a7e670d828
Update the kdump documentation to reflect the changes due to recent kernel config option changes for kexec and kdump. Signed-off-by: Maneesh Soni <maneesh@in.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
155 lines
6.5 KiB
Text
155 lines
6.5 KiB
Text
Documentation for kdump - the kexec-based crash dumping solution
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================================================================
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DESIGN
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======
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Kdump uses kexec to reboot to a second kernel whenever a dump needs to be
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taken. This second kernel is booted with very little memory. The first kernel
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reserves the section of memory that the second kernel uses. This ensures that
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on-going DMA from the first kernel does not corrupt the second kernel.
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All the necessary information about Core image is encoded in ELF format and
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stored in reserved area of memory before crash. Physical address of start of
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ELF header is passed to new kernel through command line parameter elfcorehdr=.
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On i386, the first 640 KB of physical memory is needed to boot, irrespective
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of where the kernel loads. Hence, this region is backed up by kexec just before
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rebooting into the new kernel.
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In the second kernel, "old memory" can be accessed in two ways.
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- The first one is through a /dev/oldmem device interface. A capture utility
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can read the device file and write out the memory in raw format. This is raw
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dump of memory and analysis/capture tool should be intelligent enough to
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determine where to look for the right information. ELF headers (elfcorehdr=)
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can become handy here.
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- The second interface is through /proc/vmcore. This exports the dump as an ELF
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format file which can be written out using any file copy command
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(cp, scp, etc). Further, gdb can be used to perform limited debugging on
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the dump file. This method ensures methods ensure that there is correct
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ordering of the dump pages (corresponding to the first 640 KB that has been
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relocated).
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SETUP
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=====
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1) Download the upstream kexec-tools userspace package from
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http://www.xmission.com/~ebiederm/files/kexec/kexec-tools-1.101.tar.gz.
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Apply the latest consolidated kdump patch on top of kexec-tools-1.101
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from http://lse.sourceforge.net/kdump/. This arrangment has been made
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till all the userspace patches supporting kdump are integrated with
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upstream kexec-tools userspace.
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2) Download and build the appropriate (2.6.13-rc1 onwards) vanilla kernels.
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Two kernels need to be built in order to get this feature working.
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Following are the steps to properly configure the two kernels specific
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to kexec and kdump features:
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A) First kernel or regular kernel:
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----------------------------------
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a) Enable "kexec system call" feature (in Processor type and features).
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CONFIG_KEXEC=y
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b) Enable "sysfs file system support" (in Pseudo filesystems).
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CONFIG_SYSFS=y
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c) make
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d) Boot into first kernel with the command line parameter "crashkernel=Y@X".
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Use appropriate values for X and Y. Y denotes how much memory to reserve
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for the second kernel, and X denotes at what physical address the
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reserved memory section starts. For example: "crashkernel=64M@16M".
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B) Second kernel or dump capture kernel:
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---------------------------------------
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a) For i386 architecture enable Highmem support
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CONFIG_HIGHMEM=y
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b) Enable "kernel crash dumps" feature (under "Processor type and features")
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CONFIG_CRASH_DUMP=y
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c) Make sure a suitable value for "Physical address where the kernel is
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loaded" (under "Processor type and features"). By default this value
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is 0x1000000 (16MB) and it should be same as X (See option d above),
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e.g., 16 MB or 0x1000000.
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CONFIG_PHYSICAL_START=0x1000000
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d) Enable "/proc/vmcore support" (Optional, under "Pseudo filesystems").
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CONFIG_PROC_VMCORE=y
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3) After booting to regular kernel or first kernel, load the second kernel
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using the following command:
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kexec -p <second-kernel> --args-linux --elf32-core-headers
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--append="root=<root-dev> init 1 irqpoll maxcpus=1"
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Notes:
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======
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i) <second-kernel> has to be a vmlinux image ie uncompressed elf image.
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bzImage will not work, as of now.
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ii) --args-linux has to be speicfied as if kexec it loading an elf image,
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it needs to know that the arguments supplied are of linux type.
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iii) By default ELF headers are stored in ELF64 format to support systems
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with more than 4GB memory. Option --elf32-core-headers forces generation
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of ELF32 headers. The reason for this option being, as of now gdb can
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not open vmcore file with ELF64 headers on a 32 bit systems. So ELF32
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headers can be used if one has non-PAE systems and hence memory less
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than 4GB.
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iv) Specify "irqpoll" as command line parameter. This reduces driver
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initialization failures in second kernel due to shared interrupts.
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v) <root-dev> needs to be specified in a format corresponding to the root
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device name in the output of mount command.
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vi) If you have built the drivers required to mount root file system as
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modules in <second-kernel>, then, specify
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--initrd=<initrd-for-second-kernel>.
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vii) Specify maxcpus=1 as, if during first kernel run, if panic happens on
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non-boot cpus, second kernel doesn't seem to be boot up all the cpus.
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The other option is to always built the second kernel without SMP
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support ie CONFIG_SMP=n
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4) After successfully loading the second kernel as above, if a panic occurs
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system reboots into the second kernel. A module can be written to force
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the panic or "ALT-SysRq-c" can be used initiate a crash dump for testing
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purposes.
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5) Once the second kernel has booted, write out the dump file using
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cp /proc/vmcore <dump-file>
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Dump memory can also be accessed as a /dev/oldmem device for a linear/raw
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view. To create the device, type:
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mknod /dev/oldmem c 1 12
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Use "dd" with suitable options for count, bs and skip to access specific
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portions of the dump.
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Entire memory: dd if=/dev/oldmem of=oldmem.001
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ANALYSIS
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========
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Limited analysis can be done using gdb on the dump file copied out of
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/proc/vmcore. Use vmlinux built with -g and run
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gdb vmlinux <dump-file>
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Stack trace for the task on processor 0, register display, memory display
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work fine.
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Note: gdb cannot analyse core files generated in ELF64 format for i386.
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Latest "crash" (crash-4.0-2.18) as available on Dave Anderson's site
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http://people.redhat.com/~anderson/ works well with kdump format.
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TODO
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====
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1) Provide a kernel pages filtering mechanism so that core file size is not
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insane on systems having huge memory banks.
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2) Relocatable kernel can help in maintaining multiple kernels for crashdump
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and same kernel as the first kernel can be used to capture the dump.
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CONTACT
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=======
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Vivek Goyal (vgoyal@in.ibm.com)
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Maneesh Soni (maneesh@in.ibm.com)
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