size is larger than 2G, then reserve 128M299 3003) crashkernel=size,high and crashkernel=size,low301 302 If memory above 4G is preferred, crashkernel=size,high can be used to303 fulfill that. With it, physical memory is allowed to be allocated from top,304 so could be above 4G if system has more than 4G RAM installed. Otherwise,305 memory region will be allocated below 4G if available.306 307 When crashkernel=X,high is passed, kernel could allocate physical memory308 region above 4G, low memory under 4G is needed in this case. There are309 three ways to get low memory:310 311 1) Kernel will allocate at least 256M memory below 4G automatically312 if crashkernel=Y,low is not specified.313 2) Let user specify low memory size instead.314 3) Specified value 0 will disable low memory allocation::315 316 crashkernel=0,low317 318Boot into System Kernel319-----------------------3201) Update the boot loader (such as grub, yaboot, or lilo) configuration321 files as necessary.322 3232) Boot the system kernel with the boot parameter "crashkernel=Y@X".324 325 On x86 and x86_64, use "crashkernel=Y[@X]". Most of the time, the326 start address 'X' is not necessary, kernel will search a suitable327 area. Unless an explicit start address is expected.328 329 On ppc64, use "crashkernel=128M@32M".330 331 On s390x, typically use "crashkernel=xxM". The value of xx is dependent332 on the memory consumption of the kdump system. In general this is not333 dependent on the memory size of the production system.334 335 On arm, the use of "crashkernel=Y@X" is no longer necessary; the336 kernel will automatically locate the crash kernel image within the337 first 512MB of RAM if X is not given.338 339 On arm64, use "crashkernel=Y[@X]". Note that the start address of340 the kernel, X if explicitly specified, must be aligned to 2MiB (0x200000).341 342Load the Dump-capture Kernel343============================344 345After booting to the system kernel, dump-capture kernel needs to be346loaded.347 348Based on the architecture and type of image (relocatable or not), one349can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz350of dump-capture kernel. Following is the summary.351 352For i386 and x86_64:353 354 - Use bzImage/vmlinuz if kernel is relocatable.355 - Use vmlinux if kernel is not relocatable.356 357For ppc64:358 359 - Use vmlinux360 361For s390x:362 363 - Use image or bzImage364 365For arm:366 367 - Use zImage368 369For arm64:370 371 - Use vmlinux or Image372 373If you are using an uncompressed vmlinux image then use following command374to load dump-capture kernel::375 376 kexec -p <dump-capture-kernel-vmlinux-image> \377 --initrd=<initrd-for-dump-capture-kernel> --args-linux \378 --append="root=<root-dev> <arch-specific-options>"379 380If you are using a compressed bzImage/vmlinuz, then use following command381to load dump-capture kernel::382 383 kexec -p <dump-capture-kernel-bzImage> \384 --initrd=<initrd-for-dump-capture-kernel> \385 --append="root=<root-dev> <arch-specific-options>"386 387If you are using a compressed zImage, then use following command388to load dump-capture kernel::389 390 kexec --type zImage -p <dump-capture-kernel-bzImage> \391 --initrd=<initrd-for-dump-capture-kernel> \392 --dtb=<dtb-for-dump-capture-kernel> \393 --append="root=<root-dev> <arch-specific-options>"394 395If you are using an uncompressed Image, then use following command396to load dump-capture kernel::397 398 kexec -p <dump-capture-kernel-Image> \399 --initrd=<initrd-for-dump-capture-kernel> \400 --append="root=<root-dev> <arch-specific-options>"401 402Following are the arch specific command line options to be used while403loading dump-capture kernel.404 405For i386 and x86_64:406 407 "1 irqpoll nr_cpus=1 reset_devices"408 409For ppc64:410 411 "1 maxcpus=1 noirqdistrib reset_devices"412 413For s390x:414 415 "1 nr_cpus=1 cgroup_disable=memory"416 417For arm:418 419 "1 maxcpus=1 reset_devices"420 421For arm64:422 423 "1 nr_cpus=1 reset_devices"424 425Notes on loading the dump-capture kernel:426 427* By default, the ELF headers are stored in ELF64 format to support428 systems with more than 4GB memory. On i386, kexec automatically checks if429 the physical RAM size exceeds the 4 GB limit and if not, uses ELF32.430 So, on non-PAE systems, ELF32 is always used.431 432 The --elf32-core-headers option can be used to force the generation of ELF32433 headers. This is necessary because GDB currently cannot open vmcore files434 with ELF64 headers on 32-bit systems.435 436* The "irqpoll" boot parameter reduces driver initialization failures437 due to shared interrupts in the dump-capture kernel.438 439* You must specify <root-dev> in the format corresponding to the root440 device name in the output of mount command.441 442* Boot parameter "1" boots the dump-capture kernel into single-user443 mode without networking. If you want networking, use "3".444 445* We generally don't have to bring up a SMP kernel just to capture the446 dump. Hence generally it is useful either to build a UP dump-capture447 kernel or specify maxcpus=1 option while loading dump-capture kernel.448 Note, though maxcpus always works, you had better replace it with449 nr_cpus to save memory if supported by the current ARCH, such as x86.450 451* You should enable multi-cpu support in dump-capture kernel if you intend452 to use multi-thread programs with it, such as parallel dump feature of453 makedumpfile. Otherwise, the multi-thread program may have a great454 performance degradation. To enable multi-cpu support, you should bring up an455 SMP dump-capture kernel and specify maxcpus/nr_cpus options while loading it.456 457* For s390x there are two kdump modes: If a ELF header is specified with458 the elfcorehdr= kernel parameter, it is used by the kdump kernel as it459 is done on all other architectures. If no elfcorehdr= kernel parameter is460 specified, the s390x kdump kernel dynamically creates the header. The461 second mode has the advantage that for CPU and memory hotplug, kdump has462 not to be reloaded with kexec_load().463 464* For s390x systems with many attached devices the "cio_ignore" kernel465 parameter should be used for the kdump kernel in order to prevent allocation466 of kernel memory for devices that are not relevant for kdump. The same467 applies to systems that use SCSI/FCP devices. In that case the468 "allow_lun_scan" zfcp module parameter should be set to zero before469 setting FCP devices online.470 471Kernel Panic472============473 474After successfully loading the dump-capture kernel as previously475described, the system will reboot into the dump-capture kernel if a476system crash is triggered. Trigger points are located in panic(),477die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).478 479The following conditions will execute a crash trigger point:480 481If a hard lockup is detected and "NMI watchdog" is configured, the system482will boot into the dump-capture kernel ( die_nmi() ).483 484If die() is called, and it happens to be a thread with pid 0 or 1, or die()485is called inside interrupt context or die() is called and panic_on_oops is set,486the system will boot into the dump-capture kernel.487 488On powerpc systems when a soft-reset is generated, die() is called by all cpus489and the system will boot into the dump-capture kernel.490 491For testing purposes, you can trigger a crash by using "ALT-SysRq-c",492"echo c > /proc/sysrq-trigger" or write a module to force the panic.493 494Write Out the Dump File495=======================496 497After the dump-capture kernel is booted, write out the dump file with498the following command::499 500 cp /proc/vmcore <dump-file>501 502or use scp to write out the dump file between hosts on a network, e.g::503 504 scp /proc/vmcore remote_username@remote_ip:<dump-file>505 506You can also use makedumpfile utility to write out the dump file507with specified options to filter out unwanted contents, e.g::508 509 makedumpfile -l --message-level 1 -d 31 /proc/vmcore <dump-file>510 511Analysis512========513 514Before analyzing the dump image, you should reboot into a stable kernel.515 516You can do limited analysis using GDB on the dump file copied out of517/proc/vmcore. Use the debug vmlinux built with -g and run the following518command::519 520 gdb vmlinux <dump-file>521 522Stack trace for the task on processor 0, register display, and memory523display work fine.524 525Note: GDB cannot analyze core files generated in ELF64 format for x86.526On systems with a maximum of 4GB of memory, you can generate527ELF32-format headers using the --elf32-core-headers kernel option on the528dump kernel.529 530You can also use the Crash utility to analyze dump files in Kdump531format. Crash is available at the following URL:532 533 https://github.com/crash-utility/crash534 535Crash document can be found at:536 https://crash-utility.github.io/537 538Trigger Kdump on WARN()539=======================540 541The kernel parameter, panic_on_warn, calls panic() in all WARN() paths. This542will cause a kdump to occur at the panic() call. In cases where a user wants543to specify this during runtime, /proc/sys/kernel/panic_on_warn can be set to 1544to achieve the same behaviour.545 546Trigger Kdump on add_taint()547============================548 549The kernel parameter panic_on_taint facilitates a conditional call to panic()550from within add_taint() whenever the value set in this bitmask matches with the551bit flag being set by add_taint().552This will cause a kdump to occur at the add_taint()->panic() call.553 554Contact555=======556 557- kexec@lists.infradead.org558 559GDB macros560==========561 562.. include:: gdbmacros.txt563 :literal:564