Re: oops in :snd_pcm_oss:resample_expand+0x19c/0x1f0

From: Jean-Marc Saffroy
Date: Sun Sep 24 2006 - 18:03:28 EST

On Sun, 24 Sep 2006, Andrew Morton wrote:

I keep a crash dump from kdump,

Whoa. Impressed. Which distro are you using and how did you go about this
and how much fuss was it to set up?

Well, it sure didn't work right out of the box... But it's not that hard, really: the main problems are lack of proper end-user docs and packaging, as usual. A serial console on another PC very much helped in troubleshooting problems (eg. my kdump kernel's initrd was initially too large), but it's not required (though it's always a nice thing to have anyway).

The basic instructions for setting up kdump are here:

I followed them, and also had to:
- reduce the uncompressed size of my initramfs (was 50+ MB initially, now ca. 22MB), by listing only the required modules (lsmod w/ some hand picking), see /etc/initramfs-tools/initramfs.conf
- reserve more RAM for the kdump kernel (now I use "crashkernel=96M@16M")

The distro is the AMD64 Debian etch, with two vanilla 2.6.18 kernels: one "regular" SMP kernel with CONFIG_KEXEC=y, which is what I use, and one UP kernel with CONFIG_PROC_VMCORE=y and a different load address, which is activated on a crash; other than that they are the same.

The crude script below is run at boot time and configures kdump properly for me. All oopses invoke panic ("kernel.panic_on_oops = 1" in /etc/sysctl.conf), and thus the kdump kernel is activated. This kernel boots on the very same root fs, runs the script below again, this time to save the dump and reboot to the regular kernel. I did not try to have the kdump kernel reset the vga console, so it stays in graphic mode and shows garbage until the final reboot.

Now from an oops to being back to work, I would say it takes about 2-3 minutes (the longer part is the cold reboot, then the 30" timeout on the Windows loader ;-)). I would certainly not do kernel development on my desktop (backups? what backups?), but oopses do occur from time to time (oh the joys of SMP on a desktop), and it's great to have a dump the first time I see one.

Oh and I wish I could use gdb on a kdump core. :-)

Would be nice, but this is much better than what we usually get, no?

I'm used to having LKCD for debugging at work (on IA64), it has raised my expectations. :-)

But using gdb would be so much nicer, and I'm sure we're close to being able to use it, it's probably only a matter of messing a bit with the ELF image provided by kdump (it's really a good old core dump): ie. remapping vmalloc'ed memory pages, so modules can be accessed, and ideally adding processor context for each task (currently there is one per CPU). Crash does all this stuff internally and has other nice commands, but I don't find it quite as comfortable as gdb (with a few good scripts gdb would certainly be on par with crash).

Now I hope you're converted!



#! /bin/bash
# Provides: kdump
# Required-Start:
# Required-Stop:
# Default-Start: S
# Default-Stop:
# This script configures a kdump kernel, or saves a dump and reboot.

_echo() {
echo "kdump: $@"
logger -p daemon.notice "kdump: $@"
echo "$(date) $@" >> /var/log/kdump.log

if grep -q crashkernel /proc/cmdline ; then
_echo "Configuring kdump kernel..."
_opts="$(sed -e 's,crashkernel=[^ ]*,,' /proc/cmdline)"
if kexec -p -t elf-x86_64 --args-linux \
--append="${_opts} init 1 irqpoll maxcpus=1" \
--initrd=/boot/initrd.img-kdump \
_echo "Successfully configured kdump kernel."
_echo "Failed to configure kdump kernel!"

if grep -q irqpoll /proc/cmdline ; then
_dump=/var/dump/dump-$(date +%F_%T)
_echo "Saving kernel dump under ${_dump}..."
cp /proc/vmcore "${_dump}"
ls -l /proc/vmcore "${_dump}"
chgrp adm "${_dump}"
chmod g+r "${_dump}"
_echo "Done saving kernel dump (rc=$rc), rebooting..."

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