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Kexec/Kdump HOWTO


Kexec and kdump are new features in the 2.6 mainstream kernel. These features are included in Red Hat Enterprise Linux 5. The purpose of these features is to ensure faster boot up and creation of reliable kernel vmcores for diagnostic purposes.



Kexec is a fastboot mechanism which allows booting a Linux kernel 
from the context of already running kernel without going through BIOS. 
BIOS can be very time consuming especially on 
the big servers with lots of peripherals. This can save a lot of 
time for developers who end up booting a machine numerous times.


Kdump is a new kernel crash dumping mechanism and is very reliable because the crash dump is
captured from the context of a freshly booted kernel and not from the context of the crashed kernel. 
Kdump uses kexec to boot into a second kernel whenever system crashes. 
This second kernel, often called a capture kernel, boots with very little memory and captures the dump image.

The first kernel reserves a section of memory that the second kernel uses to boot. 
Kexec enables booting the capture kernel without going through BIOS hence contents 
of first kernel's memory are preserved, which is essentially the kernel crash dump.

Kdump is supported on the i686, x86_64, ia64 and ppc64 platforms. The standard kernel and capture 
kernel are one in the same on i686, x86_64,ia64 and ppc64. If you're reading this document, 
you should already have kexec-tools installed. If not, you install it via the following command:

    # yum install kexec-tools

Now load a kernel with kexec:

    # kver=`uname -r`
    # kexec -l /boot/vmlinuz-$kver \
    --initrd=/boot/initrd-$kver.img \
        --command-line="`cat /proc/cmdline`"

NOTE: The above will boot you back into the kernel you're currently running,
if you want to load a different kernel, substitute it in place of `uname -r`.

Now reboot your system, taking note that it should bypass the BIOS:

    # reboot

Kdump Operational Flow

Given its purpose, kdump must provide both flexibility in dump capture
configuration, resiliance to errors encountered during the capture
process, and a default out-of-the box configuration that allows for
rudimentary captures to work without any addition setup.  Given these
constraints, kdump implements its capture process in two stages:

A) An initramfs, which attempts to capture the vmcore without having to touch
the systems rootfs data

B) An initscript stored on the rootfs, which captures a vmcore using a simple
copy operation to the local /var/crash directory (if a capture is needed) when
the kdump service starts.

(A) is the recommended capture approach, as it is the most likely to succeded
(its resilient to corruption in the root filesystem).  It uses the stored
configuration in /etc/kdump.conf to control its actions.  (B) serves as both
the default configuration action (when no /etc/kdump.conf configuration is
provided, the kdump initramfs acts just as the normal kernel initramfs does,
mounting the rootfs, and runing /sbin/init so that the kdump initscript can be
executed).  This is also the default behavior should the configured capture
process fail in (A).  Note that when using (B), all settings in
/etc/kdump.conf are ignored.  This is because (B) is meant to be used as a
last ditch capture effort.  We are using method (B) beacuse we are either
using the default configuration, or something went wrong with the applied
configuration.  In either case, no special settings are honored, we simply
attempt to capture the core in as simple a manner as possible.

the control flow between the two works as follows:
1) System panics
2) kdump kernel boots
3) kdump initramfs loads and runs /init
4) is a dump target configured in /etc/kdump.conf. If not, determine the
   root filesystem block device and use that as the default dump target
5) capture dump according to /etc/kdump.conf
6) was capture successful (yes goto 12, no goto 7)
7) does /etc/kdump.conf set default_action to halt (yes goto 14, no goto 8)
8) does /etc/kdump.conf set default_action to reboot (yes goto 12, no goto 9)
9) mount root filesystem, pivot_root, run /sbin/init
10) start kdump service via /etc/init.d/kdump
11) capture core via cp /proc/vmcore /var/crash/<host-addr>-<date>/vmcore
12) reboot
13) drop to shell
14) halt system

How to configure kdump:

Again, we assume if you're reading this document, you should already have kexec-tools installed. If not, you install it via the following command:

    # yum install kexec-tools

To be able to do much of anything interesting in the way of debug analysis, you'll also need to install the kernel-debuginfo package, of the same arch as your running kernel, and the crash utility:

    # yum --enablerepo=\*debuginfo install kernel-debuginfo.$(uname -m) crash

Next up, we need to modify some boot parameters to reserve a chunk of memory for the capture kernel. For i686 and x86_64, edit /etc/grub.conf, and append "crashkernel=128M" to the end of your kernel line. Similarly, append the same to the append line in /etc/yaboot.conf for ppc64. On ia64, edit /etc/elilo.conf, adding "crashkernel=256M" to the append line for your kernel. Note that the X values are such that X = the amount of memory to reserve for the capture kernel.

Note that there is an alternative form in which to specify a crashkernel memory reservation, in the event that more control is needed over the size and placement of the reserved memory. The format is:


Where range<n> specifies a range of values that are matched against the amount
of physical RAM present in the system, and the corresponding size<n> value
specifies the amount of kexec memory to reserve.  For example:


This line tells kexec to reserve 64M of ram if the system contains between
512M and 2G of physical memory.  If the system contains 2G or more of physical
memory, 128M should be reserved.

There is also a third specification for the crashkernel parameter:
This specification allows the kernel to decide how much meory to reserve for the
purposes of kdump.  It will make this determination based on the amount of
memory you have in your system, and scale the allocation accordingly.  Note that
if you have less than 4Gb of memory in your system, this specification will  opt
to not allocate any memory for the purposes of kdump.

# grub.conf generated by anaconda
# Note that you do not have to rerun grub after making changes to this file
# NOTICE:  You have a /boot partition.  This means that
#          all kernel and initrd paths are relative to /boot/, eg.
#          root (hd0,0)
#          kernel /vmlinuz-version ro root=/dev/VolGroup00/LogVol00
#          initrd /initrd-version.img
title Red Hat Enterprise Linux (2.6.18-8.el5)
        root (hd0,0)
        kernel /vmlinuz-2.6.18-8.el5 ro root=/dev/VolGroup00/LogVol00 crashkernel=128M@16M
        initrd /initrd-2.6.18-8.el5.img

# cat /etc/yaboot.conf
# yaboot.conf generated by anaconda

init-message=Welcome to Red Hat Enterprise Linux!\nHit <TAB> for boot options

        label=linux read-only
        append="root=LABEL=/ crashkernel=128M"

# cat /etc/elilo.conf

        initrd=initrd-2.6.17-1.2621.el5.img read-only
        append="-- root=LABEL=/ crashkernel=256M"

After making said changes, reboot your system, so that the X MB of memory is
left untouched by the normal system, reserved for the capture kernel. Take note
that the output of 'free -m' will show X MB less memory than without this
parameter, which is expected. You may be able to get by with less than 128M, but
testing with only 64M has proven unreliable of late. On ia64, as much as 512M
may be required.

Now that you've got that reserved memory region set up, you want to turn on
the kdump init script:

    # chkconfig kdump on

Then, start up kdump as well:

    # service kdump start

This should load your kernel-kdump image via kexec, leaving the system ready
to capture a vmcore upon crashing. To test this out, you can force-crash
your system by echo'ing a c into /proc/sysrq-trigger:

    # echo c > /proc/sysrq-trigger

You should see some panic output, followed by the system restarting into
the kdump kernel. When the boot process gets to the point where it starts
the kdump service, your vmcore should be copied out to disk (by default,
in /var/crash/<YYYY-MM-DD-HH:MM>/vmcore), then the system rebooted back into
your normal kernel.

Once back to your normal kernel, you can use the previously installed crash
kernel in conjunction with the previously installed kernel-debuginfo to
perform postmortem analysis:

    # crash /usr/lib/debug/lib/modules/2.6.17-1.2621.el5/vmlinux

    crash> bt

and so on...

Dump Triggering methods:

This section talks about the various ways, other than a Kernel Panic, in which
Kdump can be triggered. The following methods assume that Kdump is configured
on your system, with the scripts enabled as described in the section above.

1) AltSysRq C

Kdump can be triggered with the combination of the 'Alt','SysRq' and 'C'
keyboard keys. Please refer to the following link for more details:

In addition, on PowerPC boxes, Kdump can also be triggered via Hardware
Management Console(HMC) using 'Ctrl', 'O' and 'C' keyboard keys.


In case a machine has a hard hang, it is quite possible that it does not
respond to keyboard interrupts. As a result 'Alt-SysRq' keys will not help
trigger a dump. In such scenarios Nmi Watchdog feature can prove to be useful.
The following link has more details on configuring Nmi watchdog option.

Once this feature has been enabled in the kernel, any lockups will result in an
OOPs message to be generated, followed by Kdump being triggered.

3) Kernel OOPs

If we want to generate a dump everytime the Kernel OOPses, we can achieve this
by setting the 'Panic On OOPs' option as follows:

    # echo 1 > /proc/sys/kernel/panic_on_oops

This is enabled by default on RHEL5.

4) NMI(Non maskable interrupt) button

In cases where the system is in a hung state, and is not accepting keyboard
interrupts, using NMI button for triggering Kdump can be very useful. NMI
button is present on most of the newer x86 and x86_64 machines. Please refer
to the User guides/manuals to locate the button, though in most occasions it
is not very well documented. In most cases it is hidden behind a small hole
on the front or back panel of the machine. You could use a toothpick or some
other non-conducting probe to press the button.

For example, on the IBM X series 366 machine, the NMI button is located behind
a small hole on the bottom center of the rear panel.

To enable this method of dump triggering using NMI button, you will need to set
the 'unknown_nmi_panic' option as follows:

   # echo 1 > /proc/sys/kernel/unknown_nmi_panic

5) PowerPC specific methods:

On IBM PowerPC machines, issuing a soft reset invokes the XMON debugger(if
XMON is configured). To configure XMON one needs to compile the kernel with
the CONFIG_XMON and CONFIG_XMON_DEFAULT options, or by compiling with
CONFIG_XMON and booting the kernel with xmon=on option.

Following are the ways to remotely issue a soft reset on PowerPC boxes, which
would drop you to XMON. Pressing a 'X' (capital alphabet X) followed by an
'Enter' here will trigger the dump.

5.1) HMC

Hardware Management Console(HMC) available on Power4 and Power5 machines allow
partitions to be reset remotely. This is specially useful in hang situations
where the system is not accepting any keyboard inputs.

Once you have HMC configured, the following steps will enable you to trigger
Kdump via a soft reset:

On Power4
  Using GUI

    * In the right pane, right click on the partition you wish to dump.
    * Select "Operating System->Reset".
    * Select "Soft Reset".
    * Select "Yes".

  Using HMC Commandline

    # reset_partition -m <machine> -p <partition> -t soft

On Power5
  Using GUI

    * In the right pane, right click on the partition you wish to dump.
    * Select "Restart Partition".
    * Select "Dump".
    * Select "OK".

  Using HMC Commandline

    # chsysstate -m <managed system name> -n <lpar name> -o dumprestart -r lpar

5.2) Blade Management Console for Blade Center

To initiate a dump operation, go to Power/Restart option under "Blade Tasks" in
the Blade Management Console. Select the corresponding blade for which you want
to initate the dump and then click "Restart blade with NMI". This issues a
system reset and invokes xmon debugger.

Advanced Setups:

In addition to being able to capture a vmcore to your system's local file
system, kdump can be configured to capture a vmcore to a number of other
locations, including a raw disk partition, a dedicated file system, an NFS
mounted file system, or a remote system via ssh/scp. Additional options
exist for specifying the relative path under which the dump is captured,
what to do if the capture fails, and for compressing and filtering the dump
(so as to produce smaller, more manageable, vmcore files).

In theory, dumping to a location other than the local file system should be
safer than kdump's default setup, as its possible the default setup will try
dumping to a file system that has become corrupted. The raw disk partition and
dedicated file system options allow you to still dump to the local system,
but without having to remount your possibly corrupted file system(s),
thereby decreasing the chance a vmcore won't be captured. Dumping to an
NFS server or remote system via ssh/scp also has this advantage, as well
as allowing for the centralization of vmcore files, should you have several
systems from which you'd like to obtain vmcore files. Of course, note that
these configurations could present problems if your network is unreliable.

Advanced setups are configured via modifications to /etc/kdump.conf,
which out of the box, is fairly well documented itself. Any alterations to
/etc/kdump.conf should be followed by a restart of the kdump service, so
the changes can be incorporated in the kdump initrd. Restarting the kdump
service is as simple as '/sbin/service kdump restart'.

Note that kdump.conf is used as a configuration mechanism for capturing dump
files from the initramfs (in the interests of safety), the root file system is
mounted, and the init process is started, only as a last resort if the
initramfs fails to capture the vmcore.  As such, configuration made in
/etc/kdump.conf is only applicable to capture recorded in the initramfs.  If
for any reason the init process is started on the root file system, only a
simple copying of the vmcore from /proc/vmcore to /var/crash/$DATE/vmcore will
be preformed.

Raw partition

Raw partition dumping requires that a disk partition in the system, at least
as large as the amount of memory in the system, be left unformatted. Assuming
/dev/sda5 is left unformatted, kdump.conf can be configured with 'raw
/dev/sda5', and the vmcore file will be copied via dd directly onto partition
/dev/sda5. Restart the kdump service via '/sbin/service kdump restart'
to commit this change to your kdump initrd.

Dedicated file system

Similar to raw partition dumping, you can format a partition with the file
system of your choice, leaving it unmounted during normal operation. Again,
it should be at least as large as the amount of memory in the system. Assuming
/dev/sda3 has been formatted ext4, specify 'ext4 /dev/sda3' in kdump.conf,
and a vmcore file will be copied onto the file system after it has been
mounted. Dumping to a dedicated partition has the advantage that you can dump
multiple vmcores to the file system, space permitting, without overwriting
previous ones, as would be the case in a raw partition setup. Restart the
kdump service via '/sbin/service kdump restart' to commit this change to
your kdump initrd.  Note that for local file systems ext4 and ext2 are
supported as dumpable targets.  Kdump will not prevent you from specifying
other filesystems, and they will most likely work, but their operation
cannot be guaranteed.  for instance specifying a vfat filesystem or msdos
filesystem will result in a successful load of the kdump service, but during
crash recovery, the dump will fail if the system has more than 2GB of memory
(since vfat and msdos filesystems do not support more than 2GB files).
Be careful of your filesystem selection when using this target.

NFS mount

Dumping over NFS requires an NFS server configured to export a file system
with full read/write access for the root user. All operations done within
the kdump initial ramdisk are done as root, and to write out a vmcore file,
we obviously must be able to write to the NFS mount. Configuring an NFS
server is outside the scope of this document, but either the no_root_squash
or anonuid options on the NFS server side are likely of interest to permit
the kdump initrd operations write to the NFS mount as root.

Assuming your're exporting /dump on the machine,
once the mount is properly configured, specify it in kdump.conf, via 'net'. The server portion can be specified either
by host name or IP address. Note that if a host name is used, it should be a
fully qualified domain name. Following a system crash, the kdump initrd will
mount the NFS mount and copy out the vmcore to your NFS server. Restart the
kdump service via '/sbin/service kdump restart' to commit this change to
your kdump initrd.

Remote system via ssh/scp

Dumping over ssh/scp requires setting up passwordless ssh keys for every
machine you wish to have dump via this method. First up, configure kdump.conf
for ssh/scp dumping, adding a config line of 'net user@server', where 'user'
can be any user on the target system you choose, and 'server' is the host
name or IP address of the target system. Using a dedicated, restricted user
account on the target system is recommended, as there will be keyless ssh
access to this account. Note that the fully qualified domain name of the
server should also be used in this configuration.

Once kdump.conf is appropriately configured, issue the command '/sbin/service
kdump propagate' to automatically set up the ssh host keys and transmit
the necessary bits to the target server. You'll have to type in 'yes'
to accept the host key for your targer server if this is the first time
you've connected to it, and then input the target system user's password
to send over the necessary ssh key file. Restart the kdump service via
'/sbin/service kdump restart' to commit this change to your kdump initrd.


By default, local file system vmcore files are written to /var/crash/%DATE
on the local system, ssh/scp dumps to /var/crash/%HOST-%DATE on the target
system, dedicated file system partition dumps to ./var/crash/%DATE, and
NFS dumps to ./var/crash/%HOST-%DATE, the latter two both relative to
their respective mount points within the kdump initrd (usually /mnt). The
'/var/crash' portion of the path can be overridden using kdump.conf's 'path'
variable, should you wish to write the vmcore out to a different location. For
example, 'path /data/coredumps' would lead to vmcore files being written to
/data/coredumps/%DATE if you were dumping to your local file system.  Note
that the path option is ingnored if your kdump configuration results in the
core being saved from the initscripts in the root filesystem.

Kdump Post-Capture Executable

It is possible to specify a custom script or binary you wish to run following
an attempt to capture a vmcore. The executable is passed an exit code from
the capture process, which can be used to trigger different actions from
within your post-capture executable.

Extra Binaries

If you have specific binaries or scripts you want to have made available
within your kdump initrd, you can specify them by their full path, and they
will be included in your kdump initrd, along with all dependent libraries.
This may be particularly useful for those running post-capture scripts that
rely on other binaries.

Extra Modules

By default, only the bare minimum of kernel modules will be included in your
kdump initrd. Should you wish to capture your vmcore files to a non-boot-path
storage device, such as an iscsi target disk or clustered file system, you may
need to manually specify additional kernel modules to load into your kdump

Default action

By default, if a configured dump method fails, the kdump initrd falls back
to trying to dump to the local file system (i.e., into the file system(s)
you would have mounted under normal system operation). The system always
reboots following an attempted dump to your local file system, regardless
of success or failure.

However, for any of the advanced methods, if the dump fails, you can configure
the kdump initrd to skip trying to dump to the local file system, instead
immediately rebooting ('default reboot'), halting the system ('default halt')
or dropping you to a shell within the initrd ('default shell'), from which you
could try to capture the vmcore manually. Again, if the 'default' parameter is
unset, a local file system dump will be attempted, then the system will reboot.

Compression and filtering

The 'core_collector' parameter in kdump.conf allows you to specify a custom
dump capture method. The most common alternate method is makedumpfile, which
is a dump filtering and compression utility provided with kexec-tools. On
some architectures, it can drastically reduce the size of your vmcore files,
which becomes very useful on systems with large amounts of memory.

A typical setup is 'core_collector makedumpfile -c', but check the output of
'/sbin/makedumpfile --help' for a list of all available options (-i and -g
don't need to be specified, they're automatically taken care of). Note that
use of makedumpfile requires that the kernel-debuginfo package corresponding
with your running kernel be installed.

If you use other command as a 'core_collector', probably you also need to
specify its full path in 'extra_bins' by hand.

Also note that makedumpfile is only used from the initramfs.  Saving a
core from the initscript in the root filesystem is considered a last ditch
effort, only used when the initramfs has failed to save the core properly.
As such only the cp utiltiy is used in the initscripts.  The implication
here is that in order to use makedumpfile as your core collector, you must
specify a dump target in /etc/kdump.conf.


Console frame-buffers and X are not properly supported. If you typically run
with something along the lines of "vga=791" in your kernel config line or
have X running, console video will be garbled when a kernel is booted via
kexec. Note that the kdump kernel should still be able to create a dump,
and when the system reboots, video should be restored to normal.

Notes on resetting video:

Video is a notoriously difficult issue with kexec.  Video cards contain ROM code
that controls their initial configuration and setup.  This code is nominally
accessed and executed from the Bios, and otherwise not safely executable. Since
the purpose of kexec is to reboot the system without re-executing the Bios, it
is rather difficult if not impossible to reset video cards with kexec.  The
result is, that if a system crashes while running in a graphical mode (i.e.
running X), the screen may appear to become 'frozen' while the dump capture is
taking place.  A serial console will of course reveal that the system is
operating and capturing a vmcore image, but a casual observer will see the
system as hung until the dump completes and a true reboot is executed.

There are two possiblilties to work around this issue.  One is by adding
--reset-vga to the kexec command line options in /etc/sysconfig/kdump.  This
tells kdump to write some reasonable default values to the video card register
file, in the hopes of returning it to a text mode such that boot messages are
visible on the screen.  It does not work with all video cards however.
Secondly, it may be worth trying to add vga15fb.ko to the extra_modules list in
/etc/kdump.conf.  This will attempt to use the video card in framebuffer mode,
which can blank the screen prior to the start of a dump capture.

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Page last modified on July 04, 2012, at 02:03 AM