linux-kernel - Re: [PATCH 04/16] sched/fair: Optimize find_idlest

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Message-ID: <20160601195920.GE28447@twins.programming.kicks-ass.net>
Date:	Wed, 1 Jun 2016 21:59:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	mingo@...hat.com, dietmar.eggemann@....com, yuyang.du@...el.com,
	vincent.guittot@...aro.org, mgalbraith@...e.de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/16] sched/fair: Optimize find_idlest_cpu() when there
 is no choice

On Mon, May 23, 2016 at 11:58:46AM +0100, Morten Rasmussen wrote:
> In the current find_idlest_group()/find_idlest_cpu() search we end up
> calling find_idlest_cpu() in a sched_group containing only one cpu in
> the end. Checking idle-states becomes pointless when there is no
> alternative, so bail out instead.
> 
> cc: Ingo Molnar <mingo@...hat.com>
> cc: Peter Zijlstra <peterz@...radead.org>
> 
> Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
> ---
>  kernel/sched/fair.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0fe3020..564215d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5155,6 +5155,11 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
>  	int shallowest_idle_cpu = -1;
>  	int i;
>  
> +	/* Check if we have any choice */
> +	if (group->group_weight == 1) {
> +		return cpumask_first(sched_group_cpus(group));
> +	}

superfluous brackets, also isn't @this_cpu supposed to be part of the
sched_group_cpu(group) mask?