[ 107/122] sched: Fix load avg vs. cpu-hotplug

Greg Kroah-Hartman
2012-10-11T00:26:07Z

From: Peter Zijlstra <peterz@infradead.org>

commit 08bedae1d0acd8c9baf514fb69fa199d0c8345f6 upstream.

Commit f319da0c68 ("sched: Fix load avg vs cpu-hotplug") was an
incomplete fix:

In particular, the problem is that at the point it calls
calc_load_migrate() nr_running := 1 (the stopper thread), so move the
call to CPU_DEAD where we're sure that nr_running := 0.

Also note that we can call calc_load_migrate() without serialization, we
know the state of rq is stable since its cpu is dead, and we modify the
global state using appropriate atomic ops.

Suggested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1346882630.2600.59.camel@twins
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/sched/core.c | 2 ++
1 file changed, 2 insertions(+)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5604,7 +5604,9 @@ migration_call(struct notifier_block *nf
migrate_tasks(cpu);
BUG_ON(rq->nr_running != 1); /* the migration thread */
raw_spin_unlock_irqrestore(&rq->lock, flags);
+ break;

+ case CPU_DEAD:
calc_load_migrate(rq);
break;
#endif