linux-kernel - Re: [bisected] Clocksource tsc unstable git

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Date:	Wed, 27 Oct 2010 21:42:37 +0200
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	john stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...64.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...ux.intel.com" <hpa@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andreas Herrmann <andreas.herrmann3@....com>,
	heiko.carstens@...ibm.com, avi@...hat.com, mtosatti@...hat.com
Subject: Re: [bisected] Clocksource tsc unstable git

On Wed, Oct 27, 2010 at 09:36:55PM +0200, Peter Zijlstra wrote:
> On Wed, 2010-10-27 at 20:26 +0200, markus@...ppelsdorf.de wrote:
> > 
> > 34f971f6f7988be4d014eec3e3526bee6d007ffa is the first bad commit
> > commit 34f971f6f7988be4d014eec3e3526bee6d007ffa
> > Author: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Date:   Wed Sep 22 13:53:15 2010 +0200
> > 
> >     sched: Create special class for stop/migrate work
> > 
> >     In order to separate the stop/migrate work thread from the SCHED_FIFO
> >     implementation, create a special class for it that is of higher priority than
> >     SCHED_FIFO itself.
> > 
> >     This currently solves a problem where cpu-hotplug consumes so much cpu-time
> >     that the SCHED_FIFO class gets throttled, but has the bandwidth replenishment
> >     timer pending on the now dead cpu.
> > 
> >     It is also required for when we add the planned deadline scheduling class above
> >     SCHED_FIFO, as the stop/migrate thread still needs to transcent those tasks.
> > 
> >     Tested-by: Heiko Carstens <heiko.carstens@...ibm.com>
> >     Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> >     LKML-Reference: <1285165776.2275.1022.camel@...top>
> >     Signed-off-by: Ingo Molnar <mingo@...e.hu>
> > 
> > Reverting the commit solves the kvm hang issue.
> > (If this issue is related to my original tsc problem is of course open for
> > debate, but I have a strong hunch it is.) 
> 
> Too weird,.. what does the hang look like?
> 
> Can you generate a sysrq-t dump? The thing I'm looking for is the
> migration/# thread being runnable but not being current.
> 
> How can I reproduce this?

It's quite easy to reproduce on my setup. I simply run:

qemu-kvm -net nic,vlan=0,model=virtio -net user -drive file=ubuntu,if=virtio,boot=on -smp 2 -m 512

and then move the mousepointer in circles in the qemu window. It will stop and
freeze after a few seconds.
-- 
Markus
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