linux-kernel - Re: [PATCH 05/15] sched,fair: remove cfs_rqs from leaf_cfs_rq

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Message-ID: <CAKfTPtAoZwmZXthcaP6VQO-0oEOtPfTWwDPtgHfKwBt=+TO2XA@mail.gmail.com>
Date:   Wed, 28 Aug 2019 16:09:49 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>, Paul Turner <pjt@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH 05/15] sched,fair: remove cfs_rqs from leaf_cfs_rq_list
 bottom up

On Thu, 22 Aug 2019 at 04:18, Rik van Riel <riel@...riel.com> wrote:
>
> Reducing the overhead of the CPU controller is achieved by not walking
> all the sched_entities every time a task is enqueued or dequeued.
>
> One of the things being checked every single time is whether the cfs_rq
> is on the rq->leaf_cfs_rq_list.
>
> By only removing a cfs_rq from the list once it no longer has children
> on the list, we can avoid walking the sched_entity hierarchy if the bottom
> cfs_rq is on the list, once the runqueues have been flattened.
>
> Signed-off-by: Rik van Riel <riel@...riel.com>
> Suggested-by: Vincent Guittot <vincent.guittot@...aro.org>

Acked-by: Vincent Guittot <vincent.guittot@...aro.org>

> ---
>  kernel/sched/fair.c | 35 ++++++++++++++++++++++++++++++++++-
>  1 file changed, 34 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a48d0dbfc232..04b216234265 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -369,6 +369,39 @@ static inline void assert_list_leaf_cfs_rq(struct rq *rq)
>         SCHED_WARN_ON(rq->tmp_alone_branch != &rq->leaf_cfs_rq_list);
>  }
>
> +/*
> + * Because list_add_leaf_cfs_rq always places a child cfs_rq on the list
> + * immediately before a parent cfs_rq, and cfs_rqs are removed from the list
> + * bottom-up, we only have to test whether the cfs_rq before us on the list
> + * is our child.
> + */
> +static inline bool child_cfs_rq_on_list(struct cfs_rq *cfs_rq)
> +{
> +       struct cfs_rq *prev_cfs_rq;
> +       struct list_head *prev;
> +
> +       prev = cfs_rq->leaf_cfs_rq_list.prev;
> +       prev_cfs_rq = container_of(prev, struct cfs_rq, leaf_cfs_rq_list);
> +
> +       return (prev_cfs_rq->tg->parent == cfs_rq->tg);
> +}
> +
> +/*
> + * Remove a cfs_rq from the list if it has no children on the list.
> + * The scheduler iterates over the list regularly; if conditions for
> + * removal are still true, we'll get to this cfs_rq in the future.
> + */
> +static inline void list_del_leaf_cfs_rq_bottom(struct cfs_rq *cfs_rq)
> +{
> +       if (!cfs_rq->on_list)
> +               return;
> +
> +       if (child_cfs_rq_on_list(cfs_rq))
> +               return;
> +
> +       list_del_leaf_cfs_rq(cfs_rq);
> +}
> +
>  /* Iterate thr' all leaf cfs_rq's on a runqueue */
>  #define for_each_leaf_cfs_rq_safe(rq, cfs_rq, pos)                     \
>         list_for_each_entry_safe(cfs_rq, pos, &rq->leaf_cfs_rq_list,    \
> @@ -7723,7 +7756,7 @@ static void update_blocked_averages(int cpu)
>                  * decayed cfs_rqs linger on the list.
>                  */
>                 if (cfs_rq_is_decayed(cfs_rq))
> -                       list_del_leaf_cfs_rq(cfs_rq);
> +                       list_del_leaf_cfs_rq_bottom(cfs_rq);
>
>                 /* Don't need periodic decay once load/util_avg are null */
>                 if (cfs_rq_has_blocked(cfs_rq))
> --
> 2.20.1
>