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

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Message-Id: <20121010225357.276051295@linuxfoundation.org>
Date:	Thu, 11 Oct 2012 07:54:33 +0900
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	alan@...rguk.ukuu.org.uk,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...nel.org>
Subject: [ 107/122] sched: Fix load avg vs. cpu-hotplug

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

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

From: Peter Zijlstra <peterz@...radead.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@...ux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Link: http://lkml.kernel.org/r/1346882630.2600.59.camel@twins
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.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

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