linux-kernel - Re: [PATCH] sched/fair: Return the busiest group only if imbalance is meaningful

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Message-ID: <44ca58cf-3daa-bec5-2c52-eb5a6dd305b9@arm.com>
Date:   Wed, 6 Apr 2022 19:25:46 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        彭志刚 <zgpeng.linux@...il.com>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        rostedt@...dmis.org, Benjamin Segall <bsegall@...gle.com>,
        mgorman@...e.de, bristot@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Return the busiest group only if imbalance is
 meaningful

On 06/04/2022 17:33, Vincent Guittot wrote:
> On Wed, 6 Apr 2022 at 17:07, 彭志刚 <zgpeng.linux@...il.com> wrote:

[...]

> I think that this case should be covered by an additional test in
> calculate_imbalance because we should not try to pull load in groupA
> if it's already above  the average load. Something like below should
> cover this
> 
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9469,6 +9469,16 @@ static inline void calculate_imbalance(struct
> lb_env *env, struct sd_lb_stats *s
> 
>                 sds->avg_load = (sds->total_load * SCHED_CAPACITY_SCALE) /
>                                 sds->total_capacity;
> +
> +               /*
> +                * Don't pull any tasks if this group is already above the
> +                * domain average load.
> +                */
> +               if (local->avg_load >= sds->avg_load) {
> +                       env->imbalance = 0;
> +                       return;
> +               }
> +

LGTM. Like for the `local->group_type == group_overloaded` case in
find_busiest_group() where we force `goto out_balanced`.

[...]