linux-kernel - [PATCH 1/2] sched,numa: fix off-by-one in capacity check

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Date:	Mon,  4 Aug 2014 13:23:27 -0400
From:	riel@...hat.com
To:	linux-kernel@...r.kernel.org
Cc:	peterz@...radead.org, mgorman@...e.de, mingo@...nel.org,
	vincent.guittot@...aro.org, Morten.Rasmussen@....com,
	nicolas.pitre@...aro.org, efault@....de
Subject: [PATCH 1/2] sched,numa: fix off-by-one in capacity check

From: Rik van Riel <riel@...hat.com>

Commit a43455a1d572daf7b730fe12eb747d1e17411365 ensures that
task_numa_migrate will call task_numa_compare on the preferred
node all the time, even when the preferred node has no free capacity.

This could lead to a performance regression if nr_running == capacity
on both the source and the destination node. This can be avoided by
also checking for nr_running == capacity on the source node, which is
one stricter than checking .has_free_capacity.

Signed-off-by: Rik van Riel <riel@...hat.com>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bfa3c86..678ed03 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1206,7 +1206,7 @@ static void task_numa_compare(struct task_numa_env *env,
 
 	if (!cur) {
 		/* Is there capacity at our destination? */
-		if (env->src_stats.has_free_capacity &&
+		if (env->src_stats.nr_running <= env->src_stats.task_capacity &&
 		    !env->dst_stats.has_free_capacity)
 			goto unlock;
 
-- 
1.9.3

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