linux-kernel - [patch] sched: minimalist select_idle_sibling() bouncing cow syndrome fix

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Date:	Sun, 27 Jan 2013 08:50:21 +0100
From:	Mike Galbraith <bitbucket@...ine.de>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: [patch] sched: minimalist select_idle_sibling() bouncing cow
 syndrome fix


If the previous CPU is cache affine and idle, select it.

Signed-off-by: Mike Galbraith <bitbucket@...ine.de>
---
 kernel/sched/fair.c |   21 +++++++--------------
 1 file changed, 7 insertions(+), 14 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3270,25 +3270,18 @@ find_idlest_cpu(struct sched_group *grou
  */
 static int select_idle_sibling(struct task_struct *p, int target)
 {
-	int cpu = smp_processor_id();
-	int prev_cpu = task_cpu(p);
 	struct sched_domain *sd;
 	struct sched_group *sg;
-	int i;
+	int i = task_cpu(p);
 
-	/*
-	 * If the task is going to be woken-up on this cpu and if it is
-	 * already idle, then it is the right target.
-	 */
-	if (target == cpu && idle_cpu(cpu))
-		return cpu;
+	if (idle_cpu(target))
+		return target;
 
 	/*
-	 * If the task is going to be woken-up on the cpu where it previously
-	 * ran and if it is currently idle, then it the right target.
+	 * If the prevous cpu is cache affine and idle, don't be stupid.
 	 */
-	if (target == prev_cpu && idle_cpu(prev_cpu))
-		return prev_cpu;
+	if (i != target && cpus_share_cache(i, target) && idle_cpu(i))
+		return i;
 
 	/*
 	 * Otherwise, iterate the domains and find an elegible idle cpu.
@@ -3302,7 +3295,7 @@ static int select_idle_sibling(struct ta
 				goto next;
 
 			for_each_cpu(i, sched_group_cpus(sg)) {
-				if (!idle_cpu(i))
+				if (i == target || !idle_cpu(i))
 					goto next;
 			}
 


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