linux-kernel - Re: [PATCH 3/4] sched/fair: Do not migrate on wake_affine

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Message-ID: <20180212172926.GY25201@hirez.programming.kicks-ass.net>
Date:   Mon, 12 Feb 2018 18:29:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Mike Galbraith <efault@....de>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] sched/fair: Do not migrate on wake_affine_weight if
 weights are equal

On Mon, Feb 12, 2018 at 02:58:56PM +0000, Mel Gorman wrote:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c1091cb023c4..28c8d9c91955 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5747,7 +5747,16 @@ wake_affine_weight(struct sched_domain *sd, struct task_struct *p,
>  		prev_eff_load *= 100 + (sd->imbalance_pct - 100) / 2;
>  	prev_eff_load *= capacity_of(this_cpu);
>  
> -	return this_eff_load <= prev_eff_load ? this_cpu : nr_cpumask_bits;
> +	/*
> +	 * If sync, adjust the weight of prev_eff_load such that if
> +	 * prev_eff == this_eff that select_idle_sibling will consider
> +	 * stacking the wakee on top of the waker if no other CPU is
> +	 * idle.
> +	 */
> +	if (sync)
> +		prev_eff_load += 1;

So where we had <= and would consistently favour pulling the task to the
waking CPU when all else what equal, you now switch to <, such that when
things are equal we do not pull.

That makes sense I suppose.

Except for sync wakeups, where you say, if everything else is equal,
pull, which also makes sense, because sync says 'current' promises to go
away.

OK.

> +
> +	return this_eff_load < prev_eff_load ? this_cpu : nr_cpumask_bits;
>  }
>  
>  static int wake_affine(struct sched_domain *sd, struct task_struct *p,
> -- 
> 2.15.1
>