linux/Documentation/kdump/kdump.txt
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   1================================================================
   2Documentation for Kdump - The kexec-based Crash Dumping Solution
   3================================================================
   4
   5This document includes overview, setup and 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 and scp, to copy the
  17memory image to a dump file on the local disk, or across the network to
  18a remote system.
  19
  20Kdump and kexec are currently supported on the x86, x86_64, ppc64, ia64,
  21and s390x 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 to boot,
  30regardless of where the kernel loads. Therefore, kexec backs up this
  31region just before rebooting into the dump-capture kernel.
  32
  33Similarly on PPC64 machines first 32KB of physical memory is needed for
  34booting regardless of where the kernel is loaded and to support 64K page
  35size kexec backs up the first 64KB memory.
  36
  37For s390x, when kdump is triggered, the crashkernel region is exchanged
  38with the region [0, crashkernel region size] and then the kdump kernel
  39runs in [0, crashkernel region size]. Therefore no relocatable kernel is
  40needed for s390x.
  41
  42All of the necessary information about the system kernel's core image is
  43encoded in the ELF format, and stored in a reserved area of memory
  44before a crash. The physical address of the start of the ELF header is
  45passed to the dump-capture kernel through the elfcorehdr= boot
  46parameter. Optionally the size of the ELF header can also be passed
  47when using the elfcorehdr=[size[KMG]@]offset[KMG] syntax.
  48
  49
  50With 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
  52write out using file copy commands such as cp or scp. Further, you can
  53use analysis tools such as the GNU Debugger (GDB) and the Crash tool to
  54debug the dump file. This method ensures that the dump pages are correctly
  55ordered.
  56
  57
  58Setup and Installation
  59======================
  60
  61Install kexec-tools
  62-------------------
  63
  641) Login as the root user.
  65
  662) Download the kexec-tools user-space package from the following URL:
  67
  68http://kernel.org/pub/linux/utils/kernel/kexec/kexec-tools.tar.gz
  69
  70This is a symlink to the latest version.
  71
  72The latest kexec-tools git tree is available at:
  73
  74git://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
  75and
  76http://www.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
  77
  78There is also a gitweb interface available at
  79http://www.kernel.org/git/?p=utils/kernel/kexec/kexec-tools.git
  80
  81More information about kexec-tools can be found at
  82http://horms.net/projects/kexec/
  83
  843) Unpack the tarball with the tar command, as follows:
  85
  86   tar xvpzf kexec-tools.tar.gz
  87
  884) Change to the kexec-tools directory, as follows:
  89
  90   cd kexec-tools-VERSION
  91
  925) Configure the package, as follows:
  93
  94   ./configure
  95
  966) Compile the package, as follows:
  97
  98   make
  99
 1007) Install the package, as follows:
 101
 102   make install
 103
 104
 105Build the system and dump-capture kernels
 106-----------------------------------------
 107There are two possible methods of using Kdump.
 108
 1091) Build a separate custom dump-capture kernel for capturing the
 110   kernel core dump.
 111
 1122) 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 and ia64 architectures support relocatable
 116   kernel.
 117
 118Building a relocatable kernel is advantageous from the point of view that
 119one does not have to build a second kernel for capturing the dump. But
 120at the same time one might want to build a custom dump capture kernel
 121suitable to his needs.
 122
 123Following are the configuration setting required for system and
 124dump-capture kernels for enabling kdump support.
 125
 126System kernel config options
 127----------------------------
 128
 1291) Enable "kexec system call" in "Processor type and features."
 130
 131   CONFIG_KEXEC=y
 132
 1332) 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
 1453) 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
 153Dump-capture kernel config options (Arch Independent)
 154-----------------------------------------------------
 155
 1561) Enable "kernel crash dumps" support under "Processor type and
 157   features":
 158
 159   CONFIG_CRASH_DUMP=y
 160
 1612) 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
 166Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
 167--------------------------------------------------------------------
 168
 1691) On i386, enable high memory support under "Processor type and
 170   features":
 171
 172   CONFIG_HIGHMEM64G=y
 173   or
 174   CONFIG_HIGHMEM4G
 175
 1762) 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
 1853) 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
 1914) 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
 2085) Make and install the kernel and its modules. DO NOT add this kernel
 209   to the boot loader configuration files.
 210
 211Dump-capture kernel config options (Arch Dependent, ppc64)
 212----------------------------------------------------------
 213
 2141) Enable "Build a kdump crash kernel" support under "Kernel" options:
 215
 216   CONFIG_CRASH_DUMP=y
 217
 2182)   Enable "Build a relocatable kernel" support
 219
 220   CONFIG_RELOCATABLE=y
 221
 222   Make and install the kernel and its modules.
 223
 224Dump-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
 245Extended crashkernel syntax
 246===========================
 247
 248While the "crashkernel=size[@offset]" syntax is sufficient for most
 249configurations, sometimes it's handy to have the reserved memory dependent
 250on the value of System RAM -- that's mostly for distributors that pre-setup
 251the kernel command line to avoid a unbootable system after some memory has
 252been removed from the machine.
 253
 254The syntax is:
 255
 256    crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
 257    range=start-[end]
 258
 259    'start' is inclusive and 'end' is exclusive.
 260
 261For example:
 262
 263    crashkernel=512M-2G:64M,2G-:128M
 264
 265This would mean:
 266
 267    1) if the RAM is smaller than 512M, then don't reserve anything
 268       (this is the "rescue" case)
 269    2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
 270    3) if the RAM size is larger than 2G, then reserve 128M
 271
 272
 273
 274Boot into System Kernel
 275=======================
 276
 2771) Update the boot loader (such as grub, yaboot, or lilo) configuration
 278   files as necessary.
 279
 2802) Boot the system kernel with the boot parameter "crashkernel=Y@X",
 281   where Y specifies how much memory to reserve for the dump-capture kernel
 282   and X specifies the beginning of this reserved memory. For example,
 283   "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
 284   starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
 285
 286   On x86 and x86_64, use "crashkernel=64M@16M".
 287
 288   On ppc64, use "crashkernel=128M@32M".
 289
 290   On ia64, 256M@256M is a generous value that typically works.
 291   The region may be automatically placed on ia64, see the
 292   dump-capture kernel config option notes above.
 293   If use sparse memory, the size should be rounded to GRANULE boundaries.
 294
 295   On s390x, typically use "crashkernel=xxM". The value of xx is dependent
 296   on the memory consumption of the kdump system. In general this is not
 297   dependent on the memory size of the production system.
 298
 299Load the Dump-capture Kernel
 300============================
 301
 302After booting to the system kernel, dump-capture kernel needs to be
 303loaded.
 304
 305Based on the architecture and type of image (relocatable or not), one
 306can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz
 307of dump-capture kernel. Following is the summary.
 308
 309For i386 and x86_64:
 310        - Use vmlinux if kernel is not relocatable.
 311        - Use bzImage/vmlinuz if kernel is relocatable.
 312For ppc64:
 313        - Use vmlinux
 314For ia64:
 315        - Use vmlinux or vmlinuz.gz
 316For s390x:
 317        - Use image or bzImage
 318
 319
 320If you are using a uncompressed vmlinux image then use following command
 321to load dump-capture kernel.
 322
 323   kexec -p <dump-capture-kernel-vmlinux-image> \
 324   --initrd=<initrd-for-dump-capture-kernel> --args-linux \
 325   --append="root=<root-dev> <arch-specific-options>"
 326
 327If you are using a compressed bzImage/vmlinuz, then use following command
 328to load dump-capture kernel.
 329
 330   kexec -p <dump-capture-kernel-bzImage> \
 331   --initrd=<initrd-for-dump-capture-kernel> \
 332   --append="root=<root-dev> <arch-specific-options>"
 333
 334Please note, that --args-linux does not need to be specified for ia64.
 335It is planned to make this a no-op on that architecture, but for now
 336it should be omitted
 337
 338Following are the arch specific command line options to be used while
 339loading dump-capture kernel.
 340
 341For i386, x86_64 and ia64:
 342        "1 irqpoll maxcpus=1 reset_devices"
 343
 344For ppc64:
 345        "1 maxcpus=1 noirqdistrib reset_devices"
 346
 347For s390x:
 348        "1 maxcpus=1 cgroup_disable=memory"
 349
 350Notes on loading the dump-capture kernel:
 351
 352* By default, the ELF headers are stored in ELF64 format to support
 353  systems with more than 4GB memory. On i386, kexec automatically checks if
 354  the physical RAM size exceeds the 4 GB limit and if not, uses ELF32.
 355  So, on non-PAE systems, ELF32 is always used.
 356
 357  The --elf32-core-headers option can be used to force the generation of ELF32
 358  headers. This is necessary because GDB currently cannot open vmcore files
 359  with ELF64 headers on 32-bit systems.
 360
 361* The "irqpoll" boot parameter reduces driver initialization failures
 362  due to shared interrupts in the dump-capture kernel.
 363
 364* You must specify <root-dev> in the format corresponding to the root
 365  device name in the output of mount command.
 366
 367* Boot parameter "1" boots the dump-capture kernel into single-user
 368  mode without networking. If you want networking, use "3".
 369
 370* We generally don' have to bring up a SMP kernel just to capture the
 371  dump. Hence generally it is useful either to build a UP dump-capture
 372  kernel or specify maxcpus=1 option while loading dump-capture kernel.
 373
 374* For s390x there are two kdump modes: If a ELF header is specified with
 375  the elfcorehdr= kernel parameter, it is used by the kdump kernel as it
 376  is done on all other architectures. If no elfcorehdr= kernel parameter is
 377  specified, the s390x kdump kernel dynamically creates the header. The
 378  second mode has the advantage that for CPU and memory hotplug, kdump has
 379  not to be reloaded with kexec_load().
 380
 381* For s390x systems with many attached devices the "cio_ignore" kernel
 382  parameter should be used for the kdump kernel in order to prevent allocation
 383  of kernel memory for devices that are not relevant for kdump. The same
 384  applies to systems that use SCSI/FCP devices. In that case the
 385  "allow_lun_scan" zfcp module parameter should be set to zero before
 386  setting FCP devices online.
 387
 388Kernel Panic
 389============
 390
 391After successfully loading the dump-capture kernel as previously
 392described, the system will reboot into the dump-capture kernel if a
 393system crash is triggered.  Trigger points are located in panic(),
 394die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).
 395
 396The following conditions will execute a crash trigger point:
 397
 398If a hard lockup is detected and "NMI watchdog" is configured, the system
 399will boot into the dump-capture kernel ( die_nmi() ).
 400
 401If die() is called, and it happens to be a thread with pid 0 or 1, or die()
 402is called inside interrupt context or die() is called and panic_on_oops is set,
 403the system will boot into the dump-capture kernel.
 404
 405On powerpc systems when a soft-reset is generated, die() is called by all cpus
 406and the system will boot into the dump-capture kernel.
 407
 408For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
 409"echo c > /proc/sysrq-trigger" or write a module to force the panic.
 410
 411Write Out the Dump File
 412=======================
 413
 414After the dump-capture kernel is booted, write out the dump file with
 415the following command:
 416
 417   cp /proc/vmcore <dump-file>
 418
 419
 420Analysis
 421========
 422
 423Before analyzing the dump image, you should reboot into a stable kernel.
 424
 425You can do limited analysis using GDB on the dump file copied out of
 426/proc/vmcore. Use the debug vmlinux built with -g and run the following
 427command:
 428
 429   gdb vmlinux <dump-file>
 430
 431Stack trace for the task on processor 0, register display, and memory
 432display work fine.
 433
 434Note: GDB cannot analyze core files generated in ELF64 format for x86.
 435On systems with a maximum of 4GB of memory, you can generate
 436ELF32-format headers using the --elf32-core-headers kernel option on the
 437dump kernel.
 438
 439You can also use the Crash utility to analyze dump files in Kdump
 440format. Crash is available on Dave Anderson's site at the following URL:
 441
 442   http://people.redhat.com/~anderson/
 443
 444
 445Contact
 446=======
 447
 448Vivek Goyal (vgoyal@redhat.com)
 449Maneesh Soni (maneesh@in.ibm.com)
 450
 451
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