Messages in this thread | | | Subject | Re: [PATCH v2] Make sure timers have migrated before killing migration_thread | From | Peter Zijlstra <> | Date | Sun, 23 May 2010 11:13:06 +0200 |
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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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