linux-kernel - Re: [PATCH V2 1/1] sched/fair:Reduce unnecessary check preempt in the sched tick

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Message-ID: <YHaV1CGkbUgbp2ek@hirez.programming.kicks-ass.net>
Date:   Wed, 14 Apr 2021 09:12:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     qianjun.kernel@...il.com
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 1/1] sched/fair:Reduce unnecessary check preempt in
 the sched tick

On Wed, Apr 14, 2021 at 10:22:29AM +0800, qianjun.kernel@...il.com wrote:
> From: jun qian <qianjun.kernel@...il.com>
> 
> As you are already set the TIF_NEED_RESCHED, there is no need
> to check resched again.

Still no justification; does this actually help anything?

> Signed-off-by: jun qian <qianjun.kernel@...il.com>
> ---
>  kernel/sched/fair.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 794c2cb945f8..1a69b5fffe4a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4360,19 +4360,26 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>  {
>  	unsigned long ideal_runtime, delta_exec;
>  	struct sched_entity *se;
> +	struct rq *rq = rq_of(cfs_rq);
>  	s64 delta;
>  
>  	ideal_runtime = sched_slice(cfs_rq, curr);
>  	delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
>  	if (delta_exec > ideal_runtime) {
> -		resched_curr(rq_of(cfs_rq));
> +		if (!test_tsk_need_resched(rq->curr))
> +			resched_curr(rq_of(cfs_rq));
>  		/*
>  		 * The current task ran long enough, ensure it doesn't get
>  		 * re-elected due to buddy favours.
>  		 */
>  		clear_buddies(cfs_rq, curr);
>  		return;
> -	}
> +	/*
> +	 * If here with TIF_NEED_RESCHED already set from the early entity_tick,
> +	 * there is no need to check again.
> +	 */
> +	} else if (test_tsk_need_resched(rq->curr))
> +		return;

This is horrific style. And, afaict, completely unnecessary.