Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
MIME-Version: 1.0
References: <1649245580-27256-1-git-send-email-zgpeng@tencent.com>
In-Reply-To: <1649245580-27256-1-git-send-email-zgpeng@tencent.com>
From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Wed, 6 Apr 2022 17:41:41 +0200
Message-ID: <CAKfTPtDCXP8pt=F563sovxXJr-ALFuxNtE+1gWL-s7A-h4waew@mail.gmail.com>
Subject: Re: [PATCH] sched/fair: Simplify the scene where both local and
 busiest are group_fully_busy
To:     zgpeng <zgpeng.linux@gmail.com>
Cc:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Wed, 6 Apr 2022 at 13:46, zgpeng <zgpeng.linux@gmail.com> wrote:
>
> When both local and busiest group are group_fully_busy type, because
> the avg_load of the group of type group_fully_busy is not calculated, the
> avg_load value is equal to 0. In this case, load balancing will not actually
> done, because after a series of calculations in the calculate_imbalance, it
> will be considered that load balance is not required. Therefore,it is not
> necessary to enter calculate_imbalance to do some useless work.
>
> Signed-off-by: zgpeng <zgpeng@tencent.com>
> Reviewed-by: Samuel Liao <samuelliao@tencent.com>
> ---
>  kernel/sched/fair.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9f75303..cc1d6c4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9634,6 +9634,18 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
>                          * busiest doesn't have any tasks waiting to run
>                          */
>                         goto out_balanced;
> +

We are there because both local and busiest are not overloaded, local
is idle or newly_idle and there might be an opportunity to pull a
waiting task on local to use this local cpu. I wonder if we should not
cover this opportunity in calculate_imbalance instead of skipping it

> +               if (local->group_type == group_fully_busy)
> +                       /*
> +                        * If local group is group_fully_busy, the code goes here,
> +                        * the type of busiest group must also be group_fully_busy.
> +                        * Because the avg_load of the group_fully_busy type is not
> +                        * calculated at present, it is actually equal to 0. In this
> +                        * scenario, load balance is not performed. therefore, it can
> +                        * be returned directly here, and there is no need to do some
> +                        * useless work in calculate_imbalance.
> +                        */
> +                       goto out_balanced;
>         }
>
>  force_balance:
> --
> 2.9.5
>