linux-kernel - [PATCH v4 4/4] sched: adjust fix_small

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Date:	Tue, 18 Jun 2013 21:04:19 +0800
From:	Lei Wen <leiwen@...vell.com>
To:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	<mingo@...hat.com>, <leiwen@...vell.com>,
	<linux-kernel@...r.kernel.org>
Subject: [PATCH v4 4/4] sched: adjust fix_small_imbalance moving task condition

Suppose here we have such example, in current domain, there are two
cpus. CPU A with 4 task, while CPU B with 5 tasks. All tasks over two
runqueue are the same priority.

Let say the load for CPU A is 4096, and CPU B is 5120, then avg_load
should be 4608. Suppose sgp->power == SCHED_POWER_SCALE.
Then the imbalance value should be 512, less then the
busiest_load_per_task, which is 1024. From the logic we enter into
fix_small_imbalance adjustment.

For busiest_per_task==this_load_per_task, imbn's value is equal to 2.
Thus below statement would be true, and load balance would move one task
from CPU B to CPU A.
      if (sds->max_load - sds->this_load + scaled_busy_load_per_task >=
                        (scaled_busy_load_per_task * imbn)) {

When CPU B issue load balance later, it would move one task back again,
which bring infinite end...
Kill the >=, and make it as >, to stop this loop.

Signed-off-by: Lei Wen <leiwen@...vell.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 c478022..3be7844 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4717,7 +4717,7 @@ void fix_small_imbalance(struct lb_env *env, struct sd_lb_stats *sds,
 					<< SCHED_POWER_SHIFT;
 	scaled_this_load_per_task /= sds->this->sgp->power;
 
-	if (sds->max_load - sds->this_load + scaled_busy_load_per_task >=
+	if (sds->max_load - sds->this_load + scaled_busy_load_per_task >
 			(scaled_busy_load_per_task * imbn)) {
 		env->imbalance = sds->busiest_load_per_task;
 		return;
-- 
1.7.10.4

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