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

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Date:	Sun, 23 May 2010 11:13:06 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Galbraith <efault@....de>
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, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] Make sure timers have migrated before killing
 migration_thread

On Sun, 2010-05-23 at 11:07 +0200, Mike Galbraith wrote:
> On Thu, 2010-05-20 at 09:28 +0200, 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..
> 
> Since the problem seems to stem from interfering with a critical thread,
> how about create a SCHED_SYSTEM_CRITICAL flag ala SCHED_RESET_ON_FORK?
> 
> Not particularly beautiful, and completely untested (well, it compiles).

Nah, I'd rather we pull the migration thread out of SCHED_FIFO and
either schedule it explicit or add a sched_class for it. We need to do
that anyway once we go play with SCHED_DEADLINE.

But it would be very nice if we could simply order the timer and task
migration bits so that the whole problem doesn't exist in the first
place.

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