linux-kernel - Re: [Bug #12465] KVM guests stalling on 2.6.28 (bisected)

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Date:	Tue, 20 Jan 2009 12:35:46 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Kevin Shanahan <kmshanah@...b.org.au>, Avi Kivity <avi@...hat.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Kevin Shanahan <kmshanah@...xo.wumi.org.au>,
	Mike Galbraith <efault@....de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [Bug #12465] KVM guests stalling on 2.6.28 (bisected)

* Kevin Shanahan <kmshanah@...b.org.au> wrote:

> On Mon, 2009-01-19 at 22:45 +0100, Rafael J. Wysocki wrote:
> > This message has been generated automatically as a part of a report
> > of regressions introduced between 2.6.27 and 2.6.28.
> > 
> > The following bug entry is on the current list of known regressions
> > introduced between 2.6.27 and 2.6.28.  Please verify if it still should
> > be listed and let me know (either way).
> > 
> > 
> > Bug-Entry	: http://bugzilla.kernel.org/show_bug.cgi?id=12465
> > Subject		: KVM guests stalling on 2.6.28 (bisected)
> > Submitter	: Kevin Shanahan <kmshanah@...b.org.au>
> > Date		: 2009-01-17 03:37 (3 days old)
> 
> Yes, please keep this on the list.

This only seems to occur under KVM, right? I.e. you tested it with -no-kvm 
and the problem went away, correct?

This suggests some sort of KVM-specific problem. Scheduler latencies in 
the seconds that occur under normal load situations are noticed and 
reported quickly - and there are no such open regressions currently.

Avi, can you reproduce these latencies? A possibly theory would be some 
sort of guest wakeup problem/race triggered by a shift in 
preemption/scheduling patterns. Or something related to preempt-notifiers 
(which KVM is using). A genuine scheduler bug is in the cards too, but the 
KVM-only angle of this bug gives it a low probability.

	Ingo
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