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diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt index 212cf3c..08bafa8 100644 --- a/Documentation/kdump/kdump.txt +++ b/Documentation/kdump/kdump.txt @@ -1,155 +1,325 @@ -Documentation for kdump - the kexec-based crash dumping solution +================================================================ +Documentation for Kdump - The kexec-based Crash Dumping Solution ================================================================ -DESIGN -====== +This document includes overview, setup and installation, and analysis +information. -Kdump uses kexec to reboot to a second kernel whenever a dump needs to be -taken. This second kernel is booted with very little memory. The first kernel -reserves the section of memory that the second kernel uses. This ensures that -on-going DMA from the first kernel does not corrupt the second kernel. +Overview +======== -All the necessary information about Core image is encoded in ELF format and -stored in reserved area of memory before crash. Physical address of start of -ELF header is passed to new kernel through command line parameter elfcorehdr=. +Kdump uses kexec to quickly boot to a dump-capture kernel whenever a +dump of the system kernel's memory needs to be taken (for example, when +the system panics). The system kernel's memory image is preserved across +the reboot and is accessible to the dump-capture kernel. -On i386, the first 640 KB of physical memory is needed to boot, irrespective -of where the kernel loads. Hence, this region is backed up by kexec just before -rebooting into the new kernel. +You can use common Linux commands, such as cp and scp, to copy the +memory image to a dump file on the local disk, or across the network to +a remote system. -In the second kernel, "old memory" can be accessed in two ways. +Kdump and kexec are currently supported on the x86, x86_64, and ppc64 +architectures. -- The first one is through a /dev/oldmem device interface. A capture utility - can read the device file and write out the memory in raw format. This is raw - dump of memory and analysis/capture tool should be intelligent enough to - determine where to look for the right information. ELF headers (elfcorehdr=) - can become handy here. +When the system kernel boots, it reserves a small section of memory for +the dump-capture kernel. This ensures that ongoing Direct Memory Access +(DMA) from the system kernel does not corrupt the dump-capture kernel. +The kexec -p command loads the dump-capture kernel into this reserved +memory. -- The second interface is through /proc/vmcore. This exports the dump as an ELF - format file which can be written out using any file copy command - (cp, scp, etc). Further, gdb can be used to perform limited debugging on - the dump file. This method ensures methods ensure that there is correct - ordering of the dump pages (corresponding to the first 640 KB that has been - relocated). +On x86 machines, the first 640 KB of physical memory is needed to boot, +regardless of where the kernel loads. Therefore, kexec backs up this +region just before rebooting into the dump-capture kernel. -SETUP -===== +All of the necessary information about the system kernel's core image is +encoded in the ELF format, and stored in a reserved area of memory +before a crash. The physical address of the start of the ELF header is +passed to the dump-capture kernel through the elfcorehdr= boot +parameter. + +With the dump-capture kernel, you can access the memory image, or "old +memory," in two ways: + +- Through a /dev/oldmem device interface. A capture utility can read the + device file and write out the memory in raw format. This is a raw dump + of memory. Analysis and capture tools must be intelligent enough to + determine where to look for the right information. + +- Through /proc/vmcore. This exports the dump as an ELF-format file that + you can write out using file copy commands such as cp or scp. Further, + you can use analysis tools such as the GNU Debugger (GDB) and the Crash + tool to debug the dump file. This method ensures that the dump pages are + correctly ordered. + + +Setup and Installation +====================== + +Install kexec-tools and the Kdump patch +--------------------------------------- + +1) Login as the root user. + +2) Download the kexec-tools user-space package from the following URL: + + http://www.xmission.com/~ebiederm/files/kexec/kexec-tools-1.101.tar.gz + +3) Unpack the tarball with the tar command, as follows: + + tar xvpzf kexec-tools-1.101.tar.gz + +4) Download the latest consolidated Kdump patch from the following URL: + + http://lse.sourceforge.net/kdump/ + + (This location is being used until all the user-space Kdump patches + are integrated with the kexec-tools package.) + +5) Change to the kexec-tools-1.101 directory, as follows: + + cd kexec-tools-1.101 + +6) Apply the consolidated patch to the kexec-tools-1.101 source tree + with the patch command, as follows. (Modify the path to the downloaded + patch as necessary.) + + patch -p1 < /path-to-kdump-patch/kexec-tools-1.101-kdump.patch + +7) Configure the package, as follows: + + ./configure + +8) Compile the package, as follows: + + make + +9) Install the package, as follows: + + make install + + +Download and build the system and dump-capture kernels +------------------------------------------------------ + +Download the mainline (vanilla) kernel source code (2.6.13-rc1 or newer) +from http://www.kernel.org. Two kernels must be built: a system kernel +and a dump-capture kernel. Use the following steps to configure these +kernels with the necessary kexec and Kdump features: + +System kernel +------------- + +1) Enable "kexec system call" in "Processor type and features." + + CONFIG_KEXEC=y + +2) Enable "sysfs file system support" in "Filesystem" -> "Pseudo + filesystems." This is usually enabled by default. + + CONFIG_SYSFS=y + + Note that "sysfs file system support" might not appear in the "Pseudo + filesystems" menu if "Configure standard kernel features (for small + systems)" is not enabled in "General Setup." In this case, check the + .config file itself to ensure that sysfs is turned on, as follows: + + grep 'CONFIG_SYSFS' .config + +3) Enable "Compile the kernel with debug info" in "Kernel hacking." + + CONFIG_DEBUG_INFO=Y + + This causes the kernel to be built with debug symbols. The dump + analysis tools require a vmlinux with debug symbols in order to read + and analyze a dump file. + +4) Make and install the kernel and its modules. Update the boot loader + (such as grub, yaboot, or lilo) configuration files as necessary. + +5) Boot the system kernel with the boot parameter "crashkernel=Y@X", + where Y specifies how much memory to reserve for the dump-capture kernel + and X specifies the beginning of this reserved memory. For example, + "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory + starting at physical address 0x01000000 for the dump-capture kernel. + + On x86 and x86_64, use "crashkernel=64M@16M". + + On ppc64, use "crashkernel=128M@32M". + + +The dump-capture kernel +----------------------- -1) Download the upstream kexec-tools userspace package from - http://www.xmission.com/~ebiederm/files/kexec/kexec-tools-1.101.tar.gz. - - Apply the latest consolidated kdump patch on top of kexec-tools-1.101 - from http://lse.sourceforge.net/kdump/. This arrangment has been made - till all the userspace patches supporting kdump are integrated with - upstream kexec-tools userspace. - -2) Download and build the appropriate (2.6.13-rc1 onwards) vanilla kernels. - Two kernels need to be built in order to get this feature working. - Following are the steps to properly configure the two kernels specific - to kexec and kdump features: - - A) First kernel or regular kernel: - ---------------------------------- - a) Enable "kexec system call" feature (in Processor type and features). - CONFIG_KEXEC=y - b) Enable "sysfs file system support" (in Pseudo filesystems). - CONFIG_SYSFS=y - c) make - d) Boot into first kernel with the command line parameter "crashkernel=Y@X". - Use appropriate values for X and Y. Y denotes how much memory to reserve - for the second kernel, and X denotes at what physical address the - reserved memory section starts. For example: "crashkernel=64M@16M". - - - B) Second kernel or dump capture kernel: - --------------------------------------- - a) For i386 architecture enable Highmem support - CONFIG_HIGHMEM=y - b) Enable "kernel crash dumps" feature (under "Processor type and features") - CONFIG_CRASH_DUMP=y - c) Make sure a suitable value for "Physical address where the kernel is - loaded" (under "Processor type and features"). By default this value - is 0x1000000 (16MB) and it should be same as X (See option d above), - e.g., 16 MB or 0x1000000. - CONFIG_PHYSICAL_START=0x1000000 - d) Enable "/proc/vmcore support" (Optional, under "Pseudo filesystems"). - CONFIG_PROC_VMCORE=y - -3) After booting to regular kernel or first kernel, load the second kernel - using the following command: - - kexec -p <second-kernel> --args-linux --elf32-core-headers - --append="root=<root-dev> init 1 irqpoll maxcpus=1" - - Notes: - ====== - i) <second-kernel> has to be a vmlinux image ie uncompressed elf image. - bzImage will not work, as of now. - ii) --args-linux has to be speicfied as if kexec it loading an elf image, - it needs to know that the arguments supplied are of linux type. - iii) By default ELF headers are stored in ELF64 format to support systems - with more than 4GB memory. Option --elf32-core-headers forces generation - of ELF32 headers. The reason for this option being, as of now gdb can - not open vmcore file with ELF64 headers on a 32 bit systems. So ELF32 - headers can be used if one has non-PAE systems and hence memory less - than 4GB. - iv) Specify "irqpoll" as command line parameter. This reduces driver - initialization failures in second kernel due to shared interrupts. - v) <root-dev> needs to be specified in a format corresponding to the root - device name in the output of mount command. - vi) If you have built the drivers required to mount root file system as - modules in <second-kernel>, then, specify - --initrd=<initrd-for-second-kernel>. - vii) Specify maxcpus=1 as, if during first kernel run, if panic happens on - non-boot cpus, second kernel doesn't seem to be boot up all the cpus. - The other option is to always built the second kernel without SMP - support ie CONFIG_SMP=n - -4) After successfully loading the second kernel as above, if a panic occurs - system reboots into the second kernel. A module can be written to force - the panic or "ALT-SysRq-c" can be used initiate a crash dump for testing - purposes. - -5) Once the second kernel has booted, write out the dump file using +1) Under "General setup," append "-kdump" to the current string in + "Local version." + +2) On x86, enable high memory support under "Processor type and + features": + + CONFIG_HIGHMEM64G=y + or + CONFIG_HIGHMEM4G + +3) On x86 and x86_64, disable symmetric multi-processing support + under "Processor type and features": + + CONFIG_SMP=n + (If CONFIG_SMP=y, then specify maxcpus=1 on the kernel command line + when loading the dump-capture kernel, see section "Load the Dump-capture + Kernel".) + +4) On ppc64, disable NUMA support and enable EMBEDDED support: + + CONFIG_NUMA=n + CONFIG_EMBEDDED=y + CONFIG_EEH=N for the dump-capture kernel + +5) Enable "kernel crash dumps" support under "Processor type and + features": + + CONFIG_CRASH_DUMP=y + +6) Use a suitable value for "Physical address where the kernel is + loaded" (under "Processor type and features"). This only appears when + "kernel crash dumps" is enabled. By default this value is 0x1000000 + (16MB). It should be the same as X in the "crashkernel=Y@X" boot + parameter discussed above. + + On x86 and x86_64, use "CONFIG_PHYSICAL_START=0x1000000". + + On ppc64 the value is automatically set at 32MB when + CONFIG_CRASH_DUMP is set. + +6) Optionally enable "/proc/vmcore support" under "Filesystems" -> + "Pseudo filesystems". + + CONFIG_PROC_VMCORE=y + (CONFIG_PROC_VMCORE is set by default when CONFIG_CRASH_DUMP is selected.) + +7) Make and install the kernel and its modules. DO NOT add this kernel + to the boot loader configuration files. + + +Load the Dump-capture Kernel +============================ + +After booting to the system kernel, load the dump-capture kernel using +the following command: + + kexec -p <dump-capture-kernel> \ + --initrd=<initrd-for-dump-capture-kernel> --args-linux \ + --append="root=<root-dev> init 1 irqpoll" + + +Notes on loading the dump-capture kernel: + +* <dump-capture-kernel> must be a vmlinux image (that is, an + uncompressed ELF image). bzImage does not work at this time. + +* By default, the ELF headers are stored in ELF64 format to support + systems with more than 4GB memory. The --elf32-core-headers option can + be used to force the generation of ELF32 headers. This is necessary + because GDB currently cannot open vmcore files with ELF64 headers on + 32-bit systems. ELF32 headers can be used on non-PAE systems (that is, + less than 4GB of memory). + +* The "irqpoll" boot parameter reduces driver initialization failures + due to shared interrupts in the dump-capture kernel. + +* You must specify <root-dev> in the format corresponding to the root + device name in the output of mount command. + +* "init 1" boots the dump-capture kernel into single-user mode without + networking. If you want networking, use "init 3." + + +Kernel Panic +============ + +After successfully loading the dump-capture kernel as previously +described, the system will reboot into the dump-capture kernel if a +system crash is triggered. Trigger points are located in panic(), +die(), die_nmi() and in the sysrq handler (ALT-SysRq-c). + +The following conditions will execute a crash trigger point: + +If a hard lockup is detected and "NMI watchdog" is configured, the system +will boot into the dump-capture kernel ( die_nmi() ). + +If die() is called, and it happens to be a thread with pid 0 or 1, or die() +is called inside interrupt context or die() is called and panic_on_oops is set, +the system will boot into the dump-capture kernel. + +On 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. + +For testing purposes, you can trigger a crash by using "ALT-SysRq-c", +"echo c > /proc/sysrq-trigger or write a module to force the panic. + +Write Out the Dump File +======================= + +After the dump-capture kernel is booted, write out the dump file with +the following command: cp /proc/vmcore <dump-file> - Dump memory can also be accessed as a /dev/oldmem device for a linear/raw - view. To create the device, type: +You can also access dumped memory as a /dev/oldmem device for a linear +and raw view. To create the device, use the following command: - mknod /dev/oldmem c 1 12 + mknod /dev/oldmem c 1 12 - Use "dd" with suitable options for count, bs and skip to access specific - portions of the dump. +Use the dd command with suitable options for count, bs, and skip to +access specific portions of the dump. - Entire memory: dd if=/dev/oldmem of=oldmem.001 +To see the entire memory, use the following command: + dd if=/dev/oldmem of=oldmem.001 -ANALYSIS + +Analysis ======== -Limited analysis can be done using gdb on the dump file copied out of -/proc/vmcore. Use vmlinux built with -g and run - gdb vmlinux <dump-file> +Before analyzing the dump image, you should reboot into a stable kernel. + +You can do limited analysis using GDB on the dump file copied out of +/proc/vmcore. Use the debug vmlinux built with -g and run the following +command: + + gdb vmlinux <dump-file> -Stack trace for the task on processor 0, register display, memory display -work fine. +Stack trace for the task on processor 0, register display, and memory +display work fine. -Note: gdb cannot analyse core files generated in ELF64 format for i386. +Note: GDB cannot analyze core files generated in ELF64 format for x86. +On systems with a maximum of 4GB of memory, you can generate +ELF32-format headers using the --elf32-core-headers kernel option on the +dump kernel. -Latest "crash" (crash-4.0-2.18) as available on Dave Anderson's site -http://people.redhat.com/~anderson/ works well with kdump format. +You can also use the Crash utility to analyze dump files in Kdump +format. Crash is available on Dave Anderson's site at the following URL: + http://people.redhat.com/~anderson/ + + +To Do +===== -TODO -==== -1) Provide a kernel pages filtering mechanism so that core file size is not - insane on systems having huge memory banks. -2) Relocatable kernel can help in maintaining multiple kernels for crashdump - and same kernel as the first kernel can be used to capture the dump. +1) Provide a kernel pages filtering mechanism, so core file size is not + extreme on systems with huge memory banks. +2) Relocatable kernel can help in maintaining multiple kernels for + crash_dump, and the same kernel as the system kernel can be used to + capture the dump. -CONTACT + +Contact ======= + Vivek Goyal (vgoyal@in.ibm.com) Maneesh Soni (maneesh@in.ibm.com) + + +Trademark +========= + +Linux is a trademark of Linus Torvalds in the United States, other +countries, or both. |