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,
21s390x and arm 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, ia64 and arm 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
244Dump-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
252Extended crashkernel syntax
253===========================
254
255While the "crashkernel=size[@offset]" syntax is sufficient for most
256configurations, sometimes it's handy to have the reserved memory dependent
257on the value of System RAM -- that's mostly for distributors that pre-setup
258the kernel command line to avoid a unbootable system after some memory has
259been removed from the machine.
260
261The syntax is:
262
263    crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
264    range=start-[end]
265
266Please note, on arm, the offset is required.
267    crashkernel=<range1>:<size1>[,<range2>:<size2>,...]@offset
268    range=start-[end]
269
270    'start' is inclusive and 'end' is exclusive.
271
272For example:
273
274    crashkernel=512M-2G:64M,2G-:128M
275
276This would mean:
277
278    1) if the RAM is smaller than 512M, then don't reserve anything
279       (this is the "rescue" case)
280    2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
281    3) if the RAM size is larger than 2G, then reserve 128M
282
283
284
285Boot into System Kernel
286=======================
287
2881) Update the boot loader (such as grub, yaboot, or lilo) configuration
289   files as necessary.
290
2912) Boot the system kernel with the boot parameter "crashkernel=Y@X",
292   where Y specifies how much memory to reserve for the dump-capture kernel
293   and X specifies the beginning of this reserved memory. For example,
294   "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
295   starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
296
297   On x86 and x86_64, use "crashkernel=64M@16M".
298
299   On ppc64, use "crashkernel=128M@32M".
300
301   On ia64, 256M@256M is a generous value that typically works.
302   The region may be automatically placed on ia64, see the
303   dump-capture kernel config option notes above.
304   If use sparse memory, the size should be rounded to GRANULE boundaries.
305
306   On s390x, typically use "crashkernel=xxM". The value of xx is dependent
307   on the memory consumption of the kdump system. In general this is not
308   dependent on the memory size of the production system.
309
310   On arm, use "crashkernel=Y@X". Note that the start address of the kernel
311   will be aligned to 128MiB (0x08000000), so if the start address is not then
312   any space below the alignment point may be overwritten by the dump-capture kernel,
313   which means it is possible that the vmcore is not that precise as expected.
314
315
316Load the Dump-capture Kernel
317============================
318
319After booting to the system kernel, dump-capture kernel needs to be
320loaded.
321
322Based on the architecture and type of image (relocatable or not), one
323can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz
324of dump-capture kernel. Following is the summary.
325
326For i386 and x86_64:
327	- Use vmlinux if kernel is not relocatable.
328	- Use bzImage/vmlinuz if kernel is relocatable.
329For ppc64:
330	- Use vmlinux
331For ia64:
332	- Use vmlinux or vmlinuz.gz
333For s390x:
334	- Use image or bzImage
335For arm:
336	- Use zImage
337
338If you are using a uncompressed vmlinux image then use following command
339to load dump-capture kernel.
340
341   kexec -p <dump-capture-kernel-vmlinux-image> \
342   --initrd=<initrd-for-dump-capture-kernel> --args-linux \
343   --append="root=<root-dev> <arch-specific-options>"
344
345If you are using a compressed bzImage/vmlinuz, then use following command
346to load dump-capture kernel.
347
348   kexec -p <dump-capture-kernel-bzImage> \
349   --initrd=<initrd-for-dump-capture-kernel> \
350   --append="root=<root-dev> <arch-specific-options>"
351
352If you are using a compressed zImage, then use following command
353to load dump-capture kernel.
354
355   kexec --type zImage -p <dump-capture-kernel-bzImage> \
356   --initrd=<initrd-for-dump-capture-kernel> \
357   --dtb=<dtb-for-dump-capture-kernel> \
358   --append="root=<root-dev> <arch-specific-options>"
359
360
361Please note, that --args-linux does not need to be specified for ia64.
362It is planned to make this a no-op on that architecture, but for now
363it should be omitted
364
365Following are the arch specific command line options to be used while
366loading dump-capture kernel.
367
368For i386, x86_64 and ia64:
369	"1 irqpoll maxcpus=1 reset_devices"
370
371For ppc64:
372	"1 maxcpus=1 noirqdistrib reset_devices"
373
374For s390x:
375	"1 maxcpus=1 cgroup_disable=memory"
376
377For arm:
378	"1 maxcpus=1 reset_devices"
379
380Notes on loading the dump-capture kernel:
381
382* By default, the ELF headers are stored in ELF64 format to support
383  systems with more than 4GB memory. On i386, kexec automatically checks if
384  the physical RAM size exceeds the 4 GB limit and if not, uses ELF32.
385  So, on non-PAE systems, ELF32 is always used.
386
387  The --elf32-core-headers option can be used to force the generation of ELF32
388  headers. This is necessary because GDB currently cannot open vmcore files
389  with ELF64 headers on 32-bit systems.
390
391* The "irqpoll" boot parameter reduces driver initialization failures
392  due to shared interrupts in the dump-capture kernel.
393
394* You must specify <root-dev> in the format corresponding to the root
395  device name in the output of mount command.
396
397* Boot parameter "1" boots the dump-capture kernel into single-user
398  mode without networking. If you want networking, use "3".
399
400* We generally don' have to bring up a SMP kernel just to capture the
401  dump. Hence generally it is useful either to build a UP dump-capture
402  kernel or specify maxcpus=1 option while loading dump-capture kernel.
403
404* For s390x there are two kdump modes: If a ELF header is specified with
405  the elfcorehdr= kernel parameter, it is used by the kdump kernel as it
406  is done on all other architectures. If no elfcorehdr= kernel parameter is
407  specified, the s390x kdump kernel dynamically creates the header. The
408  second mode has the advantage that for CPU and memory hotplug, kdump has
409  not to be reloaded with kexec_load().
410
411* For s390x systems with many attached devices the "cio_ignore" kernel
412  parameter should be used for the kdump kernel in order to prevent allocation
413  of kernel memory for devices that are not relevant for kdump. The same
414  applies to systems that use SCSI/FCP devices. In that case the
415  "allow_lun_scan" zfcp module parameter should be set to zero before
416  setting FCP devices online.
417
418Kernel Panic
419============
420
421After successfully loading the dump-capture kernel as previously
422described, the system will reboot into the dump-capture kernel if a
423system crash is triggered.  Trigger points are located in panic(),
424die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).
425
426The following conditions will execute a crash trigger point:
427
428If a hard lockup is detected and "NMI watchdog" is configured, the system
429will boot into the dump-capture kernel ( die_nmi() ).
430
431If die() is called, and it happens to be a thread with pid 0 or 1, or die()
432is called inside interrupt context or die() is called and panic_on_oops is set,
433the system will boot into the dump-capture kernel.
434
435On powerpc systems when a soft-reset is generated, die() is called by all cpus
436and the system will boot into the dump-capture kernel.
437
438For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
439"echo c > /proc/sysrq-trigger" or write a module to force the panic.
440
441Write Out the Dump File
442=======================
443
444After the dump-capture kernel is booted, write out the dump file with
445the following command:
446
447   cp /proc/vmcore <dump-file>
448
449
450Analysis
451========
452
453Before analyzing the dump image, you should reboot into a stable kernel.
454
455You can do limited analysis using GDB on the dump file copied out of
456/proc/vmcore. Use the debug vmlinux built with -g and run the following
457command:
458
459   gdb vmlinux <dump-file>
460
461Stack trace for the task on processor 0, register display, and memory
462display work fine.
463
464Note: GDB cannot analyze core files generated in ELF64 format for x86.
465On systems with a maximum of 4GB of memory, you can generate
466ELF32-format headers using the --elf32-core-headers kernel option on the
467dump kernel.
468
469You can also use the Crash utility to analyze dump files in Kdump
470format. Crash is available on Dave Anderson's site at the following URL:
471
472   http://people.redhat.com/~anderson/
473
474Trigger Kdump on WARN()
475=======================
476
477The kernel parameter, panic_on_warn, calls panic() in all WARN() paths.  This
478will cause a kdump to occur at the panic() call.  In cases where a user wants
479to specify this during runtime, /proc/sys/kernel/panic_on_warn can be set to 1
480to achieve the same behaviour.
481
482Contact
483=======
484
485Vivek Goyal (vgoyal@redhat.com)
486Maneesh Soni (maneesh@in.ibm.com)
487
488