Kexec/Kdump HOWTO

Introduction

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.

Overview

Kexec

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

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

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. With the help of grubby, it's very easy to append

"crashkernel=128M" to the end of your kernel boot parameters. Note that the X

values are such that X = the amount of memory to reserve for the capture kernel.

And based on arch and system configuration, one might require more than 128M to

be reserved for kdump. One need to experiment and test kdump, if 128M is not

sufficient, try reserving more memory.

   # grubby --args="crashkernel=128M" --update-kernel=/boot/vmlinuz-`uname -r`

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:

crashkernel=range1:size1[,range2:size2,...][@offset]

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:

crashkernel=512M-2G:64M,2G-:128M

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.

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:

    # systemctl start kdump.service

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

    /var/crash/2006-08-23-15:34/vmcore

    crash> bt

and so on...

Saving vmcore-dmesg.txt

----------------------

Kernel log bufferes are one of the most important information available

in vmcore. Now before saving vmcore, kernel log bufferes are extracted

from /proc/vmcore and saved into a file vmcore-dmesg.txt. After

vmcore-dmesg.txt, vmcore is saved. Destination disk and directory for

vmcore-dmesg.txt is same as vmcore. Note that kernel log buffers will

not be available if dump target is raw device.

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:

http://kbase.redhat.com/faq/FAQ_43_5559.shtm

In addition, on PowerPC boxes, Kdump can also be triggered via Hardware

Management Console(HMC) using 'Ctrl', 'O' and 'C' keyboard keys.

2) NMI_WATCHDOG

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.

http://kbase.redhat.com/faq/FAQ_85_9129.shtm

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/systemctl restart kdump.service'.

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.

For both local filesystem and nfs dump the dump target must be mounted before

building kdump initramfs. That means one needs to put an entry for the dump

file system in /etc/fstab so that after reboot when kdump service starts,

it can find the dump target and build initramfs instead of failing.

Usually the dump target should be used only for kdump. If you worry about

someone uses the filesystem for something else other than dumping vmcore

you can mount it as read-only. Mkdumprd will still remount it as read-write

for creating dump directory and will move it back to read-only afterwards.

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/vg/lv_kdump is left unformatted, kdump.conf can be configured with

'raw /dev/vg/lv_kdump', and the vmcore file will be copied via dd directly

onto partition /dev/vg/lv_kdump. Restart the kdump service via

'/sbin/systemctl restart kdump.service' to commit this change to your kdump

initrd. Dump target should be persistent device name, such as lvm or device

mapper canonical name.

Dedicated file system

Similar to raw partition dumping, you can format a partition with the file

system of your choice, Again, it should be at least as large as the amount

of memory in the system. Assuming it should be at least as large as the

amount of memory in the system. Assuming /dev/vg/lv_kdump has been

formatted ext4, specify 'ext4 /dev/vg/lv_kdump' 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/systemctl restart kdump.service' 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.

It is recommended to use persistent device names or UUID/LABEL for file system

dumps. One example of persistent device is /dev/vg/<devname>.

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 nfs-server.example.com,

once the mount is properly configured, specify it in kdump.conf, via

'nfs nfs-server.example.com:/dump'. The server portion can be specified either

by host name or IP address. 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/systemctl restart kdump.service' 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 'ssh 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.

Once kdump.conf is appropriately configured, issue the command

'kdumpctl 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/systemctl restart kdump.service' to commit this change to your kdump initrd.

Path

====

"path" represents the file system path in which vmcore will be saved. In

fact kdump creates a directory $hostip-$date with-in "path" and saves

vmcore there. So practically dump is saved in $path/$hostip-$date/. To

simplify discussion further, if we say dump will be saved in $path, it

is implied that kdump will create another directory inside path and

save vmcore there.

If a dump target is specified in kdump.conf, then "path" is relative to the

specified dump target. For example, if dump target is "ext4 /dev/sda", then

dump will be saved in "$path" directory on /dev/sda.

Same is the case for nfs dump. If user specified "nfs foo.com:/export/tmp/"

as dump target, then dump will effectively be saved in

"foo.com:/export/tmp/var/crash/" directory.

Interpretation of path changes a bit if user has not specified a dump

target explicitly in kdump.conf. In this case, "path" represents the

absolute path from root. And dump target and adjusted path are arrived

at automatically depending on what's mounted in the current system.

Following are few examples.

path /var/crash/

----------------

Assuming there is no disk mounted on /var/ or on /var/crash, dump will

be saved on disk backing rootfs in directory /var/crash.

path /var/crash/ (A separate disk mounted on /var)

--------------------------------------------------

Say a disk /dev/sdb is mouted on /var. In this case dump target will

become /dev/sdb and path will become "/crash" and dump will be saved

on "sdb:/crash/" directory.

path /var/crash/ (NFS mounted on /var)

-------------------------------------

Say foo.com:/export/tmp is mounted on /var. In this case dump target is

nfs server and path will be adjusted to "/crash" and dump will be saved to

foo.com:/export/tmp/crash/ directory.

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.

Kdump Pre-Capture Executable

It is possible to specify a custom script or binary you wish to run before

capturing a vmcore. Exit status of this binary is interpreted:

0 - continue with dump process as usual

non 0 - reboot the system

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

initrd.

Default action

==============

Default action specifies what to do when dump to configured dump target

fails. By default, default action is "reboot" and that is system reboots

if attempt to save dump to dump target fails.

There are other default actions available though.

- dump_to_rootfs

	This option tries to mount root and save dump on root filesystem

	in a path specified by "path". This option will generally make

	sense when dump target is not root filesystem. For example, if

	dump is being saved over network using "ssh" then one can specify

	default to "dump_to_rootfs" to try saving dump to root filesystem

	if dump over network fails.

- shell

	Drop into a shell session inside initramfs.

- halt

	Halt system after failure

- poweroff

	Poweroff system after failure.

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 -F -l --message-level 1 -d 31',

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.

Core collector command format depends on dump target type. Typically for

filesystem (local/remote), core_collector should accept two arguments.

First one is source file and second one is target file. For ex.

ex1.

---

core_collector "cp --sparse=always"

Above will effectively be translated to:

cp --sparse=always /proc/vmcore <dest-path>/vmcore

ex2.

---

core_collector "makedumpfile -l --message-level 1 -d 31"

Above will effectively be translated to:

makedumpfile -l --message-level 1 -d 31 /proc/vmcore <dest-path>/vmcore

For dump targets like raw and ssh, in general, core collector should expect

one argument (source file) and should output the processed core on standard

output (There is one exception of "scp", discussed later). This standard

output will be saved to destination using appropriate commands.

raw dumps core_collector examples:

---------

ex3.

---

core_collector "cat"

Above will effectively be translated to.

cat /proc/vmcore | dd of=<target-device>

ex4.

---

core_collector "makedumpfile -F -l --message-level 1 -d 31"

Above will effectively be translated to.

makedumpfile -F -l --message-level 1 -d 31 | dd of=<target-device>

ssh dumps core_collector examples:

---------

ex5.

---

core_collector "cat"

Above will effectively be translated to.

cat /proc/vmcore | ssh <options> <remote-location> "dd of=path/vmcore"

ex6.

---

core_collector "makedumpfile -F -l --message-level 1 -d 31"

Above will effectively be translated to.

makedumpfile -F -l --message-level 1 -d 31 | ssh <options> <remote-location> "dd of=path/vmcore"

There is one exception to standard output rule for ssh dumps. And that is

scp. As scp can handle ssh destinations for file transfers, one can

specify "scp" as core collector for ssh targets (no output on stdout).

ex7.

----

core_collector "scp"

Above will effectively be translated to.

scp /proc/vmcore <user@host>:path/vmcore

About default core collector

----------------------------

Default core_collector for ssh/raw dump is:

"makedumpfile -F -l --message-level 1 -d 31".

Default core_collector for other targets is:

"makedumpfile -l --message-level 1 -d 31".

Even if core_collector option is commented out in kdump.conf, makedumpfile

is default core collector and kdump uses it internally.

If one does not want makedumpfile as default core_collector, then they

need to specify one using core_collector option to change the behavior.

Note: If "makedumpfile -F" is used then you will get a flattened format

vmcore.flat, you will need to use "makedumpfile -R" to rearrange the

dump data from stdard input to a normal dumpfile (readable with analysis

tools).

For example: "makedumpfile -R vmcore < vmcore.flat"

Caveats:

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.

Notes on rootfs mount:

Dracut is designed to mount rootfs by default. If rootfs mounting fails it

will refuse to go on. So kdump leaves rootfs mounting to dracut currently.

We make the assumtion that proper root= cmdline is being passed to dracut

initramfs for the time being. If you need modify "KDUMP_COMMANDLINE=" in

/etc/sysconfig/kdump, you will need to make sure that appropriate root=

options are copied from /proc/cmdline. In general it is best to append

command line options using "KDUMP_COMMANDLINE_APPEND=" instead of replacing

the original command line completely.

Debugging Tips

--------------

- One can drop into a shell before/after saving vmcore with the help of

  using kdump_pre/kdump_post hooks. Use following in one of the pre/post

  scripts to drop into a shell.

  #!/bin/bash

  _ctty=/dev/ttyS0

  setsid /bin/sh -i -l 0<>$_ctty 1<>$_ctty 2<>$_ctty

  One might have to change the terminal depending on what they are using.

- Serial console logging for virtual machines

  I generally use "virsh console <domain-name>" to get to serial console.

  I noticed after dump saving system reboots and when grub menu shows up

  some of the previously logged messages are no more there. That means

  any important debugging info at the end will be lost.

  One can log serial console as follows to make sure messages are not lost.

  virsh ttyconsole <domain-name>

  ln -s <name-of-tty> /dev/modem

  minicom -C /tmp/console-logs

  Now minicom should be logging serial console in file console-logs.