linux-kernel - Re: [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle

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Date:   Wed, 24 Nov 2021 12:13:28 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Barry Song <21cnbao@...il.com>
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, rostedt@...dmis.org,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        Barry Song <song.bao.hua@...ilicon.com>
Subject: Re: [PATCH v2] sched/fair: Remove the cost of a redundant
 cpumask_next_wrap in select_idle_cpu

On Wed, Nov 24, 2021 at 05:15:46PM +0800, Barry Song wrote:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6e476f6..8cd23f1 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6278,6 +6278,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>  		time = cpu_clock(this);
>  	}
>  
> +	--nr;
>  	for_each_cpu_wrap(cpu, cpus, target + 1) {
>  		if (has_idle_core) {
>  			i = select_idle_core(p, cpu, cpus, &idle_cpu);
> @@ -6285,11 +6286,11 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>  				return i;
>  
>  		} else {
> -			if (!--nr)
> -				return -1;
>  			idle_cpu = __select_idle_cpu(cpu, p);
>  			if ((unsigned int)idle_cpu < nr_cpumask_bits)
>  				break;
> +			if (!--nr)
> +				return -1;
>  		}
>  	}

This way nr can never be 1 for a single iteration -- it current isn't,
but that's besides the point.