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Subject[009/123] sched: kill migration thread in CPU_POST_DEAD instead of CPU_DEAD
From: Amit Arora <amitarora@in.ibm.com>

[Fixed in a different manner upstream, due to rewrites in this area]

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 <amitarora@in.ibm.com>
Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
kernel/sched.c | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -7752,14 +7752,24 @@ 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.
+ */
+ rq = cpu_rq(cpu);
+ kthread_stop(rq->migration_thread);
+ put_task_struct(rq->migration_thread);
+ rq->migration_thread = NULL;
+ break;
+
case CPU_DEAD:
case CPU_DEAD_FROZEN:
cpuset_lock(); /* around calls to cpuset_cpus_allowed_lock() */
migrate_live_tasks(cpu);
rq = cpu_rq(cpu);
- kthread_stop(rq->migration_thread);
- put_task_struct(rq->migration_thread);
- rq->migration_thread = NULL;
/* Idle task back to normal (off runqueue, low prio) */
spin_lock_irq(&rq->lock);
update_rq_clock(rq);



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