linux-kernel - [PATCH] sched: prevent sched entity from being decayed twice when both waking and migrating it

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Message-Id: <1437034317-15120-1-git-send-email-byungchul.park@lge.com>
Date:	Thu, 16 Jul 2015 17:11:57 +0900
From:	byungchul.park@....com
To:	pjt@...gle.com
Cc:	linux-kernel@...r.kernel.org,
	Byungchul Park <byungchul.park@....com>
Subject: [PATCH] sched: prevent sched entity from being decayed twice when both waking and migrating it

From: Byungchul Park <byungchul.park@....com>

hello paul,

can i ask you something?

when a sched entity is both waken and migrated, it looks being decayed twice.
did you do it on purpose?
or am i missing something? :(

thanks,
byungchul

--------------->8---------------
>From 793c963d0b29977a0f6f9330291a9ea469cc54f0 Mon Sep 17 00:00:00 2001
From: Byungchul Park <byungchul.park@....com>
Date: Thu, 16 Jul 2015 16:49:48 +0900
Subject: [PATCH] sched: prevent sched entity from being decayed twice when
 both waking and migrating it

current code is decaying load average variables with a sleep time twice,
when both waking and migrating it. the first decaying happens in a call path
"migrate_task_rq_fair() -> __synchronize_entity_decay()". the second
decaying happens in a call path "enqueue_entity_load_avg() ->
update_entity_load_avg()". so make it happen once.

Signed-off-by: Byungchul Park <byungchul.park@....com>
---
 kernel/sched/fair.c |   29 +++--------------------------
 1 file changed, 3 insertions(+), 26 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 09456fc..c86cca0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2873,32 +2873,9 @@ static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq,
 						  struct sched_entity *se,
 						  int wakeup)
 {
-	/*
-	 * We track migrations using entity decay_count <= 0, on a wake-up
-	 * migration we use a negative decay count to track the remote decays
-	 * accumulated while sleeping.
-	 *
-	 * Newly forked tasks are enqueued with se->avg.decay_count == 0, they
-	 * are seen by enqueue_entity_load_avg() as a migration with an already
-	 * constructed load_avg_contrib.
-	 */
-	if (unlikely(se->avg.decay_count <= 0)) {
+	/* we track migrations using entity decay_count == 0 */
+	if (unlikely(!se->avg.decay_count)) {
 		se->avg.last_runnable_update = rq_clock_task(rq_of(cfs_rq));
-		if (se->avg.decay_count) {
-			/*
-			 * In a wake-up migration we have to approximate the
-			 * time sleeping.  This is because we can't synchronize
-			 * clock_task between the two cpus, and it is not
-			 * guaranteed to be read-safe.  Instead, we can
-			 * approximate this using our carried decays, which are
-			 * explicitly atomically readable.
-			 */
-			se->avg.last_runnable_update -= (-se->avg.decay_count)
-							<< 20;
-			update_entity_load_avg(se, 0);
-			/* Indicate that we're now synchronized and on-rq */
-			se->avg.decay_count = 0;
-		}
 		wakeup = 0;
 	} else {
 		__synchronize_entity_decay(se);
@@ -5114,7 +5091,7 @@ migrate_task_rq_fair(struct task_struct *p, int next_cpu)
 	 * be negative here since on-rq tasks have decay-count == 0.
 	 */
 	if (se->avg.decay_count) {
-		se->avg.decay_count = -__synchronize_entity_decay(se);
+		__synchronize_entity_decay(se);
 		atomic_long_add(se->avg.load_avg_contrib,
 						&cfs_rq->removed_load);
 	}
-- 
1.7.9.5

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