linux-kernel - Re: [PATCH] sched/cfs: change initial value of runnable

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Date:   Wed, 24 Jun 2020 17:32:12 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, linux-kernel@...r.kernel.org,
        rong.a.chen@...el.com, pauld@...hat.com, hdanton@...a.com
Subject: Re: [PATCH] sched/cfs: change initial value of runnable_avg


On 24/06/20 16:44, Vincent Guittot wrote:
> 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@...el.com>
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.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;

IIRC we didn't go for this initially because hackbench behaved slightly
worse with it. Did we end up re-evaluating this? Also, how does this reaim
benchmark behave with it? I *think* the table from that regression thread
says it behaves better, but I had a hard time parsing it (seems like it got
damaged by line wrapping)

Conceptually I'm all for it, so as long as the tests back it up:
Reviewed-by: Valentin Schneider <valentin.schneider@....com>

>  
>  	if (p->sched_class != &fair_sched_class) {
>  		/*