linux-kernel - Re: KVM guest lockup

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Date:	Sat, 20 Dec 2008 13:15:23 +0100
From:	Jan Niehusmann <jan@...dor.com>
To:	linux-kernel@...r.kernel.org
Cc:	kvm@...r.kernel.org
Subject:  Re: KVM guest lockup

Max Kellermann wrote:
> I am running a KVM enabled AMD64 kernel, version 2.6.26 and 2.6.27
> (Debian kernel).  The guest has been tested with 2.6.26 up to
> 2.6.28-rc8.
> 
> Whenever I put high load on the guest (git-svn is quite good at
> triggering the problem), it becomes unresponsive after a few minutes.
> After a while, a "soft lockup" message appears on serial console, with
> a ridiculously large time span, machine still unresponsive.

You mean something like "BUG: soft lockup - CPU#0 stuck for 1179869794s!"?

I had similar lockups, also on 2.6.26 (amd64).
It turned out to be related to frequency scaling on the host. With 
frequency scaling enabled, I was able to lock up the VM within minutes 
by periodically switching between full load and idle. (running burnMMX 
for a few seconds and then sleeping for a few seconds in a loop)

After setting scaling_min_freq to the same value as scaling_max_freq, 
virtual machines became rock solid.

BTW often the lockups were not complete, but the VM resumed work after a 
while. (IIRC, ~1/2 hour, for sure much less than 1179869794s :-) )

Jan

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