linux-kernel - [PATCH] sched/fair: Make cfs_rq::decay

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Date:	Tue, 16 Dec 2014 17:25:33 +0300
From:	Kirill Tkhai <ktkhai@...allels.com>
To:	<linux-kernel@...r.kernel.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Ben Segall <bsegall@...gle.com>, Kirill Tkhai <tkhai@...dex.ru>
Subject: [PATCH] sched/fair: Make cfs_rq::decay_counter non-atomic


We update decay_counter in update_cfs_rq_blocked_load()
only. This function is always called with rq lock locked,
so we can kill atomic actions.

Signed-off-by: Kirill Tkhai <ktkhai@...allels.com>
---
 kernel/sched/fair.c  |   11 ++++++-----
 kernel/sched/sched.h |    2 +-
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5f3b5a7..af990c4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2570,7 +2570,7 @@ static __always_inline int __update_entity_runnable_avg(u64 now,
 static inline u64 __synchronize_entity_decay(struct sched_entity *se)
 {
 	struct cfs_rq *cfs_rq = cfs_rq_of(se);
-	u64 decays = atomic64_read(&cfs_rq->decay_counter);
+	u64 decays = ACCESS_ONCE(cfs_rq->decay_counter);
 
 	decays -= se->avg.decay_count;
 	if (!decays)
@@ -2767,7 +2767,8 @@ static void update_cfs_rq_blocked_load(struct cfs_rq *cfs_rq, int force_update)
 	if (decays) {
 		cfs_rq->blocked_load_avg = decay_load(cfs_rq->blocked_load_avg,
 						      decays);
-		atomic64_add(decays, &cfs_rq->decay_counter);
+		lockdep_assert_held(&rq_of(cfs_rq)->lock);
+		cfs_rq->decay_counter += decays;
 		cfs_rq->last_decay = now;
 	}
 
@@ -2837,7 +2838,7 @@ static inline void dequeue_entity_load_avg(struct cfs_rq *cfs_rq,
 	cfs_rq->runnable_load_avg -= se->avg.load_avg_contrib;
 	if (sleep) {
 		cfs_rq->blocked_load_avg += se->avg.load_avg_contrib;
-		se->avg.decay_count = atomic64_read(&cfs_rq->decay_counter);
+		se->avg.decay_count = ACCESS_ONCE(cfs_rq->decay_counter);
 	} /* migrations, e.g. sleep=0 leave decay_count == 0 */
 }
 
@@ -7876,7 +7877,7 @@ void init_cfs_rq(struct cfs_rq *cfs_rq)
 	cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime;
 #endif
 #ifdef CONFIG_SMP
-	atomic64_set(&cfs_rq->decay_counter, 1);
+	cfs_rq->decay_counter = 1;
 	atomic_long_set(&cfs_rq->removed_load, 0);
 #endif
 }
@@ -7928,7 +7929,7 @@ static void task_move_group_fair(struct task_struct *p, int queued)
 		 * contribution, but we must synchronize for ongoing future
 		 * decay.
 		 */
-		se->avg.decay_count = atomic64_read(&cfs_rq->decay_counter);
+		se->avg.decay_count = ACCESS_ONCE(cfs_rq->decay_counter);
 		cfs_rq->blocked_load_avg += se->avg.load_avg_contrib;
 #endif
 	}
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 9a2a45c..672f120 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -364,7 +364,7 @@ struct cfs_rq {
 	 * the FAIR_GROUP_SCHED case).
 	 */
 	unsigned long runnable_load_avg, blocked_load_avg;
-	atomic64_t decay_counter;
+	u64 decay_counter;
 	u64 last_decay;
 	atomic_long_t removed_load;
 



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