linux-kernel - Re: [PATCH v2] sched: revise the initial value of the util

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Date:   Thu, 5 Nov 2020 13:30:33 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Xuewen Yan <xuewen.yan@...soc.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        xuewen.yan94@...il.com
Subject: Re: [PATCH v2] sched: revise the initial value of the util_avg.

On Thu, 5 Nov 2020 at 12:22, Xuewen Yan <xuewen.yan@...soc.com> wrote:
>
> According to the original code logic:
>                 cfs_rq->avg.util_avg
> sa->util_avg  = -------------------- * se->load.weight
>                 cfs_rq->avg.load_avg

this should have been scale_load_down(se->load.weight) from the beginning

> but for fair_sched_class:
> se->load.weight = 1024 * sched_prio_to_weight[prio];

This is only true for 64bits platform otherwise scale_load and
scale_load_down are nop

>         cfs_rq->avg.util_avg
> so the  -------------------- must be extremely small, the
>         cfs_rq->avg.load_avg
> judgment condition "sa->util_avg < cap" could be established.
> It's not fair for those tasks who has smaller nice value.
>
> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> ---
>  kernel/sched/fair.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 290f9e3..079760b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -794,7 +794,11 @@ void post_init_entity_util_avg(struct task_struct *p)
>
>         if (cap > 0) {
>                 if (cfs_rq->avg.util_avg != 0) {

We should now use cpu_util() instead of cfs_rq->avg.util_avg which
takes into account other classes

> -                       sa->util_avg  = cfs_rq->avg.util_avg * se->load.weight;
> +                       if (p->sched_class == &fair_sched_class)
> +                               sa->util_avg  = cfs_rq->avg.util_avg * se_weight(se);
> +                       else
> +                               sa->util_avg  = cfs_rq->avg.util_avg * se->load.weight;

Why this else keeps using se->load.weight ?

Either we uses sa->util_avg  = cfs_rq->avg.util_avg * se_weight(se);
for all classes

Or we want a different init value for other classes. But in this case
se->load.weight is meaningless and we should simply set them to 0
although we could probably compute a value based on bandwidth for
deadline class

> +
>                         sa->util_avg /= (cfs_rq->avg.load_avg + 1);
>
>                         if (sa->util_avg > cap)
> --
> 1.9.1
>
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