linux-kernel - [PATCH] sched/fair: Fix the dealing with decay_count in __synchronize_entity

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Date:	Tue, 16 Dec 2014 23:58:29 +0800
From:	Xunlei Pang <pang.xunlei@...aro.org>
To:	linux-kernel@...r.kernel.org
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Xunlei Pang <pang.xunlei@...aro.org>
Subject: [PATCH] sched/fair: Fix the dealing with decay_count in __synchronize_entity_decay()

In __synchronize_entity_decay(), if "decays" happens to be zero, 
se->avg.decay_count will not be zeroed, holding the positive value
assigned when dequeued last time.

This is problematic in the following case:
If this runnable task is CFS-balanced to other CPUs soon afterwards, 
migrate_task_rq_fair() will treat it as a blocked task due to its
non-zero decay_count, thereby adding its load to cfs_rq->removed_load
wrongly.

Thus, we must zero se->avg.decay_count in this case as well.

Signed-off-by: Xunlei Pang <pang.xunlei@...aro.org>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index df2cdf7..ea517cd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2574,11 +2574,11 @@ static inline u64 __synchronize_entity_decay(struct sched_entity *se)
 	u64 decays = atomic64_read(&cfs_rq->decay_counter);
 
 	decays -= se->avg.decay_count;
+	se->avg.decay_count = 0;
 	if (!decays)
 		return 0;
 
 	se->avg.load_avg_contrib = decay_load(se->avg.load_avg_contrib, decays);
-	se->avg.decay_count = 0;
 
 	return decays;
 }
-- 
1.9.1

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