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

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Date:	Tue, 9 Nov 2010 14:21:38 +0100
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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>, avi@...hat.com,
	mtosatti@...hat.com
Subject: Re: [bisected] Clocksource tsc unstable git

On Tue, Nov 09, 2010 at 01:58:42PM +0100, Heiko Carstens wrote:
> 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?
> 
> I think there is a bug in pick_next_task_stop() in sched_stopclass.c:
> 
> If a stop-task scheduling class task (well... the migration thread ;) sets
> its state to TASK_INTERRUPTIBLE and then gets preempted it will never
> scheduled again, because pick_next_task_stop() ignores all tasks with a
> state != TASK_RUNNING:
> ...
> 
> I would guess something like the below would probably fix it.
> Does that make any sense or did I miss something obvious?

You've missed the fact that Peter already has a patch that fixes the
problem, but he never bothered to post it in this thread.

http://article.gmane.org/gmane.linux.kernel/1058018

-- 
Markus
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