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1Documentation for kdump - the kexec-based crash dumping solution 1================================================================
2Documentation for Kdump - The kexec-based Crash Dumping Solution
2================================================================ 3================================================================
3 4
4DESIGN 5This document includes overview, setup and installation, and analysis
5====== 6information.
6 7
7Kdump uses kexec to reboot to a second kernel whenever a dump needs to be 8Overview
8taken. This second kernel is booted with very little memory. The first kernel 9========
9reserves the section of memory that the second kernel uses. This ensures that
10on-going DMA from the first kernel does not corrupt the second kernel.
11 10
12All the necessary information about Core image is encoded in ELF format and 11Kdump uses kexec to quickly boot to a dump-capture kernel whenever a
13stored in reserved area of memory before crash. Physical address of start of 12dump of the system kernel's memory needs to be taken (for example, when
14ELF header is passed to new kernel through command line parameter elfcorehdr=. 13the system panics). The system kernel's memory image is preserved across
14the reboot and is accessible to the dump-capture kernel.
15 15
16On i386, the first 640 KB of physical memory is needed to boot, irrespective 16You can use common Linux commands, such as cp and scp, to copy the
17of where the kernel loads. Hence, this region is backed up by kexec just before 17memory image to a dump file on the local disk, or across the network to
18rebooting into the new kernel. 18a remote system.
19 19
20In the second kernel, "old memory" can be accessed in two ways. 20Kdump and kexec are currently supported on the x86, x86_64, and ppc64
21architectures.
21 22
22- The first one is through a /dev/oldmem device interface. A capture utility 23When the system kernel boots, it reserves a small section of memory for
23 can read the device file and write out the memory in raw format. This is raw 24the dump-capture kernel. This ensures that ongoing Direct Memory Access
24 dump of memory and analysis/capture tool should be intelligent enough to 25(DMA) from the system kernel does not corrupt the dump-capture kernel.
25 determine where to look for the right information. ELF headers (elfcorehdr=) 26The kexec -p command loads the dump-capture kernel into this reserved
26 can become handy here. 27memory.
27 28
28- The second interface is through /proc/vmcore. This exports the dump as an ELF 29On x86 machines, the first 640 KB of physical memory is needed to boot,
29 format file which can be written out using any file copy command 30regardless of where the kernel loads. Therefore, kexec backs up this
30 (cp, scp, etc). Further, gdb can be used to perform limited debugging on 31region just before rebooting into the dump-capture kernel.
31 the dump file. This method ensures methods ensure that there is correct
32 ordering of the dump pages (corresponding to the first 640 KB that has been
33 relocated).
34 32
35SETUP 33All of the necessary information about the system kernel's core image is
36===== 34encoded in the ELF format, and stored in a reserved area of memory
35before a crash. The physical address of the start of the ELF header is
36passed to the dump-capture kernel through the elfcorehdr= boot
37parameter.
38
39With the dump-capture kernel, you can access the memory image, or "old
40memory," in two ways:
41
42- Through a /dev/oldmem device interface. A capture utility can read the
43 device file and write out the memory in raw format. This is a raw dump
44 of memory. Analysis and capture tools must be intelligent enough to
45 determine where to look for the right information.
46
47- Through /proc/vmcore. This exports the dump as an ELF-format file that
48 you can write out using file copy commands such as cp or scp. Further,
49 you can use analysis tools such as the GNU Debugger (GDB) and the Crash
50 tool to debug the dump file. This method ensures that the dump pages are
51 correctly ordered.
52
53
54Setup and Installation
55======================
56
57Install kexec-tools and the Kdump patch
58---------------------------------------
59
601) Login as the root user.
61
622) Download the kexec-tools user-space package from the following URL:
63
64 http://www.xmission.com/~ebiederm/files/kexec/kexec-tools-1.101.tar.gz
65
663) Unpack the tarball with the tar command, as follows:
67
68 tar xvpzf kexec-tools-1.101.tar.gz
69
704) Download the latest consolidated Kdump patch from the following URL:
71
72 http://lse.sourceforge.net/kdump/
73
74 (This location is being used until all the user-space Kdump patches
75 are integrated with the kexec-tools package.)
76
775) Change to the kexec-tools-1.101 directory, as follows:
78
79 cd kexec-tools-1.101
80
816) Apply the consolidated patch to the kexec-tools-1.101 source tree
82 with the patch command, as follows. (Modify the path to the downloaded
83 patch as necessary.)
84
85 patch -p1 < /path-to-kdump-patch/kexec-tools-1.101-kdump.patch
86
877) Configure the package, as follows:
88
89 ./configure
90
918) Compile the package, as follows:
92
93 make
94
959) Install the package, as follows:
96
97 make install
98
99
100Download and build the system and dump-capture kernels
101------------------------------------------------------
102
103Download the mainline (vanilla) kernel source code (2.6.13-rc1 or newer)
104from http://www.kernel.org. Two kernels must be built: a system kernel
105and a dump-capture kernel. Use the following steps to configure these
106kernels with the necessary kexec and Kdump features:
107
108System kernel
109-------------
110
1111) Enable "kexec system call" in "Processor type and features."
112
113 CONFIG_KEXEC=y
114
1152) Enable "sysfs file system support" in "Filesystem" -> "Pseudo
116 filesystems." This is usually enabled by default.
117
118 CONFIG_SYSFS=y
119
120 Note that "sysfs file system support" might not appear in the "Pseudo
121 filesystems" menu if "Configure standard kernel features (for small
122 systems)" is not enabled in "General Setup." In this case, check the
123 .config file itself to ensure that sysfs is turned on, as follows:
124
125 grep 'CONFIG_SYSFS' .config
126
1273) Enable "Compile the kernel with debug info" in "Kernel hacking."
128
129 CONFIG_DEBUG_INFO=Y
130
131 This causes the kernel to be built with debug symbols. The dump
132 analysis tools require a vmlinux with debug symbols in order to read
133 and analyze a dump file.
134
1354) Make and install the kernel and its modules. Update the boot loader
136 (such as grub, yaboot, or lilo) configuration files as necessary.
137
1385) Boot the system kernel with the boot parameter "crashkernel=Y@X",
139 where Y specifies how much memory to reserve for the dump-capture kernel
140 and X specifies the beginning of this reserved memory. For example,
141 "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
142 starting at physical address 0x01000000 for the dump-capture kernel.
143
144 On x86 and x86_64, use "crashkernel=64M@16M".
145
146 On ppc64, use "crashkernel=128M@32M".
147
148
149The dump-capture kernel
150-----------------------
37 151
381) Download the upstream kexec-tools userspace package from 1521) Under "General setup," append "-kdump" to the current string in
39 http://www.xmission.com/~ebiederm/files/kexec/kexec-tools-1.101.tar.gz. 153 "Local version."
40 154
41 Apply the latest consolidated kdump patch on top of kexec-tools-1.101 1552) On x86, enable high memory support under "Processor type and
42 from http://lse.sourceforge.net/kdump/. This arrangment has been made 156 features":
43 till all the userspace patches supporting kdump are integrated with 157
44 upstream kexec-tools userspace. 158 CONFIG_HIGHMEM64G=y
45 159 or
462) Download and build the appropriate (2.6.13-rc1 onwards) vanilla kernels. 160 CONFIG_HIGHMEM4G
47 Two kernels need to be built in order to get this feature working. 161
48 Following are the steps to properly configure the two kernels specific 1623) On x86 and x86_64, disable symmetric multi-processing support
49 to kexec and kdump features: 163 under "Processor type and features":
50 164
51 A) First kernel or regular kernel: 165 CONFIG_SMP=n
52 ---------------------------------- 166 (If CONFIG_SMP=y, then specify maxcpus=1 on the kernel command line
53 a) Enable "kexec system call" feature (in Processor type and features). 167 when loading the dump-capture kernel, see section "Load the Dump-capture
54 CONFIG_KEXEC=y 168 Kernel".)
55 b) Enable "sysfs file system support" (in Pseudo filesystems). 169
56 CONFIG_SYSFS=y 1704) On ppc64, disable NUMA support and enable EMBEDDED support:
57 c) make 171
58 d) Boot into first kernel with the command line parameter "crashkernel=Y@X". 172 CONFIG_NUMA=n
59 Use appropriate values for X and Y. Y denotes how much memory to reserve 173 CONFIG_EMBEDDED=y
60 for the second kernel, and X denotes at what physical address the 174 CONFIG_EEH=N for the dump-capture kernel
61 reserved memory section starts. For example: "crashkernel=64M@16M". 175
62 1765) Enable "kernel crash dumps" support under "Processor type and
63 177 features":
64 B) Second kernel or dump capture kernel: 178
65 --------------------------------------- 179 CONFIG_CRASH_DUMP=y
66 a) For i386 architecture enable Highmem support 180
67 CONFIG_HIGHMEM=y 1816) Use a suitable value for "Physical address where the kernel is
68 b) Enable "kernel crash dumps" feature (under "Processor type and features") 182 loaded" (under "Processor type and features"). This only appears when
69 CONFIG_CRASH_DUMP=y 183 "kernel crash dumps" is enabled. By default this value is 0x1000000
70 c) Make sure a suitable value for "Physical address where the kernel is 184 (16MB). It should be the same as X in the "crashkernel=Y@X" boot
71 loaded" (under "Processor type and features"). By default this value 185 parameter discussed above.
72 is 0x1000000 (16MB) and it should be same as X (See option d above), 186
73 e.g., 16 MB or 0x1000000. 187 On x86 and x86_64, use "CONFIG_PHYSICAL_START=0x1000000".
74 CONFIG_PHYSICAL_START=0x1000000 188
75 d) Enable "/proc/vmcore support" (Optional, under "Pseudo filesystems"). 189 On ppc64 the value is automatically set at 32MB when
76 CONFIG_PROC_VMCORE=y 190 CONFIG_CRASH_DUMP is set.
77 191
783) After booting to regular kernel or first kernel, load the second kernel 1926) Optionally enable "/proc/vmcore support" under "Filesystems" ->
79 using the following command: 193 "Pseudo filesystems".
80 194
81 kexec -p <second-kernel> --args-linux --elf32-core-headers 195 CONFIG_PROC_VMCORE=y
82 --append="root=<root-dev> init 1 irqpoll maxcpus=1" 196 (CONFIG_PROC_VMCORE is set by default when CONFIG_CRASH_DUMP is selected.)
83 197
84 Notes: 1987) Make and install the kernel and its modules. DO NOT add this kernel
85 ====== 199 to the boot loader configuration files.
86 i) <second-kernel> has to be a vmlinux image ie uncompressed elf image. 200
87 bzImage will not work, as of now. 201
88 ii) --args-linux has to be speicfied as if kexec it loading an elf image, 202Load the Dump-capture Kernel
89 it needs to know that the arguments supplied are of linux type. 203============================
90 iii) By default ELF headers are stored in ELF64 format to support systems 204
91 with more than 4GB memory. Option --elf32-core-headers forces generation 205After booting to the system kernel, load the dump-capture kernel using
92 of ELF32 headers. The reason for this option being, as of now gdb can 206the following command:
93 not open vmcore file with ELF64 headers on a 32 bit systems. So ELF32 207
94 headers can be used if one has non-PAE systems and hence memory less 208 kexec -p <dump-capture-kernel> \
95 than 4GB. 209 --initrd=<initrd-for-dump-capture-kernel> --args-linux \
96 iv) Specify "irqpoll" as command line parameter. This reduces driver 210 --append="root=<root-dev> init 1 irqpoll"
97 initialization failures in second kernel due to shared interrupts. 211
98 v) <root-dev> needs to be specified in a format corresponding to the root 212
99 device name in the output of mount command. 213Notes on loading the dump-capture kernel:
100 vi) If you have built the drivers required to mount root file system as 214
101 modules in <second-kernel>, then, specify 215* <dump-capture-kernel> must be a vmlinux image (that is, an
102 --initrd=<initrd-for-second-kernel>. 216 uncompressed ELF image). bzImage does not work at this time.
103 vii) Specify maxcpus=1 as, if during first kernel run, if panic happens on 217
104 non-boot cpus, second kernel doesn't seem to be boot up all the cpus. 218* By default, the ELF headers are stored in ELF64 format to support
105 The other option is to always built the second kernel without SMP 219 systems with more than 4GB memory. The --elf32-core-headers option can
106 support ie CONFIG_SMP=n 220 be used to force the generation of ELF32 headers. This is necessary
107 221 because GDB currently cannot open vmcore files with ELF64 headers on
1084) After successfully loading the second kernel as above, if a panic occurs 222 32-bit systems. ELF32 headers can be used on non-PAE systems (that is,
109 system reboots into the second kernel. A module can be written to force 223 less than 4GB of memory).
110 the panic or "ALT-SysRq-c" can be used initiate a crash dump for testing 224
111 purposes. 225* The "irqpoll" boot parameter reduces driver initialization failures
112 226 due to shared interrupts in the dump-capture kernel.
1135) Once the second kernel has booted, write out the dump file using 227
228* You must specify <root-dev> in the format corresponding to the root
229 device name in the output of mount command.
230
231* "init 1" boots the dump-capture kernel into single-user mode without
232 networking. If you want networking, use "init 3."
233
234
235Kernel Panic
236============
237
238After successfully loading the dump-capture kernel as previously
239described, the system will reboot into the dump-capture kernel if a
240system crash is triggered. Trigger points are located in panic(),
241die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).
242
243The following conditions will execute a crash trigger point:
244
245If a hard lockup is detected and "NMI watchdog" is configured, the system
246will boot into the dump-capture kernel ( die_nmi() ).
247
248If die() is called, and it happens to be a thread with pid 0 or 1, or die()
249is called inside interrupt context or die() is called and panic_on_oops is set,
250the system will boot into the dump-capture kernel.
251
252On powererpc systems when a soft-reset is generated, die() is called by all cpus and the system system will boot into the dump-capture kernel.
253
254For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
255"echo c > /proc/sysrq-trigger or write a module to force the panic.
256
257Write Out the Dump File
258=======================
259
260After the dump-capture kernel is booted, write out the dump file with
261the following command:
114 262
115 cp /proc/vmcore <dump-file> 263 cp /proc/vmcore <dump-file>
116 264
117 Dump memory can also be accessed as a /dev/oldmem device for a linear/raw 265You can also access dumped memory as a /dev/oldmem device for a linear
118 view. To create the device, type: 266and raw view. To create the device, use the following command:
119 267
120 mknod /dev/oldmem c 1 12 268 mknod /dev/oldmem c 1 12
121 269
122 Use "dd" with suitable options for count, bs and skip to access specific 270Use the dd command with suitable options for count, bs, and skip to
123 portions of the dump. 271access specific portions of the dump.
124 272
125 Entire memory: dd if=/dev/oldmem of=oldmem.001 273To see the entire memory, use the following command:
126 274
275 dd if=/dev/oldmem of=oldmem.001
127 276
128ANALYSIS 277
278Analysis
129======== 279========
130Limited analysis can be done using gdb on the dump file copied out of
131/proc/vmcore. Use vmlinux built with -g and run
132 280
133 gdb vmlinux <dump-file> 281Before analyzing the dump image, you should reboot into a stable kernel.
282
283You can do limited analysis using GDB on the dump file copied out of
284/proc/vmcore. Use the debug vmlinux built with -g and run the following
285command:
286
287 gdb vmlinux <dump-file>
134 288
135Stack trace for the task on processor 0, register display, memory display 289Stack trace for the task on processor 0, register display, and memory
136work fine. 290display work fine.
137 291
138Note: gdb cannot analyse core files generated in ELF64 format for i386. 292Note: GDB cannot analyze core files generated in ELF64 format for x86.
293On systems with a maximum of 4GB of memory, you can generate
294ELF32-format headers using the --elf32-core-headers kernel option on the
295dump kernel.
139 296
140Latest "crash" (crash-4.0-2.18) as available on Dave Anderson's site 297You can also use the Crash utility to analyze dump files in Kdump
141http://people.redhat.com/~anderson/ works well with kdump format. 298format. Crash is available on Dave Anderson's site at the following URL:
142 299
300 http://people.redhat.com/~anderson/
301
302
303To Do
304=====
143 305
144TODO 3061) Provide a kernel pages filtering mechanism, so core file size is not
145==== 307 extreme on systems with huge memory banks.
1461) Provide a kernel pages filtering mechanism so that core file size is not
147 insane on systems having huge memory banks.
1482) Relocatable kernel can help in maintaining multiple kernels for crashdump
149 and same kernel as the first kernel can be used to capture the dump.
150 308
3092) Relocatable kernel can help in maintaining multiple kernels for
310 crash_dump, and the same kernel as the system kernel can be used to
311 capture the dump.
151 312
152CONTACT 313
314Contact
153======= 315=======
316
154Vivek Goyal (vgoyal@in.ibm.com) 317Vivek Goyal (vgoyal@in.ibm.com)
155Maneesh Soni (maneesh@in.ibm.com) 318Maneesh Soni (maneesh@in.ibm.com)
319
320
321Trademark
322=========
323
324Linux is a trademark of Linus Torvalds in the United States, other
325countries, or both.