sched/fair: Fix the dealing with decay_count in __synchronize_entity_decay()

Message ID
State Accepted
Commit 638476007d13534b2ed4134bf0279ef44071140b
Headers show

Commit Message

pang.xunlei Dec. 16, 2014, 3:58 p.m.
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

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

Signed-off-by: Xunlei Pang <>
 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;