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From:   Vincent Guittot <vincent.guittot@linaro.org>
To:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, linux-kernel@vger.kernel.org,
        rong.a.chen@intel.com
Cc:     valentin.schneider@arm.com, pauld@redhat.com, hdanton@sina.com,
        Vincent Guittot <vincent.guittot@linaro.org>
Subject: [PATCH] sched/cfs: change initial value of runnable_avg
Date:   Wed, 24 Jun 2020 17:44:22 +0200
Message-Id: <20200624154422.29166-1-vincent.guittot@linaro.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Some performance regression on reaim benchmark have been raised with
  commit 070f5e860ee2 ("sched/fair: Take into account runnable_avg to classify group")

The problem comes from the init value of runnable_avg which is initialized
with max value. This can be a problem if the newly forked task is finally
a short task because the group of CPUs is wrongly set to overloaded and
tasks are pulled less agressively.

Set initial value of runnable_avg equals to util_avg to reflect that there
is no waiting time so far.

Fixes: 070f5e860ee2 ("sched/fair: Take into account runnable_avg to classify group")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 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 0424a0af5f87..45e467bf42fc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -806,7 +806,7 @@ void post_init_entity_util_avg(struct task_struct *p)
 		}
 	}
 
-	sa->runnable_avg = cpu_scale;
+	sa->runnable_avg = sa->util_avg;
 
 	if (p->sched_class != &fair_sched_class) {
 		/*
-- 
2.17.1