sched: Make sure timers have migrated before killing the migration_thread

sched: Make sure timers have migrated before killing the migration_thread

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 which we just marked dead!

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).

Signed-off-by: Amit Arora <[email protected]>
Signed-off-by: Gautham R Shenoy <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
LKML-Reference: <20100525132346[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>