linux-kernel - [PATCH] sched: Skip looking at skip if next or last is set

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Message-ID: <20130218130146.GA7472@linux.vnet.ibm.com>
Date:	Mon, 18 Feb 2013 18:31:46 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Cc:	Mike Galbraith <efault@....de>
Subject: [PATCH] sched: Skip looking at skip if next or last is set

pick_next_entity() prefers next, then last. However code checks if the
left entity can be skipped even if next / last is set.

Check if left entity should be skipped only if next/last is not set.

Signed-off-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
---
 kernel/sched/fair.c |   31 +++++++++++++++----------------
 1 files changed, 15 insertions(+), 16 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fdee793..cc97b12 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1900,27 +1900,26 @@ static struct sched_entity *pick_next_entity(struct cfs_rq *cfs_rq)
 	struct sched_entity *left = se;
 
 	/*
-	 * Avoid running the skip buddy, if running something else can
-	 * be done without getting too unfair.
+	 * Someone really wants next to run. If it's not unfair, run it.
 	 */
-	if (cfs_rq->skip == se) {
-		struct sched_entity *second = __pick_next_entity(se);
+	if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
+		se = cfs_rq->next;
+	} else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1) {
+		/*
+		 * Prefer last buddy, try to return the CPU to a preempted
+		 * task.
+		 */
+		se = cfs_rq->last;
+	} else if (cfs_rq->skip == left) {
+		/*
+		 * Avoid running the skip buddy, if running something else
+		 * can be done without getting too unfair.
+		 */
+		struct sched_entity *second = __pick_next_entity(left);
 		if (second && wakeup_preempt_entity(second, left) < 1)
 			se = second;
 	}
 
-	/*
-	 * Prefer last buddy, try to return the CPU to a preempted task.
-	 */
-	if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
-		se = cfs_rq->last;
-
-	/*
-	 * Someone really wants this to run. If it's not unfair, run it.
-	 */
-	if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
-		se = cfs_rq->next;
-
 	clear_buddies(cfs_rq, se);
 
 	return se;

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