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

[Greg Kroah-Hartman]

3.6-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Link: http://lkml.kernel.org/r/1346882630.2600.59.camel@twins
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 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


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