Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 ================================================================ 2 Documentation for Kdump - The kexec-based Crash Dumping Solution 3 ================================================================ 4 5 This document includes overview, setup and installation, and analysis 6 information. 7 8 Overview 9 ======== 10 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. 15 16 You can use common commands, such as cp and scp, to copy the 17 memory image to a dump file on the local disk, or across the network to 18 a remote system. 19 20 Kdump and kexec are currently supported on the x86, x86_64, ppc64, ia64, 21 s390x, arm and arm64 architectures. 22 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 27 memory. 28 29 On x86 machines, the first 640 KB of physical memory is needed to boot, 30 regardless of where the kernel loads. Therefore, kexec backs up this 31 region just before rebooting into the dump-capture kernel. 32 33 Similarly on PPC64 machines first 32KB of physical memory is needed for 34 booting regardless of where the kernel is loaded and to support 64K page 35 size kexec backs up the first 64KB memory. 36 37 For s390x, when kdump is triggered, the crashkernel region is exchanged 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 40 needed for s390x. 41 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 44 before a crash. The physical address of the start of the ELF header is 45 passed to the dump-capture kernel through the elfcorehdr= boot 46 parameter. Optionally the size of the ELF header can also be passed 47 when using the elfcorehdr=[size[KMG]@]offset[KMG] syntax. 48 49 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 52 write out using file copy commands such as cp or scp. Further, you can 53 use analysis tools such as the GNU Debugger (GDB) and the Crash tool to 54 debug the dump file. This method ensures that the dump pages are correctly 55 ordered. 56 57 58 Setup and Installation 59 ====================== 60 61 Install kexec-tools 62 ------------------- 63 64 1) Login as the root user. 65 66 2) Download the kexec-tools user-space package from the following URL: 67 68 http://kernel.org/pub/linux/utils/kernel/kexec/kexec-tools.tar.gz 69 70 This is a symlink to the latest version. 71 72 The latest kexec-tools git tree is available at: 73 74 git://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git 75 and 76 http://www.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git 77 78 There is also a gitweb interface available at 79 http://www.kernel.org/git/?p=utils/kernel/kexec/kexec-tools.git 80 81 More information about kexec-tools can be found at 82 http://horms.net/projects/kexec/ 83 84 3) Unpack the tarball with the tar command, as follows: 85 86 tar xvpzf kexec-tools.tar.gz 87 88 4) Change to the kexec-tools directory, as follows: 89 90 cd kexec-tools-VERSION 91 92 5) Configure the package, as follows: 93 94 ./configure 95 96 6) Compile the package, as follows: 97 98 make 99 100 7) Install the package, as follows: 101 102 make install 103 104 105 Build the system and dump-capture kernels 106 ----------------------------------------- 107 There are two possible methods of using Kdump. 108 109 1) Build a separate custom dump-capture kernel for capturing the 110 kernel core dump. 111 112 2) Or use the system kernel binary itself as dump-capture kernel and there is 113 no need to build a separate dump-capture kernel. This is possible 114 only with the architectures which support a relocatable kernel. As 115 of today, i386, x86_64, ppc64, ia64, arm and arm64 architectures support 116 relocatable kernel. 117 118 Building a relocatable kernel is advantageous from the point of view that 119 one does not have to build a second kernel for capturing the dump. But 120 at the same time one might want to build a custom dump capture kernel 121 suitable to his needs. 122 123 Following are the configuration setting required for system and 124 dump-capture kernels for enabling kdump support. 125 126 System kernel config options 127 ---------------------------- 128 129 1) Enable "kexec system call" in "Processor type and features." 130 131 CONFIG_KEXEC=y 132 133 2) Enable "sysfs file system support" in "Filesystem" -> "Pseudo 134 filesystems." This is usually enabled by default. 135 136 CONFIG_SYSFS=y 137 138 Note that "sysfs file system support" might not appear in the "Pseudo 139 filesystems" menu if "Configure standard kernel features (for small 140 systems)" is not enabled in "General Setup." In this case, check the 141 .config file itself to ensure that sysfs is turned on, as follows: 142 143 grep 'CONFIG_SYSFS' .config 144 145 3) Enable "Compile the kernel with debug info" in "Kernel hacking." 146 147 CONFIG_DEBUG_INFO=Y 148 149 This causes the kernel to be built with debug symbols. The dump 150 analysis tools require a vmlinux with debug symbols in order to read 151 and analyze a dump file. 152 153 Dump-capture kernel config options (Arch Independent) 154 ----------------------------------------------------- 155 156 1) Enable "kernel crash dumps" support under "Processor type and 157 features": 158 159 CONFIG_CRASH_DUMP=y 160 161 2) Enable "/proc/vmcore support" under "Filesystems" -> "Pseudo filesystems". 162 163 CONFIG_PROC_VMCORE=y 164 (CONFIG_PROC_VMCORE is set by default when CONFIG_CRASH_DUMP is selected.) 165 166 Dump-capture kernel config options (Arch Dependent, i386 and x86_64) 167 -------------------------------------------------------------------- 168 169 1) On i386, enable high memory support under "Processor type and 170 features": 171 172 CONFIG_HIGHMEM64G=y 173 or 174 CONFIG_HIGHMEM4G 175 176 2) On i386 and x86_64, disable symmetric multi-processing support 177 under "Processor type and features": 178 179 CONFIG_SMP=n 180 181 (If CONFIG_SMP=y, then specify maxcpus=1 on the kernel command line 182 when loading the dump-capture kernel, see section "Load the Dump-capture 183 Kernel".) 184 185 3) If one wants to build and use a relocatable kernel, 186 Enable "Build a relocatable kernel" support under "Processor type and 187 features" 188 189 CONFIG_RELOCATABLE=y 190 191 4) Use a suitable value for "Physical address where the kernel is 192 loaded" (under "Processor type and features"). This only appears when 193 "kernel crash dumps" is enabled. A suitable value depends upon 194 whether kernel is relocatable or not. 195 196 If you are using a relocatable kernel use CONFIG_PHYSICAL_START=0x100000 197 This will compile the kernel for physical address 1MB, but given the fact 198 kernel is relocatable, it can be run from any physical address hence 199 kexec boot loader will load it in memory region reserved for dump-capture 200 kernel. 201 202 Otherwise it should be the start of memory region reserved for 203 second kernel using boot parameter "crashkernel=Y@X". Here X is 204 start of memory region reserved for dump-capture kernel. 205 Generally X is 16MB (0x1000000). So you can set 206 CONFIG_PHYSICAL_START=0x1000000 207 208 5) Make and install the kernel and its modules. DO NOT add this kernel 209 to the boot loader configuration files. 210 211 Dump-capture kernel config options (Arch Dependent, ppc64) 212 ---------------------------------------------------------- 213 214 1) Enable "Build a kdump crash kernel" support under "Kernel" options: 215 216 CONFIG_CRASH_DUMP=y 217 218 2) Enable "Build a relocatable kernel" support 219 220 CONFIG_RELOCATABLE=y 221 222 Make and install the kernel and its modules. 223 224 Dump-capture kernel config options (Arch Dependent, ia64) 225 ---------------------------------------------------------- 226 227 - No specific options are required to create a dump-capture kernel 228 for ia64, other than those specified in the arch independent section 229 above. This means that it is possible to use the system kernel 230 as a dump-capture kernel if desired. 231 232 The crashkernel region can be automatically placed by the system 233 kernel at run time. This is done by specifying the base address as 0, 234 or omitting it all together. 235 236 crashkernel=256M@0 237 or 238 crashkernel=256M 239 240 If the start address is specified, note that the start address of the 241 kernel will be aligned to 64Mb, so if the start address is not then 242 any space below the alignment point will be wasted. 243 244 Dump-capture kernel config options (Arch Dependent, arm) 245 ---------------------------------------------------------- 246 247 - To use a relocatable kernel, 248 Enable "AUTO_ZRELADDR" support under "Boot" options: 249 250 AUTO_ZRELADDR=y 251 252 Dump-capture kernel config options (Arch Dependent, arm64) 253 ---------------------------------------------------------- 254 255 - Please note that kvm of the dump-capture kernel will not be enabled 256 on non-VHE systems even if it is configured. This is because the CPU 257 will not be reset to EL2 on panic. 258 259 Extended crashkernel syntax 260 =========================== 261 262 While the "crashkernel=size[@offset]" syntax is sufficient for most 263 configurations, sometimes it's handy to have the reserved memory dependent 264 on the value of System RAM -- that's mostly for distributors that pre-setup 265 the kernel command line to avoid a unbootable system after some memory has 266 been removed from the machine. 267 268 The syntax is: 269 270 crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset] 271 range=start-[end] 272 273 For example: 274 275 crashkernel=512M-2G:64M,2G-:128M 276 277 This would mean: 278 279 1) if the RAM is smaller than 512M, then don't reserve anything 280 (this is the "rescue" case) 281 2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M 282 3) if the RAM size is larger than 2G, then reserve 128M 283 284 285 286 Boot into System Kernel 287 ======================= 288 289 1) Update the boot loader (such as grub, yaboot, or lilo) configuration 290 files as necessary. 291 292 2) Boot the system kernel with the boot parameter "crashkernel=Y@X", 293 where Y specifies how much memory to reserve for the dump-capture kernel 294 and X specifies the beginning of this reserved memory. For example, 295 "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory 296 starting at physical address 0x01000000 (16MB) for the dump-capture kernel. 297 298 On x86 and x86_64, use "crashkernel=64M@16M". 299 300 On ppc64, use "crashkernel=128M@32M". 301 302 On ia64, 256M@256M is a generous value that typically works. 303 The region may be automatically placed on ia64, see the 304 dump-capture kernel config option notes above. 305 If use sparse memory, the size should be rounded to GRANULE boundaries. 306 307 On s390x, typically use "crashkernel=xxM". The value of xx is dependent 308 on the memory consumption of the kdump system. In general this is not 309 dependent on the memory size of the production system. 310 311 On arm, the use of "crashkernel=Y@X" is no longer necessary; the 312 kernel will automatically locate the crash kernel image within the 313 first 512MB of RAM if X is not given. 314 315 On arm64, use "crashkernel=Y[@X]". Note that the start address of 316 the kernel, X if explicitly specified, must be aligned to 2MiB (0x200000). 317 318 Load the Dump-capture Kernel 319 ============================ 320 321 After booting to the system kernel, dump-capture kernel needs to be 322 loaded. 323 324 Based on the architecture and type of image (relocatable or not), one 325 can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz 326 of dump-capture kernel. Following is the summary. 327 328 For i386 and x86_64: 329 - Use vmlinux if kernel is not relocatable. 330 - Use bzImage/vmlinuz if kernel is relocatable. 331 For ppc64: 332 - Use vmlinux 333 For ia64: 334 - Use vmlinux or vmlinuz.gz 335 For s390x: 336 - Use image or bzImage 337 For arm: 338 - Use zImage 339 For arm64: 340 - Use vmlinux or Image 341 342 If you are using an uncompressed vmlinux image then use following command 343 to load dump-capture kernel. 344 345 kexec -p <dump-capture-kernel-vmlinux-image> \ 346 --initrd=<initrd-for-dump-capture-kernel> --args-linux \ 347 --append="root=<root-dev> <arch-specific-options>" 348 349 If you are using a compressed bzImage/vmlinuz, then use following command 350 to load dump-capture kernel. 351 352 kexec -p <dump-capture-kernel-bzImage> \ 353 --initrd=<initrd-for-dump-capture-kernel> \ 354 --append="root=<root-dev> <arch-specific-options>" 355 356 If you are using a compressed zImage, then use following command 357 to load dump-capture kernel. 358 359 kexec --type zImage -p <dump-capture-kernel-bzImage> \ 360 --initrd=<initrd-for-dump-capture-kernel> \ 361 --dtb=<dtb-for-dump-capture-kernel> \ 362 --append="root=<root-dev> <arch-specific-options>" 363 364 If you are using an uncompressed Image, then use following command 365 to load dump-capture kernel. 366 367 kexec -p <dump-capture-kernel-Image> \ 368 --initrd=<initrd-for-dump-capture-kernel> \ 369 --append="root=<root-dev> <arch-specific-options>" 370 371 Please note, that --args-linux does not need to be specified for ia64. 372 It is planned to make this a no-op on that architecture, but for now 373 it should be omitted 374 375 Following are the arch specific command line options to be used while 376 loading dump-capture kernel. 377 378 For i386, x86_64 and ia64: 379 "1 irqpoll maxcpus=1 reset_devices" 380 381 For ppc64: 382 "1 maxcpus=1 noirqdistrib reset_devices" 383 384 For s390x: 385 "1 maxcpus=1 cgroup_disable=memory" 386 387 For arm: 388 "1 maxcpus=1 reset_devices" 389 390 For arm64: 391 "1 maxcpus=1 reset_devices" 392 393 Notes on loading the dump-capture kernel: 394 395 * By default, the ELF headers are stored in ELF64 format to support 396 systems with more than 4GB memory. On i386, kexec automatically checks if 397 the physical RAM size exceeds the 4 GB limit and if not, uses ELF32. 398 So, on non-PAE systems, ELF32 is always used. 399 400 The --elf32-core-headers option can be used to force the generation of ELF32 401 headers. This is necessary because GDB currently cannot open vmcore files 402 with ELF64 headers on 32-bit systems. 403 404 * The "irqpoll" boot parameter reduces driver initialization failures 405 due to shared interrupts in the dump-capture kernel. 406 407 * You must specify <root-dev> in the format corresponding to the root 408 device name in the output of mount command. 409 410 * Boot parameter "1" boots the dump-capture kernel into single-user 411 mode without networking. If you want networking, use "3". 412 413 * We generally don' have to bring up a SMP kernel just to capture the 414 dump. Hence generally it is useful either to build a UP dump-capture 415 kernel or specify maxcpus=1 option while loading dump-capture kernel. 416 Note, though maxcpus always works, you had better replace it with 417 nr_cpus to save memory if supported by the current ARCH, such as x86. 418 419 * You should enable multi-cpu support in dump-capture kernel if you intend 420 to use multi-thread programs with it, such as parallel dump feature of 421 makedumpfile. Otherwise, the multi-thread program may have a great 422 performance degradation. To enable multi-cpu support, you should bring up an 423 SMP dump-capture kernel and specify maxcpus/nr_cpus, disable_cpu_apicid=[X] 424 options while loading it. 425 426 * For s390x there are two kdump modes: If a ELF header is specified with 427 the elfcorehdr= kernel parameter, it is used by the kdump kernel as it 428 is done on all other architectures. If no elfcorehdr= kernel parameter is 429 specified, the s390x kdump kernel dynamically creates the header. The 430 second mode has the advantage that for CPU and memory hotplug, kdump has 431 not to be reloaded with kexec_load(). 432 433 * For s390x systems with many attached devices the "cio_ignore" kernel 434 parameter should be used for the kdump kernel in order to prevent allocation 435 of kernel memory for devices that are not relevant for kdump. The same 436 applies to systems that use SCSI/FCP devices. In that case the 437 "allow_lun_scan" zfcp module parameter should be set to zero before 438 setting FCP devices online. 439 440 Kernel Panic 441 ============ 442 443 After successfully loading the dump-capture kernel as previously 444 described, the system will reboot into the dump-capture kernel if a 445 system crash is triggered. Trigger points are located in panic(), 446 die(), die_nmi() and in the sysrq handler (ALT-SysRq-c). 447 448 The following conditions will execute a crash trigger point: 449 450 If a hard lockup is detected and "NMI watchdog" is configured, the system 451 will boot into the dump-capture kernel ( die_nmi() ). 452 453 If die() is called, and it happens to be a thread with pid 0 or 1, or die() 454 is called inside interrupt context or die() is called and panic_on_oops is set, 455 the system will boot into the dump-capture kernel. 456 457 On powerpc systems when a soft-reset is generated, die() is called by all cpus 458 and the system will boot into the dump-capture kernel. 459 460 For testing purposes, you can trigger a crash by using "ALT-SysRq-c", 461 "echo c > /proc/sysrq-trigger" or write a module to force the panic. 462 463 Write Out the Dump File 464 ======================= 465 466 After the dump-capture kernel is booted, write out the dump file with 467 the following command: 468 469 cp /proc/vmcore <dump-file> 470 471 472 Analysis 473 ======== 474 475 Before analyzing the dump image, you should reboot into a stable kernel. 476 477 You can do limited analysis using GDB on the dump file copied out of 478 /proc/vmcore. Use the debug vmlinux built with -g and run the following 479 command: 480 481 gdb vmlinux <dump-file> 482 483 Stack trace for the task on processor 0, register display, and memory 484 display work fine. 485 486 Note: GDB cannot analyze core files generated in ELF64 format for x86. 487 On systems with a maximum of 4GB of memory, you can generate 488 ELF32-format headers using the --elf32-core-headers kernel option on the 489 dump kernel. 490 491 You can also use the Crash utility to analyze dump files in Kdump 492 format. Crash is available on Dave Anderson's site at the following URL: 493 494 http://people.redhat.com/~anderson/ 495 496 Trigger Kdump on WARN() 497 ======================= 498 499 The kernel parameter, panic_on_warn, calls panic() in all WARN() paths. This 500 will cause a kdump to occur at the panic() call. In cases where a user wants 501 to specify this during runtime, /proc/sys/kernel/panic_on_warn can be set to 1 502 to achieve the same behaviour. 503 504 Contact 505 ======= 506 507 Vivek Goyal (vgoyal@redhat.com) 508 Maneesh Soni (maneesh@in.ibm.com)