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Subject: Re: [PATCH RFC] sched/fair: simplfy the work when reweighting entity
To:     Jiang Biao <benbjiang@gmail.com>, mingo@redhat.com,
        peterz@infradead.org, juri.lelli@redhat.com,
        vincent.guittot@linaro.org
Cc:     rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        linux-kernel@vger.kernel.org, Jiang Biao <benbjiang@tencent.com>
References: <20200804071222.31675-1-benbjiang@tencent.com>
From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
Message-ID: <f9eab9a5-c7c2-dbf3-834b-abd9c376f92c@arm.com>
Date:   Wed, 5 Aug 2020 18:21:15 +0200
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On 04/08/2020 09:12, Jiang Biao wrote:
> If a se is on_rq when reweighting entity, all we need should be
> updating the load of cfs_rq, other dequeue/enqueue works could be
> redundant, such as,
> * account_numa_dequeue/account_numa_enqueue
> * list_del/list_add from/into cfs_tasks
> * nr_running--/nr_running++

I think this could make sense. Have you spotted a code path where this
gives you a change?

I guess only for a task on the rq, so: entity_is_task(se) && se->on_rq

> Just simplfy the work. Could be helpful for the hot path.

IMHO hotpath is update_cfs_group() -> reweight_entity() but this is only
called for '!entity_is_task(se)'.

See

 3290         if (!gcfs_rq)
 3291                 return;

in update_cfs_group().

The 'entity_is_task(se)' case is

set_load_weight(struct task_struct *p, ...) -> reweight_task(p, ...) ->
reweight_entity(..., &p->se, ...)

but here !se->on_rq.

> Signed-off-by: Jiang Biao <benbjiang@tencent.com>
> ---
>  kernel/sched/fair.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 04fa8dbcfa4d..18a8fc7bd0de 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3086,7 +3086,7 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
>  		/* commit outstanding execution time */
>  		if (cfs_rq->curr == se)
>  			update_curr(cfs_rq);
> -		account_entity_dequeue(cfs_rq, se);
> +		update_load_sub(&cfs_rq->load, se->load.weight);
>  	}
>  	dequeue_load_avg(cfs_rq, se);
>  
> @@ -3102,7 +3102,7 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
>  
>  	enqueue_load_avg(cfs_rq, se);
>  	if (se->on_rq)
> -		account_entity_enqueue(cfs_rq, se);
> +		update_load_add(&cfs_rq->load, se->load.weight);
>  
>  }