linux-kernel - Re: [PATCH] sched/fair: Optimize dequeue_task

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Date:   Tue, 11 Aug 2020 18:55:05 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Jiang Biao <benbjiang@...il.com>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org
Cc:     rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        linux-kernel@...r.kernel.org, Jiang Biao <benbjiang@...cent.com>
Subject: Re: [PATCH] sched/fair: Optimize dequeue_task_fair()

On 11/08/2020 10:43, Jiang Biao wrote:
> Similar optimization as what has been done in commit,
> 7d148be69e3a(sched/fair: Optimize enqueue_task_fair())
> 
> dequeue_task_fair jumps to dequeue_throttle label when cfs_rq_of(se) is
> throttled which means that se can't be NULL. We can move the label after
> the if (!se) statement and remove the if(!se) statment as se is always
> NULL when reaching this point.
> 
> Besides, trying to keep the same pattern with enqueue_task_fair can make
> it more readable.
> 
> Signed-off-by: Jiang Biao <benbjiang@...cent.com>
> ---
>  kernel/sched/fair.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 04fa8dbcfa4d..cbbeafdfa8b7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5618,10 +5618,10 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  
>  	}
>  
> -dequeue_throttle:
> -	if (!se)
> -		sub_nr_running(rq, 1);
> +	/* At this point se is NULL and we are at root level*/
> +	sub_nr_running(rq, 1);
>  
> +dequeue_throttle:
>  	/* balance early to pull high priority tasks */
>  	if (unlikely(!was_sched_idle && sched_idle_rq(rq)))
>  		rq->next_balance = jiffies;

There is already a similar patch in master.

423d02e1463b - sched/fair: Optimize dequeue_task_fair() (2020-06-25 Peng
Wang)