linux-kernel - Re: [PATCH v2] Make sure timers have migrated before killing migration

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Date:	Tue, 25 May 2010 22:19:29 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	"Amit K. Arora" <aarora@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Darren Hart <dvhltc@...ibm.com>,
	Brian King <brking@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] Make sure timers have migrated before killing
 migration_thread

On Thu, 20 May 2010, Peter Zijlstra wrote:

> On Wed, 2010-05-19 at 17:43 +0530, Amit K. Arora wrote:
> > Alternate Solution considered : Another option considered was to
> > increase the priority of the hrtimer cpu offline notifier, such that it
> > gets to run before scheduler's migration cpu offline notifier. In this
> > way we are sure that the timers will get migrated before migration_call
> > tries to kill migration_thread. But, this can have some non-obvious
> > implications, suggested Srivatsa.
> 
> 
> > On Wed, May 19, 2010 at 11:31:55AM +0200, Peter Zijlstra wrote:
> > > The other problem is more urgent though, CPU_POST_DEAD runs outside of
> > > the hotplug lock and thus the above becomes a race where we could
> > > possible kill off the migration thread of a newly brought up cpu:
> > > 
> > >  cpu0 - down 2
> > >  cpu1 - up 2 (allocs a new migration thread, and leaks the old one)
> > >  cpu0 - post_down 2 - frees the migration thread -- oops!
> > 
> > Ok. So, how about adding a check in CPU_UP_PREPARE event handling too ?
> > The cpuset_lock will synchronize, and thus avoid race between killing of
> > migration_thread in up_prepare and post_dead events. 
> > 
> > Here is the updated patch. If you don't like this one too, do you mind
> > suggesting an alternate approach to tackle the problem ? Thanks !
> 
> Right, so this isn't pretty at all..
> 
> Ingo, the comment near the migration_notifier says that migration_call
> should happen before all else, but can you see anything that would break
> if we let the timer migration happen first?
> 
> Thomas?

That should work, though what is killing the scheduler per rq hrtimers
_before_ we migrate stuff ? We don't want to migrate them, right ?

Thanks,

	tglx
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