147 lines
5.8 KiB
Plaintext
147 lines
5.8 KiB
Plaintext
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 taken.
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This second kernel is booted with very little memory. The first kernel reserves
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the section of memory that the second kernel uses. This ensures that on-going
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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 http://www.xmission.com/~ebiederm/files/kexec/kexec-tools-1.101.tar.gz
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and apply http://lse.sourceforge.net/kdump/patches/kexec-tools-1.101-kdump.patch
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and after that build the source.
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2) Download and build the appropriate (2.6.13-rc1 onwards) vanilla kernel.
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Two kernels need to be built in order to get this feature working.
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A) First kernel:
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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) This kernel's physical load address should be the default value of
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0x100000 (0x100000, 1 MB) (in Processor type and features).
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CONFIG_PHYSICAL_START=0x100000
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c) Enable "sysfs file system support" (in Pseudo filesystems).
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CONFIG_SYSFS=y
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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 reserved
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memory section starts. For example: "crashkernel=64M@16M".
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B) Second kernel:
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a) Enable "kernel crash dumps" feature (in Processor type and features).
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CONFIG_CRASH_DUMP=y
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b) Specify a suitable value for "Physical address where the kernel is
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loaded" (in Processor type and features). Typically this value
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should be same as X (See option d) above, e.g., 16 MB or 0x1000000.
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CONFIG_PHYSICAL_START=0x1000000
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c) Enable "/proc/vmcore support" (Optional, in Pseudo filesystems).
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CONFIG_PROC_VMCORE=y
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d) Disable SMP support and build a UP kernel (Until it is fixed).
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CONFIG_SMP=n
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e) Enable "Local APIC support on uniprocessors".
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CONFIG_X86_UP_APIC=y
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f) Enable "IO-APIC support on uniprocessors"
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CONFIG_X86_UP_IOAPIC=y
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Note: i) Options a) and b) depend upon "Configure standard kernel features
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(for small systems)" (under General setup).
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ii) Option a) also depends on CONFIG_HIGHMEM (under Processor
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type and features).
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iii) Both option a) and b) are under "Processor type and features".
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3) Boot into the first kernel. You are now ready to try out kexec-based crash
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dumps.
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4) Load the second kernel to be booted using:
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kexec -p <second-kernel> --args-linux --elf32-core-headers
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--append="root=<root-dev> init 1 irqpoll"
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Note: i) <second-kernel> has to be a vmlinux image. bzImage will not work,
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as of now.
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ii) By default ELF headers are stored in ELF64 format. Option
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--elf32-core-headers forces generation of ELF32 headers. gdb can
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not open ELF64 headers on 32 bit systems. So creating ELF32
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headers can come handy for users who have got non-PAE systems and
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hence have memory less than 4GB.
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iii) 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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iv) <root-dev> needs to be specified in a format corresponding to
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the root device name in the output of mount command.
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v) If you have built the drivers required to mount root file
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system as modules in <second-kernel>, then, specify
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--initrd=<initrd-for-second-kernel>.
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5) System reboots into the second kernel when a panic occurs. A module can be
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written to force the panic or "ALT-SysRq-c" can be used initiate a crash
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dump for testing purposes.
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6) 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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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) Modify "crash" tool to make it recognize this 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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