How to use kdump to debug kernel crashes

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Kernel and kdump

Kdump is a new kernel crash dumping mechanism and is very reliable because 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 capture kenrel, 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.

How to Use Kdump (i386)

Step 1: Configuring Kdump

- Install latest Fedora Core 5 Test 3 from http://torrent.fedoraproject.org/ or from http://download.fedora.redhat.com/pub/fedora/linux/core/test/4.92/

- Install "kernel-kdump", "kexec-tools" and "kernel-debuginfo" packages. Use following command line to install the packages.

"yum install kernel-kdump kexec-tools kernel-debuginfo"

- Boot first kernel with additional command line option "crashkernel=64M@16M". Edit /boot/grub/menu.lst and add "crashkernel=64M@16M" command line option. An example command line might look like as follows.

"kernel /vmlinuz-2.6.15-1.1955_FC5smp ro root=/dev/VolGroup00/LogVol00 rhgb console=tty0 console=ttyS0,115200 crashkernel=64M@16M"


Notes: 1. Above shown parameter reserves 64MB of physical memory starting at 16MB. This reserved memory is used to preload and run the capture kernel.

2. Init scripts take care of pre-loading the capture kernel at the system bootup time.

2. It is recommended to either setup a serial console or switch to run level 3 (init 3) for testing purposes. The reason being that kdump does not reset the console if you are in X or framebuffer mode, and no message might be visible on console after system crash.

Step 2: Capturing the Dump

Normally kernel panic() will trigger booting into capture kernel but for testing purposes one can simulate the trigger in one of the following ways.

- Trigger through /proc interface


- Trigger by inserting a module which calls panic().

System will boot into capture kernel. Dump will be automatically saved in /var/crash/<dumpdir> and system will boot back into regular kernel.


Step 3: Dump Analysis

- Open the vmcore using crash tool.


Note: <vmcore-dir> will be created under /var/crash depending on date and time of crash. For example, /var/crash/2006-02-17-17:02/vmcore.

More Documentation

- Kernel Source (Documentation/kdump/kdump.txt). - http://lse.sourceforge.net/kdump/


Kdump Setup (x86_64)

Step 1: Configuring Kdump

- Install latest Fedora Core 5 Test 3 from http://torrent.fedoraproject.org/ or from http://download.fedora.redhat.com/pub/fedora/linux/core/test/4.92/

- Install "kernel-kdump", "kexec-tools" and "kernel-debuginfo" packages. Use following command line to install the packages.

"yum install kernel-kdump kexec-tools kernel-debuginfo"

- Boot first kernel with additional command line option "crashkernel=64M@16M". Edit /boot/grub/menu.lst and add "crashkernel=64M@16M" command line option. An example command line might look like as follows.

"kernel /vmlinuz-2.6.15-1.1955_FC5smp ro root=/dev/VolGroup00/LogVol00 rhgb console=tty0 console=ttyS0,115200 crashkernel=64M@16M"


Notes: 1. Above shown parameter reserves 64MB of physical memory starting at 16MB. This reserved memory is used to preload and run the capture kernel.

2. Init scripts take care of pre-loading the capture kernel at the system bootup time.

2. It is recommended to either setup a serial console or switch to run level 3 (init 3) for testing purposes. The reason being that kdump does not reset the console if you are in X or framebuffer mode, and no message might be visible on console after system crash.

Step 2: Capturing the Dump

Normally kernel panic() will trigger booting into capture kernel but for testing purposes one can simulate the trigger in one of the following ways.

- Trigger through /proc interface


- Trigger by inserting a module which calls panic().

System will boot into capture kernel. Dump will be automatically saved in /var/crash/<dumpdir> and system will boot back into regular kernel.


Step 3: Dump Analysis

- Existing "crash" might not be new enough to open the crash dump. Download latest crash source from http://people.redhat.com/~anderson Build and install the "crash".

- Open the vmcore using crash tool.


Note: <vmcore-dir> will be created under /var/crash depending on date and time of crash. For example, /var/crash/2006-02-17-17:02/vmcore.

More Documentation

- Kernel Source (Documentation/kdump/kdump.txt). - http://lse.sourceforge.net/kdump/