Re: [bisected] Clocksource tsc unstable git

Markus Trippelsdorf
2010-10-27T19:42:42Z

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?

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