linux-kernel - [PATCH v2] sched: Accelerate "pick_next

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Date:	Tue, 17 Jan 2012 10:36:30 +0800
From:	Michael Wang <wangyun@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH v2] sched: Accelerate "pick_next_entity" under special condition

From: wangyun <wangyun@...ux.vnet.ibm.com>

In original code, we get the next entity in this way:

	if(condition1)
		result=value1;
	if(condition2)
		result=value2;
	if(condition3)
		result=value3;
	return result;

So if condition3 is true, we will get value3, but still
need to check condition1 and condition2, this will waste
our time.

This patch will change the way like:

	if(condition3) {
		result=value3;
		goto out;
	}
	if(condition2) {
		result=value2;
		goto out;
	}
	if(condition1) {
		result=value1;
		goto out;
	}

	out:
	return result;

So we can avoid check condition2 and condition1 when
condition3 is true now.

v2:
	1. do not use ugly macro any more.
	2. add more description.

Signed-off-by: Michael Wang <wangyun@...ux.vnet.ibm.com>
---
 kernel/sched/fair.c |   26 +++++++++++++++-----------
 1 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 84adb2d..e8a72b2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1308,29 +1308,33 @@ 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 this to run. If it's not unfair, run it.
 	 */
-	if (cfs_rq->skip == se) {
-		struct sched_entity *second = __pick_next_entity(se);
-		if (second && wakeup_preempt_entity(second, left) < 1)
-			se = second;
+	if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
+		se = cfs_rq->next;
+		goto out;
 	}

 	/*
 	 * Prefer last buddy, try to return the CPU to a preempted task.
 	 */
-	if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
+	if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1) {
 		se = cfs_rq->last;
+		goto out;
+	}

 	/*
-	 * Someone really wants this to run. If it's not unfair, run it.
+	 * Avoid running the skip buddy, if running something else can
+	 * be done without getting too unfair.
 	 */
-	if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
-		se = cfs_rq->next;
+	if (cfs_rq->skip == se) {
+		struct sched_entity *second = __pick_next_entity(se);
+		if (second && wakeup_preempt_entity(second, left) < 1)
+			se = second;
+	}

+out:
 	clear_buddies(cfs_rq, se);
-
 	return se;
 }

-- 
1.7.4.1

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