Re: Hard lockup with 2.6.24.7-rt26 on x86

[Clark Williams <williams@redhat.com>]

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On Mon, 2 Feb 2009 21:55:36 +0100
m.luescher@vtxmail.ch wrote:

> Dear Linux rt users
> 
> On two different desktop PC (both Core 2 Duo) and on a notebook (Core Duo) the kernel (2.6.24.7-rt26, CONFIG_X86_32) locks up hard under a certain rt load. The failure can be reproduced reliably with the following command:
> 
> sudo ./cyclictest -p99 -t10 -n -i250
> 
> During the test I made some additional observations:
> - The lockup is really hard: there is no output on a netconsole and even the magic sysrequest keys to reboot the system (Alt+SysRQ+b) do not work anymore.
> - with a lower realtime priority the system seems to run at least more stable (e.g. sudo ./cyclictest -p95 -t10 -n -i250)
> - with the vanilla 2.6.26.6-rt11 kernel I observed probably the same crash
> - the problem also appears on Ubuntu Hardy Heron with the latest official kernel (2.6.24.23.25 with rt21 patch)
> - the problem did not appear on Ubuntu Hardy Heron with older rt-kernel versions (also 2.6.24 but based on rt3 patch)
> 
> So at the moment I am really clueless and I would be happy for any hint on how I can further debug this crash.
> 
> Best regards
> Matthias

My guess would be that since you're running 10 threads on two
processors at the highest available priority, you're starving all the
hard IRQ threads (as well as soft irq and other kernel threads).

This is one of those power-tool moments where you can lock up the
system with the wrong workload/priority combination. 

Can you lock up the system running at -p49 (I  suspect all your [IRQ-X]
threads are running at priority 50)?

Clark
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