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

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Date:   Thu, 25 Jun 2020 11:24:28 +0200
From:   Holger Hoffstätte <holger@...lied-asynchrony.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>, 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
Cc:     valentin.schneider@....com, pauld@...hat.com, hdanton@...a.com
Subject: Re: [PATCH] sched/cfs: change initial value of runnable_avg

On 2020-06-24 17:44, 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;
>   
>   	if (p->sched_class != &fair_sched_class) {
>   		/*
> 

Something is wrong here. I woke up my machine from suspend-to-RAM this morning
and saw that a completely idle machine had a loadavg of ~7. According to my
monitoring system this happened to be the loadavg right before I suspended.
I've reverted this, rebooted, created a loadavg >0, suspended and after wake up
loadavg again correctly ranges between 0 and whatever, as expected.

-h