linux-kernel - Re: crash with CFS v4 and qemu/kvm (was: [patch] CFS scheduler, v4)

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Date:	Tue, 24 Apr 2007 12:54:12 +0200
From:	Christian Hesse <mail@...thworm.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org, Con Kolivas <kernel@...ivas.org>,
	Nick Piggin <npiggin@...e.de>, Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Williams <pwil3058@...pond.net.au>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Willy Tarreau <w@....eu>,
	Gene Heskett <gene.heskett@...il.com>,
	kvm-devel@...ts.sourceforge.net, Avi Kivity <avi@...ranet.com>
Subject: Re: crash with CFS v4 and qemu/kvm (was: [patch] CFS scheduler, v4)

On Monday 23 April 2007, Ingo Molnar wrote:
> * Christian Hesse <mail@...thworm.de> wrote:
> > On Friday 20 April 2007, Ingo Molnar wrote:
> > > i'm pleased to announce release -v4 of the CFS patchset.
> >
> > Hi Ingo, hi Avi, hi all,
> >
> > I'm trying to use kvm-20 with cfs v4 and get a crash:
> >
> > eworm@...o:~$ /usr/local/kvm/bin/qemu -snapshot
> > /mnt/data/virtual/qemu/winxp.img kvm_run: failed entry, reason 7
> > kvm_run returned -8
> >
> > It works (though it is a bit slow) if I start qemu with strace, so for
> > me it looks like a race condition?
>
> hm. Can you work it around with:
>
>    echo 0 > /proc/sys/kernel/sched_granularity_ns
>
> ?
>
> If yes then this is a wakeup race: some piece of code relies on the
> upstream scheduler preempting the waker task immediately in 99% of the
> cases.
>
> and you might want to test -v5 too which i released earlier today. It
> has no bugfix in this area though, so it will likely still trigger this
> race - but it will also hopefully be even more pleasant to use than -v4
> ;-)

Hi Ingo,

This was kvm's fault. It works perfectly now without modifications to the 
scheduler. If anybody is interested in details please see the kvm mailing 
list archives.
-- 
Regards,
Chris

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