[tip: sched/urgent] sched/cfs: change initial value of runnable_avg

[tip-bot2 for Vincent Guittot]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     68f7b5cc835de7d5b6c7696533c126018171e793
Gitweb:        https://git.kernel.org/tip/68f7b5cc835de7d5b6c7696533c126018171e793
Author:        Vincent Guittot <vincent.guittot@xxxxxxxxxx>
AuthorDate:    Wed, 24 Jun 2020 17:44:22 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 25 Jun 2020 13:45:38 +02:00

sched/cfs: change initial value of runnable_avg

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@xxxxxxxxx>
Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200624154422.29166-1-vincent.guittot@xxxxxxxxxx
---
 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 cbcb2f7..658aa7a 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) {
 		/*