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

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Date:	Mon, 23 Apr 2007 12:18:39 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christian Hesse <mail@...thworm.de>
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)


* 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 
;-)

	Ingo
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