linux-kernel - Re: [PATCH v2] sched/fair: Correctly insert cfs

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Date:   Thu, 3 Jun 2021 14:51:50 +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 v2] sched/fair: Correctly insert cfs_rq's to list on unthrottle

On Thu, 3 Jun 2021 at 13:41, Odin Ugedal <odin@...d.al> wrote:
>
> This fixes an issue where fairness is decreased since cfs_rq's can
> end up not being decayed properly. For two sibling control groups with
> the same priority, this can often lead to a load ratio of 99/1 (!!).
>
> This happen because when a cfs_rq is throttled, all the descendant cfs_rq's
> will be removed from the leaf list. When they initial cfs_rq is
> unthrottled, it will currently only re add descendant cfs_rq's if they
> have one or more entities enqueued. This is not a perfect heuristic.
>
> Insted, we insert all cfs_rq's that contain one or more enqueued
> entities, or contributes to the load of the task group.
>
> Can often lead to sutiations like this for equally weighted control
> groups:
>
> $ ps u -C stress
> USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
> root       10009 88.8  0.0   3676   100 pts/1    R+   11:04   0:13 stress --cpu 1
> root       10023  3.0  0.0   3676   104 pts/1    R+   11:04   0:00 stress --cpu 1
>
> Fixes: 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()")
> Signed-off-by: Odin Ugedal <odin@...d.al>
> ---
>
> Original thread: https://lore.kernel.org/lkml/20210518125202.78658-3-odin@uged.al/
> Changes since v1:
>  - Replaced cfs_rq field with using tg_load_avg_contrib
>  - Went from 3 to 1 pathces; one is merged and one is replaced
>    by a new patchset.
>
>  kernel/sched/fair.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 794c2cb945f8..0f1b39ca5ca8 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4719,8 +4719,11 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
>                 cfs_rq->throttled_clock_task_time += rq_clock_task(rq) -
>                                              cfs_rq->throttled_clock_task;
>
> -               /* Add cfs_rq with already running entity in the list */
> -               if (cfs_rq->nr_running >= 1)
> +               /*
> +                * Add cfs_rq with tg load avg contribution or one or more
> +                * already running entities to the list
> +                */
> +               if (cfs_rq->tg_load_avg_contrib || cfs_rq->nr_running)

Out of curiosity, why did you decide to use
cfs_rq->tg_load_avg_contrib instead of !cfs_rq_is_decayed(cfs_rq)
which is used to delete the cfs_rq from the list when updating blocked
load ?

>                         list_add_leaf_cfs_rq(cfs_rq);
>         }
>
> --
> 2.31.1
>