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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:46 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 and arm 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 and arm architectures support relocatable
116	   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	Extended crashkernel syntax
253	===========================
254	
255	While the "crashkernel=size[@offset]" syntax is sufficient for most
256	configurations, sometimes it's handy to have the reserved memory dependent
257	on the value of System RAM -- that's mostly for distributors that pre-setup
258	the kernel command line to avoid a unbootable system after some memory has
259	been removed from the machine.
260	
261	The syntax is:
262	
263	    crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
264	    range=start-[end]
265	
266	For example:
267	
268	    crashkernel=512M-2G:64M,2G-:128M
269	
270	This would mean:
271	
272	    1) if the RAM is smaller than 512M, then don't reserve anything
273	       (this is the "rescue" case)
274	    2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
275	    3) if the RAM size is larger than 2G, then reserve 128M
276	
277	
278	
279	Boot into System Kernel
280	=======================
281	
282	1) Update the boot loader (such as grub, yaboot, or lilo) configuration
283	   files as necessary.
284	
285	2) Boot the system kernel with the boot parameter "crashkernel=Y@X",
286	   where Y specifies how much memory to reserve for the dump-capture kernel
287	   and X specifies the beginning of this reserved memory. For example,
288	   "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
289	   starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
290	
291	   On x86 and x86_64, use "crashkernel=64M@16M".
292	
293	   On ppc64, use "crashkernel=128M@32M".
294	
295	   On ia64, 256M@256M is a generous value that typically works.
296	   The region may be automatically placed on ia64, see the
297	   dump-capture kernel config option notes above.
298	   If use sparse memory, the size should be rounded to GRANULE boundaries.
299	
300	   On s390x, typically use "crashkernel=xxM". The value of xx is dependent
301	   on the memory consumption of the kdump system. In general this is not
302	   dependent on the memory size of the production system.
303	
304	   On arm, the use of "crashkernel=Y@X" is no longer necessary; the
305	   kernel will automatically locate the crash kernel image within the
306	   first 512MB of RAM if X is not given.
307	
308	
309	Load the Dump-capture Kernel
310	============================
311	
312	After booting to the system kernel, dump-capture kernel needs to be
313	loaded.
314	
315	Based on the architecture and type of image (relocatable or not), one
316	can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz
317	of dump-capture kernel. Following is the summary.
318	
319	For i386 and x86_64:
320		- Use vmlinux if kernel is not relocatable.
321		- Use bzImage/vmlinuz if kernel is relocatable.
322	For ppc64:
323		- Use vmlinux
324	For ia64:
325		- Use vmlinux or vmlinuz.gz
326	For s390x:
327		- Use image or bzImage
328	For arm:
329		- Use zImage
330	
331	If you are using a uncompressed vmlinux image then use following command
332	to load dump-capture kernel.
333	
334	   kexec -p <dump-capture-kernel-vmlinux-image> \
335	   --initrd=<initrd-for-dump-capture-kernel> --args-linux \
336	   --append="root=<root-dev> <arch-specific-options>"
337	
338	If you are using a compressed bzImage/vmlinuz, then use following command
339	to load dump-capture kernel.
340	
341	   kexec -p <dump-capture-kernel-bzImage> \
342	   --initrd=<initrd-for-dump-capture-kernel> \
343	   --append="root=<root-dev> <arch-specific-options>"
344	
345	If you are using a compressed zImage, then use following command
346	to load dump-capture kernel.
347	
348	   kexec --type zImage -p <dump-capture-kernel-bzImage> \
349	   --initrd=<initrd-for-dump-capture-kernel> \
350	   --dtb=<dtb-for-dump-capture-kernel> \
351	   --append="root=<root-dev> <arch-specific-options>"
352	
353	
354	Please note, that --args-linux does not need to be specified for ia64.
355	It is planned to make this a no-op on that architecture, but for now
356	it should be omitted
357	
358	Following are the arch specific command line options to be used while
359	loading dump-capture kernel.
360	
361	For i386, x86_64 and ia64:
362		"1 irqpoll maxcpus=1 reset_devices"
363	
364	For ppc64:
365		"1 maxcpus=1 noirqdistrib reset_devices"
366	
367	For s390x:
368		"1 maxcpus=1 cgroup_disable=memory"
369	
370	For arm:
371		"1 maxcpus=1 reset_devices"
372	
373	Notes on loading the dump-capture kernel:
374	
375	* By default, the ELF headers are stored in ELF64 format to support
376	  systems with more than 4GB memory. On i386, kexec automatically checks if
377	  the physical RAM size exceeds the 4 GB limit and if not, uses ELF32.
378	  So, on non-PAE systems, ELF32 is always used.
379	
380	  The --elf32-core-headers option can be used to force the generation of ELF32
381	  headers. This is necessary because GDB currently cannot open vmcore files
382	  with ELF64 headers on 32-bit systems.
383	
384	* The "irqpoll" boot parameter reduces driver initialization failures
385	  due to shared interrupts in the dump-capture kernel.
386	
387	* You must specify <root-dev> in the format corresponding to the root
388	  device name in the output of mount command.
389	
390	* Boot parameter "1" boots the dump-capture kernel into single-user
391	  mode without networking. If you want networking, use "3".
392	
393	* We generally don' have to bring up a SMP kernel just to capture the
394	  dump. Hence generally it is useful either to build a UP dump-capture
395	  kernel or specify maxcpus=1 option while loading dump-capture kernel.
396	
397	* For s390x there are two kdump modes: If a ELF header is specified with
398	  the elfcorehdr= kernel parameter, it is used by the kdump kernel as it
399	  is done on all other architectures. If no elfcorehdr= kernel parameter is
400	  specified, the s390x kdump kernel dynamically creates the header. The
401	  second mode has the advantage that for CPU and memory hotplug, kdump has
402	  not to be reloaded with kexec_load().
403	
404	* For s390x systems with many attached devices the "cio_ignore" kernel
405	  parameter should be used for the kdump kernel in order to prevent allocation
406	  of kernel memory for devices that are not relevant for kdump. The same
407	  applies to systems that use SCSI/FCP devices. In that case the
408	  "allow_lun_scan" zfcp module parameter should be set to zero before
409	  setting FCP devices online.
410	
411	Kernel Panic
412	============
413	
414	After successfully loading the dump-capture kernel as previously
415	described, the system will reboot into the dump-capture kernel if a
416	system crash is triggered.  Trigger points are located in panic(),
417	die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).
418	
419	The following conditions will execute a crash trigger point:
420	
421	If a hard lockup is detected and "NMI watchdog" is configured, the system
422	will boot into the dump-capture kernel ( die_nmi() ).
423	
424	If die() is called, and it happens to be a thread with pid 0 or 1, or die()
425	is called inside interrupt context or die() is called and panic_on_oops is set,
426	the system will boot into the dump-capture kernel.
427	
428	On powerpc systems when a soft-reset is generated, die() is called by all cpus
429	and the system will boot into the dump-capture kernel.
430	
431	For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
432	"echo c > /proc/sysrq-trigger" or write a module to force the panic.
433	
434	Write Out the Dump File
435	=======================
436	
437	After the dump-capture kernel is booted, write out the dump file with
438	the following command:
439	
440	   cp /proc/vmcore <dump-file>
441	
442	
443	Analysis
444	========
445	
446	Before analyzing the dump image, you should reboot into a stable kernel.
447	
448	You can do limited analysis using GDB on the dump file copied out of
449	/proc/vmcore. Use the debug vmlinux built with -g and run the following
450	command:
451	
452	   gdb vmlinux <dump-file>
453	
454	Stack trace for the task on processor 0, register display, and memory
455	display work fine.
456	
457	Note: GDB cannot analyze core files generated in ELF64 format for x86.
458	On systems with a maximum of 4GB of memory, you can generate
459	ELF32-format headers using the --elf32-core-headers kernel option on the
460	dump kernel.
461	
462	You can also use the Crash utility to analyze dump files in Kdump
463	format. Crash is available on Dave Anderson's site at the following URL:
464	
465	   http://people.redhat.com/~anderson/
466	
467	Trigger Kdump on WARN()
468	=======================
469	
470	The kernel parameter, panic_on_warn, calls panic() in all WARN() paths.  This
471	will cause a kdump to occur at the panic() call.  In cases where a user wants
472	to specify this during runtime, /proc/sys/kernel/panic_on_warn can be set to 1
473	to achieve the same behaviour.
474	
475	Contact
476	=======
477	
478	Vivek Goyal (vgoyal@redhat.com)
479	Maneesh Soni (maneesh@in.ibm.com)
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