linux-kernel - Re: [RFC PATCH 3/4] sched/fair: delay update_tg_load_avg() for cfs

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Date:   Wed, 19 Jul 2023 11:47:06 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Aaron Lu <aaron.lu@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Daniel Jordan <daniel.m.jordan@...cle.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>,
        Valentin Schneider <vschneid@...hat.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Nitin Tekchandani <nitin.tekchandani@...el.com>,
        Yu Chen <yu.c.chen@...el.com>,
        Waiman Long <longman@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 3/4] sched/fair: delay update_tg_load_avg() for
 cfs_rq's removed load

On Wed, 19 Jul 2023 at 10:01, Aaron Lu <aaron.lu@...el.com> wrote:
>
> On Wed, Jul 19, 2023 at 01:18:26PM +0800, Aaron Lu wrote:
> > Alternatively, I can remove some callsites of update_tg_load_avg() like
> > you suggested below and only call update_tg_load_avg() when cfs_rq is
> > decayed(really just decayed, not when it detected it has removed load
> > pending or load propagated from its children). This way it would give us
> > similar result as above(roughly once per ms).
>
> Something like this: (I think this is better since it removed those
> unnecessary calls to update_tg_load_avg(), although it is inline but
> still)
>
>
> From bc749aaefa6bed36aa946921a4006b3dddb69b77 Mon Sep 17 00:00:00 2001
> From: Aaron Lu <aaron.lu@...el.com>
> Date: Wed, 19 Jul 2023 13:54:48 +0800
> Subject: [PATCH] sched/fair: only update_tg_load_avg() when cfs_rq decayed
>
> ---
>  kernel/sched/fair.c | 22 +++++++---------------
>  1 file changed, 7 insertions(+), 15 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a80a73909dc2..7d5b7352b8b5 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3913,16 +3913,16 @@ static inline void add_tg_cfs_propagate(struct cfs_rq *cfs_rq, long runnable_sum
>  }
>
>  /* Update task and its cfs_rq load average */
> -static inline int propagate_entity_load_avg(struct sched_entity *se)
> +static inline void propagate_entity_load_avg(struct sched_entity *se)
>  {
>         struct cfs_rq *cfs_rq, *gcfs_rq;
>
>         if (entity_is_task(se))
> -               return 0;
> +               return;
>
>         gcfs_rq = group_cfs_rq(se);
>         if (!gcfs_rq->propagate)
> -               return 0;
> +               return;
>
>         gcfs_rq->propagate = 0;
>
> @@ -3936,8 +3936,6 @@ static inline int propagate_entity_load_avg(struct sched_entity *se)
>
>         trace_pelt_cfs_tp(cfs_rq);
>         trace_pelt_se_tp(se);
> -
> -       return 1;
>  }
>
>  /*
> @@ -3974,9 +3972,8 @@ static inline bool skip_blocked_update(struct sched_entity *se)
>
>  static inline void update_tg_load_avg(struct cfs_rq *cfs_rq) {}
>
> -static inline int propagate_entity_load_avg(struct sched_entity *se)
> +static inline void propagate_entity_load_avg(struct sched_entity *se)
>  {
> -       return 0;
>  }
>
>  static inline void add_tg_cfs_propagate(struct cfs_rq *cfs_rq, long runnable_sum) {}
> @@ -4086,7 +4083,7 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
>  {
>         unsigned long removed_load = 0, removed_util = 0, removed_runnable = 0;
>         struct sched_avg *sa = &cfs_rq->avg;
> -       int decayed = 0;
> +       int decayed;
>
>         if (cfs_rq->removed.nr) {
>                 unsigned long r;
> @@ -4134,11 +4131,9 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
>                  */
>                 add_tg_cfs_propagate(cfs_rq,
>                         -(long)(removed_runnable * divider) >> SCHED_CAPACITY_SHIFT);
> -
> -               decayed = 1;
>         }
>
> -       decayed |= __update_load_avg_cfs_rq(now, cfs_rq);
> +       decayed = __update_load_avg_cfs_rq(now, cfs_rq);
>         u64_u32_store_copy(sa->last_update_time,
>                            cfs_rq->last_update_time_copy,
>                            sa->last_update_time);
> @@ -4252,7 +4247,7 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
>                 __update_load_avg_se(now, cfs_rq, se);
>
>         decayed  = update_cfs_rq_load_avg(now, cfs_rq);
> -       decayed |= propagate_entity_load_avg(se);
> +       propagate_entity_load_avg(se);

but then you  also skip the call to cfs_rq_util_change()
>
>         if (!se->avg.last_update_time && (flags & DO_ATTACH)) {
>
> @@ -4264,15 +4259,12 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
>                  * IOW we're enqueueing a task on a new CPU.
>                  */
>                 attach_entity_load_avg(cfs_rq, se);
> -               update_tg_load_avg(cfs_rq);
> -
>         } else if (flags & DO_DETACH) {
>                 /*
>                  * DO_DETACH means we're here from dequeue_entity()
>                  * and we are migrating task out of the CPU.
>                  */
>                 detach_entity_load_avg(cfs_rq, se);
> -               update_tg_load_avg(cfs_rq);
>         } else if (decayed) {
>                 cfs_rq_util_change(cfs_rq, 0);
>
> --
> 2.41.0
>