Re: [bisected] Clocksource tsc unstable git

Markus Trippelsdorf
2010-11-09T14:23:13Z

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@trippelsdorf.de wrote:
> > >
> > > 34f971f6f7988be4d014eec3e3526bee6d007ffa is the first bad commit
> > > commit 34f971f6f7988be4d014eec3e3526bee6d007ffa
> > > Author: Peter Zijlstra <a.p.zijlstra@chello.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@de.ibm.com>
> > > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > > LKML-Reference: <1285165776.2275.1022.camel@laptop>
> > > Signed-off-by: Ingo Molnar <mingo@elte.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 missed the fact that Peter already has a patch that fixes the
problem, but never bothered to post it in this thread.

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

--
Markus