linux-kernel - [PATCH v3] sched/fair: Do not set skip buddy up the sched hierarchy

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Date:   Wed, 18 Dec 2019 16:14:18 -0800
From:   Josh Don <joshdon@...gle.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel@...r.kernel.org, Paul Turner <pjt@...gle.com>,
        Josh Don <joshdon@...gle.com>
Subject: [PATCH v3] sched/fair: Do not set skip buddy up the sched hierarchy

From: Venkatesh Pallipadi <venki@...gle.com>

Setting the skip buddy up the hierarchy effectively means that a thread
calling yield will also end up skipping the other tasks in its hierarchy
as well. However, a yielding thread shouldn't end up causing this
behavior on behalf of its entire hierarchy.

For typical uses of yield, setting the skip buddy up the hierarchy is
counter-productive, as that results in CPU being yielded to a task in
some other cgroup.

So, limit the skip effect only to the task requesting it.

Co-developed-by: Josh Don <joshdon@...gle.com>
Signed-off-by: Josh Don <joshdon@...gle.com>
---
v2: Only clear skip buddy on the current cfs_rq

v3: Modify comment describing the justification for this change.

 kernel/sched/fair.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 08a233e97a01..0056b57d52cb 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4051,13 +4051,10 @@ static void __clear_buddies_next(struct sched_entity *se)
 
 static void __clear_buddies_skip(struct sched_entity *se)
 {
-	for_each_sched_entity(se) {
-		struct cfs_rq *cfs_rq = cfs_rq_of(se);
-		if (cfs_rq->skip != se)
-			break;
+	struct cfs_rq *cfs_rq = cfs_rq_of(se);
 
+	if (cfs_rq->skip == se)
 		cfs_rq->skip = NULL;
-	}
 }
 
 static void clear_buddies(struct cfs_rq *cfs_rq, struct sched_entity *se)
@@ -6552,8 +6549,12 @@ static void set_next_buddy(struct sched_entity *se)
 
 static void set_skip_buddy(struct sched_entity *se)
 {
-	for_each_sched_entity(se)
-		cfs_rq_of(se)->skip = se;
+	/*
+	 * Only set the skip buddy for the task requesting it. Setting the skip
+	 * buddy up the hierarchy would result in skipping all other tasks in
+	 * the hierarchy as well.
+	 */
+	cfs_rq_of(se)->skip = se;
 }
 
 /*
-- 
2.24.1.735.g03f4e72817-goog