477 lines
16 KiB
Plaintext
477 lines
16 KiB
Plaintext
================================================================
|
|
Documentation for Kdump - The kexec-based Crash Dumping Solution
|
|
================================================================
|
|
|
|
This document includes overview, setup and installation, and analysis
|
|
information.
|
|
|
|
Overview
|
|
========
|
|
|
|
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.
|
|
|
|
You can use common 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.
|
|
|
|
Kdump and kexec are currently supported on the x86, x86_64, ppc64, ia64,
|
|
and s390x architectures.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
Similarly on PPC64 machines first 32KB of physical memory is needed for
|
|
booting regardless of where the kernel is loaded and to support 64K page
|
|
size kexec backs up the first 64KB memory.
|
|
|
|
For s390x, when kdump is triggered, the crashkernel region is exchanged
|
|
with the region [0, crashkernel region size] and then the kdump kernel
|
|
runs in [0, crashkernel region size]. Therefore no relocatable kernel is
|
|
needed for s390x.
|
|
|
|
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. Optionally the size of the ELF header can also be passed
|
|
when using the elfcorehdr=[size[KMG]@]offset[KMG] syntax.
|
|
|
|
|
|
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
|
|
-------------------
|
|
|
|
1) Login as the root user.
|
|
|
|
2) Download the kexec-tools user-space package from the following URL:
|
|
|
|
http://kernel.org/pub/linux/utils/kernel/kexec/kexec-tools.tar.gz
|
|
|
|
This is a symlink to the latest version.
|
|
|
|
The latest kexec-tools git tree is available at:
|
|
|
|
git://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
|
|
and
|
|
http://www.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
|
|
|
|
There is also a gitweb interface available at
|
|
http://www.kernel.org/git/?p=utils/kernel/kexec/kexec-tools.git
|
|
|
|
More information about kexec-tools can be found at
|
|
http://www.kernel.org/pub/linux/utils/kernel/kexec/README.html
|
|
|
|
3) Unpack the tarball with the tar command, as follows:
|
|
|
|
tar xvpzf kexec-tools.tar.gz
|
|
|
|
4) Change to the kexec-tools directory, as follows:
|
|
|
|
cd kexec-tools-VERSION
|
|
|
|
5) Configure the package, as follows:
|
|
|
|
./configure
|
|
|
|
6) Compile the package, as follows:
|
|
|
|
make
|
|
|
|
7) Install the package, as follows:
|
|
|
|
make install
|
|
|
|
|
|
Build the system and dump-capture kernels
|
|
-----------------------------------------
|
|
There are two possible methods of using Kdump.
|
|
|
|
1) Build a separate custom dump-capture kernel for capturing the
|
|
kernel core dump.
|
|
|
|
2) Or use the system kernel binary itself as dump-capture kernel and there is
|
|
no need to build a separate dump-capture kernel. This is possible
|
|
only with the architectures which support a relocatable kernel. As
|
|
of today, i386, x86_64, ppc64 and ia64 architectures support relocatable
|
|
kernel.
|
|
|
|
Building a relocatable kernel is advantageous from the point of view that
|
|
one does not have to build a second kernel for capturing the dump. But
|
|
at the same time one might want to build a custom dump capture kernel
|
|
suitable to his needs.
|
|
|
|
Following are the configuration setting required for system and
|
|
dump-capture kernels for enabling kdump support.
|
|
|
|
System kernel config options
|
|
----------------------------
|
|
|
|
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.
|
|
|
|
Dump-capture kernel config options (Arch Independent)
|
|
-----------------------------------------------------
|
|
|
|
1) Enable "kernel crash dumps" support under "Processor type and
|
|
features":
|
|
|
|
CONFIG_CRASH_DUMP=y
|
|
|
|
2) 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.)
|
|
|
|
Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
|
|
--------------------------------------------------------------------
|
|
|
|
1) On i386, enable high memory support under "Processor type and
|
|
features":
|
|
|
|
CONFIG_HIGHMEM64G=y
|
|
or
|
|
CONFIG_HIGHMEM4G
|
|
|
|
2) On i386 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".)
|
|
|
|
3) If one wants to build and use a relocatable kernel,
|
|
Enable "Build a relocatable kernel" support under "Processor type and
|
|
features"
|
|
|
|
CONFIG_RELOCATABLE=y
|
|
|
|
4) 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. A suitable value depends upon
|
|
whether kernel is relocatable or not.
|
|
|
|
If you are using a relocatable kernel use CONFIG_PHYSICAL_START=0x100000
|
|
This will compile the kernel for physical address 1MB, but given the fact
|
|
kernel is relocatable, it can be run from any physical address hence
|
|
kexec boot loader will load it in memory region reserved for dump-capture
|
|
kernel.
|
|
|
|
Otherwise it should be the start of memory region reserved for
|
|
second kernel using boot parameter "crashkernel=Y@X". Here X is
|
|
start of memory region reserved for dump-capture kernel.
|
|
Generally X is 16MB (0x1000000). So you can set
|
|
CONFIG_PHYSICAL_START=0x1000000
|
|
|
|
5) Make and install the kernel and its modules. DO NOT add this kernel
|
|
to the boot loader configuration files.
|
|
|
|
Dump-capture kernel config options (Arch Dependent, ppc64)
|
|
----------------------------------------------------------
|
|
|
|
1) Enable "Build a kdump crash kernel" support under "Kernel" options:
|
|
|
|
CONFIG_CRASH_DUMP=y
|
|
|
|
2) Enable "Build a relocatable kernel" support
|
|
|
|
CONFIG_RELOCATABLE=y
|
|
|
|
Make and install the kernel and its modules.
|
|
|
|
Dump-capture kernel config options (Arch Dependent, ia64)
|
|
----------------------------------------------------------
|
|
|
|
- No specific options are required to create a dump-capture kernel
|
|
for ia64, other than those specified in the arch independent section
|
|
above. This means that it is possible to use the system kernel
|
|
as a dump-capture kernel if desired.
|
|
|
|
The crashkernel region can be automatically placed by the system
|
|
kernel at run time. This is done by specifying the base address as 0,
|
|
or omitting it all together.
|
|
|
|
crashkernel=256M@0
|
|
or
|
|
crashkernel=256M
|
|
|
|
If the start address is specified, note that the start address of the
|
|
kernel will be aligned to 64Mb, so if the start address is not then
|
|
any space below the alignment point will be wasted.
|
|
|
|
|
|
Extended crashkernel syntax
|
|
===========================
|
|
|
|
While the "crashkernel=size[@offset]" syntax is sufficient for most
|
|
configurations, sometimes it's handy to have the reserved memory dependent
|
|
on the value of System RAM -- that's mostly for distributors that pre-setup
|
|
the kernel command line to avoid a unbootable system after some memory has
|
|
been removed from the machine.
|
|
|
|
The syntax is:
|
|
|
|
crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
|
|
range=start-[end]
|
|
|
|
'start' is inclusive and 'end' is exclusive.
|
|
|
|
For example:
|
|
|
|
crashkernel=512M-2G:64M,2G-:128M
|
|
|
|
This would mean:
|
|
|
|
1) if the RAM is smaller than 512M, then don't reserve anything
|
|
(this is the "rescue" case)
|
|
2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
|
|
3) if the RAM size is larger than 2G, then reserve 128M
|
|
|
|
|
|
|
|
Boot into System Kernel
|
|
=======================
|
|
|
|
1) Update the boot loader (such as grub, yaboot, or lilo) configuration
|
|
files as necessary.
|
|
|
|
2) 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 (16MB) for the dump-capture kernel.
|
|
|
|
On x86 and x86_64, use "crashkernel=64M@16M".
|
|
|
|
On ppc64, use "crashkernel=128M@32M".
|
|
|
|
On ia64, 256M@256M is a generous value that typically works.
|
|
The region may be automatically placed on ia64, see the
|
|
dump-capture kernel config option notes above.
|
|
|
|
On s390x, typically use "crashkernel=xxM". The value of xx is dependent
|
|
on the memory consumption of the kdump system. In general this is not
|
|
dependent on the memory size of the production system.
|
|
|
|
Load the Dump-capture Kernel
|
|
============================
|
|
|
|
After booting to the system kernel, dump-capture kernel needs to be
|
|
loaded.
|
|
|
|
Based on the architecture and type of image (relocatable or not), one
|
|
can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz
|
|
of dump-capture kernel. Following is the summary.
|
|
|
|
For i386 and x86_64:
|
|
- Use vmlinux if kernel is not relocatable.
|
|
- Use bzImage/vmlinuz if kernel is relocatable.
|
|
For ppc64:
|
|
- Use vmlinux
|
|
For ia64:
|
|
- Use vmlinux or vmlinuz.gz
|
|
For s390x:
|
|
- Use image or bzImage
|
|
|
|
|
|
If you are using a uncompressed vmlinux image then use following command
|
|
to load dump-capture kernel.
|
|
|
|
kexec -p <dump-capture-kernel-vmlinux-image> \
|
|
--initrd=<initrd-for-dump-capture-kernel> --args-linux \
|
|
--append="root=<root-dev> <arch-specific-options>"
|
|
|
|
If you are using a compressed bzImage/vmlinuz, then use following command
|
|
to load dump-capture kernel.
|
|
|
|
kexec -p <dump-capture-kernel-bzImage> \
|
|
--initrd=<initrd-for-dump-capture-kernel> \
|
|
--append="root=<root-dev> <arch-specific-options>"
|
|
|
|
Please note, that --args-linux does not need to be specified for ia64.
|
|
It is planned to make this a no-op on that architecture, but for now
|
|
it should be omitted
|
|
|
|
Following are the arch specific command line options to be used while
|
|
loading dump-capture kernel.
|
|
|
|
For i386, x86_64 and ia64:
|
|
"1 irqpoll maxcpus=1 reset_devices"
|
|
|
|
For ppc64:
|
|
"1 maxcpus=1 noirqdistrib reset_devices"
|
|
|
|
For s390x:
|
|
"1 maxcpus=1 cgroup_disable=memory"
|
|
|
|
Notes on loading the dump-capture kernel:
|
|
|
|
* By default, the ELF headers are stored in ELF64 format to support
|
|
systems with more than 4GB memory. On i386, kexec automatically checks if
|
|
the physical RAM size exceeds the 4 GB limit and if not, uses ELF32.
|
|
So, on non-PAE systems, ELF32 is always used.
|
|
|
|
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.
|
|
|
|
* 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.
|
|
|
|
* Boot parameter "1" boots the dump-capture kernel into single-user
|
|
mode without networking. If you want networking, use "3".
|
|
|
|
* We generally don' have to bring up a SMP kernel just to capture the
|
|
dump. Hence generally it is useful either to build a UP dump-capture
|
|
kernel or specify maxcpus=1 option while loading dump-capture kernel.
|
|
|
|
* For s390x there are two kdump modes: If a ELF header is specified with
|
|
the elfcorehdr= kernel parameter, it is used by the kdump kernel as it
|
|
is done on all other architectures. If no elfcorehdr= kernel parameter is
|
|
specified, the s390x kdump kernel dynamically creates the header. The
|
|
second mode has the advantage that for CPU and memory hotplug, kdump has
|
|
not to be reloaded with kexec_load().
|
|
|
|
* For s390x systems with many attached devices the "cio_ignore" kernel
|
|
parameter should be used for the kdump kernel in order to prevent allocation
|
|
of kernel memory for devices that are not relevant for kdump. The same
|
|
applies to systems that use SCSI/FCP devices. In that case the
|
|
"allow_lun_scan" zfcp module parameter should be set to zero before
|
|
setting FCP devices online.
|
|
|
|
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 powerpc systems when a soft-reset is generated, die() is called by all cpus
|
|
and the 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>
|
|
|
|
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
|
|
|
|
Use the dd command with suitable options for count, bs, and skip to
|
|
access specific portions of the dump.
|
|
|
|
To see the entire memory, use the following command:
|
|
|
|
dd if=/dev/oldmem of=oldmem.001
|
|
|
|
|
|
Analysis
|
|
========
|
|
|
|
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, and memory
|
|
display work fine.
|
|
|
|
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.
|
|
|
|
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
|
|
=====
|
|
|
|
1) Provide relocatable kernels for all architectures to help in maintaining
|
|
multiple kernels for crash_dump, and the same kernel as the system kernel
|
|
can be used to capture the dump.
|
|
|
|
|
|
Contact
|
|
=======
|
|
|
|
Vivek Goyal (vgoyal@redhat.com)
|
|
Maneesh Soni (maneesh@in.ibm.com)
|
|
|