Re: [PATCH] Make sure timers have migrated before killingmigration_thread

From: Peter Zijlstra
Date: Wed May 19 2010 - 05:32:24 EST


On Wed, 2010-05-19 at 14:35 +0530, Amit K. Arora wrote:
> Problem : In a stress test where some heavy tests were running along with
> regular CPU offlining and onlining, a hang was observed. The system seems to
> be hung at a point where migration_call() tries to kill the migration_thread
> of the dying CPU, which just got moved to the current CPU. This migration
> thread does not get a chance to run (and die) since rt_throttled is set to 1
> on current, and it doesn't get cleared as the hrtimer which is supposed to
> reset the rt bandwidth (sched_rt_period_timer) is tied to the CPU being
> offlined.
>
> Solution : This patch pushes the killing of migration thread to "CPU_POST_DEAD"
> event. By then all the timers (including sched_rt_period_timer) should have got
> migrated (along with other callbacks).
>
> 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.
>
> Testing : Without the patch the stress tests didn't last for even 12
> hours. And yes, the problem was reproducible. With the patch applied the
> tests ran successfully for more than 48 hours.
>
> Thanks!
> --
> Regards,
> Amit Arora
>
> Signed-off-by: Amit Arora <aarora@xxxxxxxxxx>
> Signed-off-by: Gautham R Shenoy <ego@xxxxxxxxxx>
> --
> diff -Nuarp linux-2.6.34.org/kernel/sched.c linux-2.6.34/kernel/sched.c
> --- linux-2.6.34.org/kernel/sched.c 2010-05-18 22:56:21.000000000 -0700
> +++ linux-2.6.34/kernel/sched.c 2010-05-18 22:58:31.000000000 -0700
> @@ -5942,14 +5942,26 @@ migration_call(struct notifier_block *nf
> cpu_rq(cpu)->migration_thread = NULL;
> break;
>
> + case CPU_POST_DEAD:
> + /*
> + Bring the migration thread down in CPU_POST_DEAD event,
> + since the timers should have got migrated by now and thus
> + we should not see a deadlock between trying to kill the
> + migration thread and the sched_rt_period_timer.
> + */

Faulty comment style that, please use:

/*
* text
* goes
* here
*/

> + cpuset_lock();
> + rq = cpu_rq(cpu);
> + kthread_stop(rq->migration_thread);
> + put_task_struct(rq->migration_thread);
> + rq->migration_thread = NULL;
> + cpuset_unlock();
> + break;
> +

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!


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