linux-kernel - Re: [PATCH 2/3] sched/fair: Correctly insert cfs

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Message-ID: <CAKfTPtA6AyL2f-KqHXecZrYKmZ9r9mT=Ks6BeNLjV9dfbSZJxQ@mail.gmail.com>
Date:   Fri, 28 May 2021 17:27:04 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Odin Ugedal <odin@...d.al>
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>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] sched/fair: Correctly insert cfs_rq's to list on unthrottle

On Fri, 28 May 2021 at 17:07, Odin Ugedal <odin@...d.al> wrote:
>
> Hi,
>
> > What would be the other condition in addition to the current one
> > :cfs_rq->nr_running >= 1 ?
>
> The condition is that if it has load, we should add it (I don't have
> 100% control on util_avg and runnable_avg tho.). Using
> "!cfs_rq_is_decayed()" is another way, but imo. that is a bit
> overkill.

normally tg_load_avg_contrib should be null when cfs_rq_is_decayed()

>
> > We need to add a cfs_rq in the list if it still contributes to the
> > tg->load_avg and the split of the share. Can't we add a condition for
> > this instead of adding a new field ?
>
> Yes, using cfs_rq->tg_load_avg_contrib as below would also work the
> same way. I still think being explicit that we insert it if we have
> removed it is cleaner in a way, as it makes it consistent with the
> other use of list_add_leaf_cfs_rq() and list_del_leaf_cfs_rq(), but

The reason of this list is to ensure that the load of all cfs_rq are
periodically updated  as it is then used to share the runtime between
groups so we should keep to use the rule whenever possible.

> that is about preference I guess. I do however think that using
> tg_load_avg_contrib will work just fine, as it should always be
> positive in case the cfs_rq has some load. I can resent v2 of this
> patch using this instead;
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ad7556f99b4a..969ae7f930f5 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4720,7 +4720,7 @@ static int tg_unthrottle_up(struct task_group
> *tg, void *data)
>                                              cfs_rq->throttled_clock_task;
>
>                 /* Add cfs_rq with already running entity in the list */
> -               if (cfs_rq->nr_running >= 1)
> +               if (cfs_rq->tg_load_avg_contrib)

we probably need to keep (cfs_rq->nr_running >= 1) as we can have case
where tg_load_avg_contrib is null but a task is enqueued

>                         list_add_leaf_cfs_rq(cfs_rq);
>         }