445 lines
15 KiB
Plaintext
445 lines
15 KiB
Plaintext
================================================================
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Documentation for Kdump - The kexec-based Crash Dumping Solution
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================================================================
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This document includes overview, setup and installation, and analysis
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information.
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Overview
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========
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Kdump uses kexec to quickly boot to a dump-capture kernel whenever a
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dump of the system kernel's memory needs to be taken (for example, when
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the system panics). The system kernel's memory image is preserved across
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the reboot and is accessible to the dump-capture kernel.
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You can use common commands, such as cp and scp, to copy the
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memory image to a dump file on the local disk, or across the network to
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a remote system.
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Kdump and kexec are currently supported on the x86, x86_64, ppc64 and ia64
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architectures.
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When the system kernel boots, it reserves a small section of memory for
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the dump-capture kernel. This ensures that ongoing Direct Memory Access
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(DMA) from the system kernel does not corrupt the dump-capture kernel.
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The kexec -p command loads the dump-capture kernel into this reserved
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memory.
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On x86 machines, the first 640 KB of physical memory is needed to boot,
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regardless of where the kernel loads. Therefore, kexec backs up this
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region just before rebooting into the dump-capture kernel.
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Similarly on PPC64 machines first 32KB of physical memory is needed for
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booting regardless of where the kernel is loaded and to support 64K page
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size kexec backs up the first 64KB memory.
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All of the necessary information about the system kernel's core image is
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encoded in the ELF format, and stored in a reserved area of memory
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before a crash. The physical address of the start of the ELF header is
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passed to the dump-capture kernel through the elfcorehdr= boot
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parameter.
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With the dump-capture kernel, you can access the memory image, or "old
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memory," in two ways:
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- Through a /dev/oldmem device interface. A capture utility can read the
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device file and write out the memory in raw format. This is a raw dump
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of memory. Analysis and capture tools must be intelligent enough to
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determine where to look for the right information.
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- Through /proc/vmcore. This exports the dump as an ELF-format file that
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you can write out using file copy commands such as cp or scp. Further,
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you can use analysis tools such as the GNU Debugger (GDB) and the Crash
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tool to debug the dump file. This method ensures that the dump pages are
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correctly ordered.
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Setup and Installation
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======================
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Install kexec-tools
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-------------------
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1) Login as the root user.
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2) Download the kexec-tools user-space package from the following URL:
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http://www.kernel.org/pub/linux/kernel/people/horms/kexec-tools/kexec-tools.tar.gz
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This is a symlink to the latest version.
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The latest kexec-tools git tree is available at:
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git://git.kernel.org/pub/scm/linux/kernel/git/horms/kexec-tools.git
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or
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http://www.kernel.org/git/?p=linux/kernel/git/horms/kexec-tools.git
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More information about kexec-tools can be found at
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http://www.kernel.org/pub/linux/kernel/people/horms/kexec-tools/README.html
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3) Unpack the tarball with the tar command, as follows:
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tar xvpzf kexec-tools.tar.gz
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4) Change to the kexec-tools directory, as follows:
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cd kexec-tools-VERSION
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5) Configure the package, as follows:
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./configure
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6) Compile the package, as follows:
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make
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7) Install the package, as follows:
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make install
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Build the system and dump-capture kernels
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-----------------------------------------
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There are two possible methods of using Kdump.
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1) Build a separate custom dump-capture kernel for capturing the
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kernel core dump.
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2) Or use the system kernel binary itself as dump-capture kernel and there is
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no need to build a separate dump-capture kernel. This is possible
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only with the architectures which support a relocatable kernel. As
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of today, i386, x86_64, ppc64 and ia64 architectures support relocatable
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kernel.
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Building a relocatable kernel is advantageous from the point of view that
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one does not have to build a second kernel for capturing the dump. But
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at the same time one might want to build a custom dump capture kernel
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suitable to his needs.
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Following are the configuration setting required for system and
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dump-capture kernels for enabling kdump support.
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System kernel config options
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----------------------------
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1) Enable "kexec system call" in "Processor type and features."
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CONFIG_KEXEC=y
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2) Enable "sysfs file system support" in "Filesystem" -> "Pseudo
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filesystems." This is usually enabled by default.
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CONFIG_SYSFS=y
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Note that "sysfs file system support" might not appear in the "Pseudo
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filesystems" menu if "Configure standard kernel features (for small
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systems)" is not enabled in "General Setup." In this case, check the
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.config file itself to ensure that sysfs is turned on, as follows:
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grep 'CONFIG_SYSFS' .config
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3) Enable "Compile the kernel with debug info" in "Kernel hacking."
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CONFIG_DEBUG_INFO=Y
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This causes the kernel to be built with debug symbols. The dump
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analysis tools require a vmlinux with debug symbols in order to read
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and analyze a dump file.
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Dump-capture kernel config options (Arch Independent)
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-----------------------------------------------------
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1) Enable "kernel crash dumps" support under "Processor type and
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features":
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CONFIG_CRASH_DUMP=y
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2) Enable "/proc/vmcore support" under "Filesystems" -> "Pseudo filesystems".
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CONFIG_PROC_VMCORE=y
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(CONFIG_PROC_VMCORE is set by default when CONFIG_CRASH_DUMP is selected.)
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Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
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--------------------------------------------------------------------
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1) On i386, enable high memory support under "Processor type and
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features":
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CONFIG_HIGHMEM64G=y
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or
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CONFIG_HIGHMEM4G
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2) On i386 and x86_64, disable symmetric multi-processing support
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under "Processor type and features":
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CONFIG_SMP=n
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(If CONFIG_SMP=y, then specify maxcpus=1 on the kernel command line
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when loading the dump-capture kernel, see section "Load the Dump-capture
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Kernel".)
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3) If one wants to build and use a relocatable kernel,
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Enable "Build a relocatable kernel" support under "Processor type and
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features"
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CONFIG_RELOCATABLE=y
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4) Use a suitable value for "Physical address where the kernel is
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loaded" (under "Processor type and features"). This only appears when
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"kernel crash dumps" is enabled. A suitable value depends upon
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whether kernel is relocatable or not.
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If you are using a relocatable kernel use CONFIG_PHYSICAL_START=0x100000
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This will compile the kernel for physical address 1MB, but given the fact
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kernel is relocatable, it can be run from any physical address hence
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kexec boot loader will load it in memory region reserved for dump-capture
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kernel.
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Otherwise it should be the start of memory region reserved for
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second kernel using boot parameter "crashkernel=Y@X". Here X is
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start of memory region reserved for dump-capture kernel.
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Generally X is 16MB (0x1000000). So you can set
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CONFIG_PHYSICAL_START=0x1000000
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5) Make and install the kernel and its modules. DO NOT add this kernel
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to the boot loader configuration files.
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Dump-capture kernel config options (Arch Dependent, ppc64)
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----------------------------------------------------------
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1) Enable "Build a kdump crash kernel" support under "Kernel" options:
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CONFIG_CRASH_DUMP=y
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2) Enable "Build a relocatable kernel" support
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CONFIG_RELOCATABLE=y
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Make and install the kernel and its modules.
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Dump-capture kernel config options (Arch Dependent, ia64)
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----------------------------------------------------------
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- No specific options are required to create a dump-capture kernel
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for ia64, other than those specified in the arch independent section
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above. This means that it is possible to use the system kernel
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as a dump-capture kernel if desired.
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The crashkernel region can be automatically placed by the system
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kernel at run time. This is done by specifying the base address as 0,
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or omitting it all together.
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crashkernel=256M@0
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or
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crashkernel=256M
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If the start address is specified, note that the start address of the
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kernel will be aligned to 64Mb, so if the start address is not then
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any space below the alignment point will be wasted.
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Extended crashkernel syntax
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===========================
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While the "crashkernel=size[@offset]" syntax is sufficient for most
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configurations, sometimes it's handy to have the reserved memory dependent
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on the value of System RAM -- that's mostly for distributors that pre-setup
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the kernel command line to avoid a unbootable system after some memory has
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been removed from the machine.
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The syntax is:
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crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
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range=start-[end]
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'start' is inclusive and 'end' is exclusive.
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For example:
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crashkernel=512M-2G:64M,2G-:128M
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This would mean:
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1) if the RAM is smaller than 512M, then don't reserve anything
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(this is the "rescue" case)
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2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
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3) if the RAM size is larger than 2G, then reserve 128M
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Boot into System Kernel
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=======================
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1) Update the boot loader (such as grub, yaboot, or lilo) configuration
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files as necessary.
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2) Boot the system kernel with the boot parameter "crashkernel=Y@X",
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where Y specifies how much memory to reserve for the dump-capture kernel
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and X specifies the beginning of this reserved memory. For example,
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"crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
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starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
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On x86 and x86_64, use "crashkernel=64M@16M".
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On ppc64, use "crashkernel=128M@32M".
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On ia64, 256M@256M is a generous value that typically works.
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The region may be automatically placed on ia64, see the
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dump-capture kernel config option notes above.
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Load the Dump-capture Kernel
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============================
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After booting to the system kernel, dump-capture kernel needs to be
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loaded.
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Based on the architecture and type of image (relocatable or not), one
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can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz
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of dump-capture kernel. Following is the summary.
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For i386 and x86_64:
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- Use vmlinux if kernel is not relocatable.
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- Use bzImage/vmlinuz if kernel is relocatable.
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For ppc64:
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- Use vmlinux
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For ia64:
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- Use vmlinux or vmlinuz.gz
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If you are using a uncompressed vmlinux image then use following command
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to load dump-capture kernel.
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kexec -p <dump-capture-kernel-vmlinux-image> \
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--initrd=<initrd-for-dump-capture-kernel> --args-linux \
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--append="root=<root-dev> <arch-specific-options>"
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If you are using a compressed bzImage/vmlinuz, then use following command
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to load dump-capture kernel.
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kexec -p <dump-capture-kernel-bzImage> \
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--initrd=<initrd-for-dump-capture-kernel> \
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--append="root=<root-dev> <arch-specific-options>"
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Please note, that --args-linux does not need to be specified for ia64.
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It is planned to make this a no-op on that architecture, but for now
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it should be omitted
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Following are the arch specific command line options to be used while
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loading dump-capture kernel.
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For i386, x86_64 and ia64:
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"1 irqpoll maxcpus=1 reset_devices"
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For ppc64:
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"1 maxcpus=1 noirqdistrib reset_devices"
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Notes on loading the dump-capture kernel:
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* By default, the ELF headers are stored in ELF64 format to support
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systems with more than 4GB memory. On i386, kexec automatically checks if
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the physical RAM size exceeds the 4 GB limit and if not, uses ELF32.
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So, on non-PAE systems, ELF32 is always used.
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The --elf32-core-headers option can be used to force the generation of ELF32
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headers. This is necessary because GDB currently cannot open vmcore files
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with ELF64 headers on 32-bit systems.
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* The "irqpoll" boot parameter reduces driver initialization failures
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due to shared interrupts in the dump-capture kernel.
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* You must specify <root-dev> in the format corresponding to the root
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device name in the output of mount command.
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* Boot parameter "1" boots the dump-capture kernel into single-user
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mode without networking. If you want networking, use "3".
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* We generally don' have to bring up a SMP kernel just to capture the
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dump. Hence generally it is useful either to build a UP dump-capture
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kernel or specify maxcpus=1 option while loading dump-capture kernel.
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Kernel Panic
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============
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After successfully loading the dump-capture kernel as previously
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described, the system will reboot into the dump-capture kernel if a
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system crash is triggered. Trigger points are located in panic(),
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die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).
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The following conditions will execute a crash trigger point:
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If a hard lockup is detected and "NMI watchdog" is configured, the system
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will boot into the dump-capture kernel ( die_nmi() ).
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If die() is called, and it happens to be a thread with pid 0 or 1, or die()
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is called inside interrupt context or die() is called and panic_on_oops is set,
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the system will boot into the dump-capture kernel.
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On powerpc systems when a soft-reset is generated, die() is called by all cpus
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and the system will boot into the dump-capture kernel.
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For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
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"echo c > /proc/sysrq-trigger" or write a module to force the panic.
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Write Out the Dump File
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=======================
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After the dump-capture kernel is booted, write out the dump file with
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the following command:
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cp /proc/vmcore <dump-file>
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You can also access dumped memory as a /dev/oldmem device for a linear
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and raw view. To create the device, use the following command:
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mknod /dev/oldmem c 1 12
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Use the dd command with suitable options for count, bs, and skip to
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access specific portions of the dump.
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To see the entire memory, use the following command:
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dd if=/dev/oldmem of=oldmem.001
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Analysis
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========
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Before analyzing the dump image, you should reboot into a stable kernel.
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You can do limited analysis using GDB on the dump file copied out of
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/proc/vmcore. Use the debug vmlinux built with -g and run the following
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command:
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gdb vmlinux <dump-file>
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Stack trace for the task on processor 0, register display, and memory
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display work fine.
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Note: GDB cannot analyze core files generated in ELF64 format for x86.
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On systems with a maximum of 4GB of memory, you can generate
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ELF32-format headers using the --elf32-core-headers kernel option on the
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dump kernel.
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You can also use the Crash utility to analyze dump files in Kdump
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format. Crash is available on Dave Anderson's site at the following URL:
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http://people.redhat.com/~anderson/
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To Do
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=====
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1) Provide relocatable kernels for all architectures to help in maintaining
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multiple kernels for crash_dump, and the same kernel as the system kernel
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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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