Re: [bisected] Clocksource tsc unstable git

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

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?