Commit dc851a0f authored by David Wilder's avatar David Wilder Committed by Linus Torvalds
Browse files

[PATCH] Updated kdump documentation



Cc: Vivek Goyal <vgoyal@in.ibm.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 8ea2c2ec
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.
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