1================================================================ 2Documentation for Kdump - The kexec-based Crash Dumping Solution 3================================================================ 4 5This document includes overview, setup, installation, and analysis 6information. 7 8Overview 9======== 10 11Kdump uses kexec to quickly boot to a dump-capture kernel whenever a 12dump of the system kernel's memory needs to be taken (for example, when 13the system panics). The system kernel's memory image is preserved across 14the reboot and is accessible to the dump-capture kernel. 15 16You can use common commands, such as cp, scp or makedumpfile to copy 17the memory image to a dump file on the local disk, or across the network 18to a remote system. 19 20Kdump and kexec are currently supported on the x86, x86_64, ppc64, ia64, 21s390x, arm and arm64 architectures. 22 23When the system kernel boots, it reserves a small section of memory for 24the dump-capture kernel. This ensures that ongoing Direct Memory Access 25(DMA) from the system kernel does not corrupt the dump-capture kernel. 26The kexec -p command loads the dump-capture kernel into this reserved 27memory. 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 whole 31low 1M is reserved to avoid any later kernel or device driver writing 32data into this area. Like this, the low 1M can be reused as system RAM 33by kdump kernel without extra handling. 34 35On PPC64 machines first 32KB of physical memory is needed for booting 36regardless of where the kernel is loaded and to support 64K page size 37kexec backs up the first 64KB memory. 38 39For s390x, when kdump is triggered, the crashkernel region is exchanged 40with the region [0, crashkernel region size] and then the kdump kernel 41runs in [0, crashkernel region size]. Therefore no relocatable kernel is 42needed for s390x. 43 44All of the necessary information about the system kernel's core image is 45encoded in the ELF format, and stored in a reserved area of memory 46before a crash. The physical address of the start of the ELF header is 47passed to the dump-capture kernel through the elfcorehdr= boot 48parameter. Optionally the size of the ELF header can also be passed 49when using the elfcorehdr=[size[KMG]@]offset[KMG] syntax. 50 51With 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 53write out using file copy commands such as cp or scp. You can also use 54makedumpfile utility to analyze and write out filtered contents with 55options, 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 Crash 57tool to debug the dump file. This method ensures that the dump pages are 58correctly ordered. 59 60Setup and Installation 61====================== 62 63Install kexec-tools 64------------------- 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.gz 71 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.git 77- http://www.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git 78 79There is also a gitweb interface available at 80http://www.kernel.org/git/?p=utils/kernel/kexec/kexec-tools.git 81 82More information about kexec-tools can be found at 83http://horms.net/projects/kexec/ 84 853) Unpack the tarball with the tar command, as follows:: 86 87 tar xvpzf kexec-tools.tar.gz 88 894) Change to the kexec-tools directory, as follows:: 90 91 cd kexec-tools-VERSION 92 935) Configure the package, as follows:: 94 95 ./configure 96 976) Compile the package, as follows:: 98 99 make 100 1017) Install the package, as follows:: 102 103 make install 104 105 106Build the system and dump-capture kernels 107----------------------------------------- 108There are two possible methods of using Kdump. 109 1101) Build a separate custom dump-capture kernel for capturing the 111 kernel core dump. 112 1132) 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, ia64, arm and arm64 architectures support 117 relocatable kernel. 118 119Building a relocatable kernel is advantageous from the point of view that 120one does not have to build a second kernel for capturing the dump. But 121at the same time one might want to build a custom dump capture kernel 122suitable to his needs. 123 124Following are the configuration setting required for system and 125dump-capture kernels for enabling kdump support. 126 127System kernel config options 128---------------------------- 129 1301) Enable "kexec system call" or "kexec file based system call" in 131 "Processor type and features.":: 132 133 CONFIG_KEXEC=y or CONFIG_KEXEC_FILE=y 134 135 And both of them will select KEXEC_CORE:: 136 137 CONFIG_KEXEC_CORE=y 138 139 Subsequently, CRASH_CORE is selected by KEXEC_CORE:: 140 141 CONFIG_CRASH_CORE=y 142 1432) Enable "sysfs file system support" in "Filesystem" -> "Pseudo 144 filesystems." This is usually enabled by default:: 145 146 CONFIG_SYSFS=y 147 148 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:: 152 153 grep 'CONFIG_SYSFS' .config 154 1553) Enable "Compile the kernel with debug info" in "Kernel hacking.":: 156 157 CONFIG_DEBUG_INFO=Y 158 159 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. 162 163Dump-capture kernel config options (Arch Independent) 164----------------------------------------------------- 165 1661) Enable "kernel crash dumps" support under "Processor type and 167 features":: 168 169 CONFIG_CRASH_DUMP=y 170 1712) Enable "/proc/vmcore support" under "Filesystems" -> "Pseudo filesystems":: 172 173 CONFIG_PROC_VMCORE=y 174 175 (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 and 181 features":: 182 183 CONFIG_HIGHMEM64G=y 184 185 or:: 186 187 CONFIG_HIGHMEM4G 188 1892) 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. 192 193 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. 197 198 With CONFIG_SMP=n, the above things are not related. 199 2003) A relocatable kernel is suggested to be built by default. If not yet, 201 enable "Build a relocatable kernel" support under "Processor type and 202 features":: 203 204 CONFIG_RELOCATABLE=y 205 2064) 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. 210 211 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 215 kernel. 216 217 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 222 2235) Make and install the kernel and its modules. DO NOT add this kernel 224 to the boot loader configuration files. 225 226Dump-capture kernel config options (Arch Dependent, ppc64) 227---------------------------------------------------------- 228 2291) Enable "Build a kdump crash kernel" support under "Kernel" options:: 230 231 CONFIG_CRASH_DUMP=y 232 2332) Enable "Build a relocatable kernel" support:: 234 235 CONFIG_RELOCATABLE=y 236 237 Make and install the kernel and its modules. 238 239Dump-capture kernel config options (Arch Dependent, ia64) 240---------------------------------------------------------- 241 242- No specific options are required to create a dump-capture kernel 243 for ia64, other than those specified in the arch independent section 244 above. This means that it is possible to use the system kernel 245 as a dump-capture kernel if desired. 246 247 The crashkernel region can be automatically placed by the system 248 kernel at runtime. This is done by specifying the base address as 0, 249 or omitting it all together:: 250 251 crashkernel=256M@0 252 253 or:: 254 255 crashkernel=256M 256 257Dump-capture kernel config options (Arch Dependent, arm) 258---------------------------------------------------------- 259 260- To use a relocatable kernel, 261 Enable "AUTO_ZRELADDR" support under "Boot" options:: 262 263 AUTO_ZRELADDR=y 264 265Dump-capture kernel config options (Arch Dependent, arm64) 266---------------------------------------------------------- 267 268- Please note that kvm of the dump-capture kernel will not be enabled 269 on non-VHE systems even if it is configured. This is because the CPU 270 will not be reset to EL2 on panic. 271 272crashkernel syntax 273=========================== 2741) crashkernel=size@offset 275 276 Here 'size' specifies how much memory to reserve for the dump-capture kernel 277 and 'offset' specifies the beginning of this reserved memory. For example, 278 "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory 279 starting at physical address 0x01000000 (16MB) for the dump-capture kernel. 280 281 The crashkernel region can be automatically placed by the system 282 kernel at run time. This is done by specifying the base address as 0, 283 or omitting it all together:: 284 285 crashkernel=256M@0 286 287 or:: 288 289 crashkernel=256M 290 291 If the start address is specified, note that the start address of the 292 kernel will be aligned to a value (which is Arch dependent), so if the 293 start address is not then any space below the alignment point will be 294 wasted. 295 2962) range1:size1[,range2:size2,...][@offset] 297 298 While the "crashkernel=size[@offset]" syntax is sufficient for most 299 configurations, sometimes it's handy to have the reserved memory dependent 300 on the value of System RAM -- that's mostly for distributors that pre-setup 301 the kernel command line to avoid a unbootable system after some memory has 302 been removed from the machine. 303 304 The syntax is:: 305 306 crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset] 307 range=start-[end] 308 309 For example:: 310 311 crashkernel=512M-2G:64M,2G-:128M 312 313 This would mean: 314 315 1) if the RAM is smaller than 512M, then don't reserve anything 316 (this is the "rescue" case) 317 2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M 318 3) if the RAM size is larger than 2G, then reserve 128M 319 3203) crashkernel=size,high and crashkernel=size,low 321 322 If memory above 4G is preferred, crashkernel=size,high can be used to 323 fulfill that. With it, physical memory is allowed to be allocated from top, 324 so could be above 4G if system has more than 4G RAM installed. Otherwise, 325 memory region will be allocated below 4G if available. 326 327 When crashkernel=X,high is passed, kernel could allocate physical memory 328 region above 4G, low memory under 4G is needed in this case. There are 329 three ways to get low memory: 330 331 1) Kernel will allocate at least 256M memory below 4G automatically 332 if crashkernel=Y,low is not specified. 333 2) Let user specify low memory size instead. 334 3) Specified value 0 will disable low memory allocation:: 335 336 crashkernel=0,low 337 338Boot into System Kernel 339----------------------- 3401) Update the boot loader (such as grub, yaboot, or lilo) configuration 341 files as necessary. 342 3432) Boot the system kernel with the boot parameter "crashkernel=Y@X". 344 345 On x86 and x86_64, use "crashkernel=Y[@X]". Most of the time, the 346 start address 'X' is not necessary, kernel will search a suitable 347 area. Unless an explicit start address is expected. 348 349 On ppc64, use "crashkernel=128M@32M". 350 351 On ia64, 256M@256M is a generous value that typically works. 352 The region may be automatically placed on ia64, see the 353 dump-capture kernel config option notes above. 354 If use sparse memory, the size should be rounded to GRANULE boundaries. 355 356 On s390x, typically use "crashkernel=xxM". The value of xx is dependent 357 on the memory consumption of the kdump system. In general this is not 358 dependent on the memory size of the production system. 359 360 On arm, the use of "crashkernel=Y@X" is no longer necessary; the 361 kernel will automatically locate the crash kernel image within the 362 first 512MB of RAM if X is not given. 363 364 On arm64, use "crashkernel=Y[@X]". Note that the start address of 365 the kernel, X if explicitly specified, must be aligned to 2MiB (0x200000). 366 367Load the Dump-capture Kernel 368============================ 369 370After booting to the system kernel, dump-capture kernel needs to be 371loaded. 372 373Based on the architecture and type of image (relocatable or not), one 374can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz 375of dump-capture kernel. Following is the summary. 376 377For i386 and x86_64: 378 379 - Use bzImage/vmlinuz if kernel is relocatable. 380 - Use vmlinux if kernel is not relocatable. 381 382For ppc64: 383 384 - Use vmlinux 385 386For ia64: 387 388 - Use vmlinux or vmlinuz.gz 389 390For s390x: 391 392 - Use image or bzImage 393 394For arm: 395 396 - Use zImage 397 398For arm64: 399 400 - Use vmlinux or Image 401 402If you are using an uncompressed vmlinux image then use following command 403to load dump-capture kernel:: 404 405 kexec -p <dump-capture-kernel-vmlinux-image> \ 406 --initrd=<initrd-for-dump-capture-kernel> --args-linux \ 407 --append="root=<root-dev> <arch-specific-options>" 408 409If you are using a compressed bzImage/vmlinuz, then use following command 410to load dump-capture kernel:: 411 412 kexec -p <dump-capture-kernel-bzImage> \ 413 --initrd=<initrd-for-dump-capture-kernel> \ 414 --append="root=<root-dev> <arch-specific-options>" 415 416If you are using a compressed zImage, then use following command 417to load dump-capture kernel:: 418 419 kexec --type zImage -p <dump-capture-kernel-bzImage> \ 420 --initrd=<initrd-for-dump-capture-kernel> \ 421 --dtb=<dtb-for-dump-capture-kernel> \ 422 --append="root=<root-dev> <arch-specific-options>" 423 424If you are using an uncompressed Image, then use following command 425to load dump-capture kernel:: 426 427 kexec -p <dump-capture-kernel-Image> \ 428 --initrd=<initrd-for-dump-capture-kernel> \ 429 --append="root=<root-dev> <arch-specific-options>" 430 431Please note, that --args-linux does not need to be specified for ia64. 432It is planned to make this a no-op on that architecture, but for now 433it should be omitted 434 435Following are the arch specific command line options to be used while 436loading dump-capture kernel. 437 438For i386, x86_64 and ia64: 439 440 "1 irqpoll nr_cpus=1 reset_devices" 441 442For ppc64: 443 444 "1 maxcpus=1 noirqdistrib reset_devices" 445 446For s390x: 447 448 "1 nr_cpus=1 cgroup_disable=memory" 449 450For arm: 451 452 "1 maxcpus=1 reset_devices" 453 454For arm64: 455 456 "1 nr_cpus=1 reset_devices" 457 458Notes on loading the dump-capture kernel: 459 460* By default, the ELF headers are stored in ELF64 format to support 461 systems with more than 4GB memory. On i386, kexec automatically checks if 462 the physical RAM size exceeds the 4 GB limit and if not, uses ELF32. 463 So, on non-PAE systems, ELF32 is always used. 464 465 The --elf32-core-headers option can be used to force the generation of ELF32 466 headers. This is necessary because GDB currently cannot open vmcore files 467 with ELF64 headers on 32-bit systems. 468 469* The "irqpoll" boot parameter reduces driver initialization failures 470 due to shared interrupts in the dump-capture kernel. 471 472* You must specify <root-dev> in the format corresponding to the root 473 device name in the output of mount command. 474 475* Boot parameter "1" boots the dump-capture kernel into single-user 476 mode without networking. If you want networking, use "3". 477 478* We generally don't have to bring up a SMP kernel just to capture the 479 dump. Hence generally it is useful either to build a UP dump-capture 480 kernel or specify maxcpus=1 option while loading dump-capture kernel. 481 Note, though maxcpus always works, you had better replace it with 482 nr_cpus to save memory if supported by the current ARCH, such as x86. 483 484* You should enable multi-cpu support in dump-capture kernel if you intend 485 to use multi-thread programs with it, such as parallel dump feature of 486 makedumpfile. Otherwise, the multi-thread program may have a great 487 performance degradation. To enable multi-cpu support, you should bring up an 488 SMP dump-capture kernel and specify maxcpus/nr_cpus, disable_cpu_apicid=[X] 489 options while loading it. 490 491* For s390x there are two kdump modes: If a ELF header is specified with 492 the elfcorehdr= kernel parameter, it is used by the kdump kernel as it 493 is done on all other architectures. If no elfcorehdr= kernel parameter is 494 specified, the s390x kdump kernel dynamically creates the header. The 495 second mode has the advantage that for CPU and memory hotplug, kdump has 496 not to be reloaded with kexec_load(). 497 498* For s390x systems with many attached devices the "cio_ignore" kernel 499 parameter should be used for the kdump kernel in order to prevent allocation 500 of kernel memory for devices that are not relevant for kdump. The same 501 applies to systems that use SCSI/FCP devices. In that case the 502 "allow_lun_scan" zfcp module parameter should be set to zero before 503 setting FCP devices online. 504 505Kernel Panic 506============ 507 508After successfully loading the dump-capture kernel as previously 509described, the system will reboot into the dump-capture kernel if a 510system crash is triggered. Trigger points are located in panic(), 511die(), die_nmi() and in the sysrq handler (ALT-SysRq-c). 512 513The following conditions will execute a crash trigger point: 514 515If a hard lockup is detected and "NMI watchdog" is configured, the system 516will boot into the dump-capture kernel ( die_nmi() ). 517 518If die() is called, and it happens to be a thread with pid 0 or 1, or die() 519is called inside interrupt context or die() is called and panic_on_oops is set, 520the system will boot into the dump-capture kernel. 521 522On powerpc systems when a soft-reset is generated, die() is called by all cpus 523and the system will boot into the dump-capture kernel. 524 525For testing purposes, you can trigger a crash by using "ALT-SysRq-c", 526"echo c > /proc/sysrq-trigger" or write a module to force the panic. 527 528Write Out the Dump File 529======================= 530 531After the dump-capture kernel is booted, write out the dump file with 532the following command:: 533 534 cp /proc/vmcore <dump-file> 535 536or use scp to write out the dump file between hosts on a network, e.g:: 537 538 scp /proc/vmcore remote_username@remote_ip:<dump-file> 539 540You can also use makedumpfile utility to write out the dump file 541with specified options to filter out unwanted contents, e.g:: 542 543 makedumpfile -l --message-level 1 -d 31 /proc/vmcore <dump-file> 544 545Analysis 546======== 547 548Before analyzing the dump image, you should reboot into a stable kernel. 549 550You can do limited analysis using GDB on the dump file copied out of 551/proc/vmcore. Use the debug vmlinux built with -g and run the following 552command:: 553 554 gdb vmlinux <dump-file> 555 556Stack trace for the task on processor 0, register display, and memory 557display work fine. 558 559Note: GDB cannot analyze core files generated in ELF64 format for x86. 560On systems with a maximum of 4GB of memory, you can generate 561ELF32-format headers using the --elf32-core-headers kernel option on the 562dump kernel. 563 564You can also use the Crash utility to analyze dump files in Kdump 565format. Crash is available at the following URL: 566 567 https://github.com/crash-utility/crash 568 569Crash document can be found at: 570 https://crash-utility.github.io/ 571 572Trigger Kdump on WARN() 573======================= 574 575The kernel parameter, panic_on_warn, calls panic() in all WARN() paths. This 576will cause a kdump to occur at the panic() call. In cases where a user wants 577to specify this during runtime, /proc/sys/kernel/panic_on_warn can be set to 1 578to achieve the same behaviour. 579 580Trigger Kdump on add_taint() 581============================ 582 583The kernel parameter panic_on_taint facilitates a conditional call to panic() 584from within add_taint() whenever the value set in this bitmask matches with the 585bit flag being set by add_taint(). 586This will cause a kdump to occur at the add_taint()->panic() call. 587 588Contact 589======= 590 591- kexec@lists.infradead.org 592 593GDB macros 594========== 595 596.. include:: gdbmacros.txt 597 :literal: 598