linux-kernel - Re: [PATCH 3/3] sched: limit cpu search and rotate search window for scalability

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Message-ID: <20180424125349.GU4082@hirez.programming.kicks-ass.net>
Date:   Tue, 24 Apr 2018 14:53:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     subhra mazumdar <subhra.mazumdar@...cle.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        daniel.lezcano@...aro.org, steven.sistare@...cle.com,
        dhaval.giani@...cle.com, rohit.k.jain@...cle.com
Subject: Re: [PATCH 3/3] sched: limit cpu search and rotate search window for
 scalability

On Mon, Apr 23, 2018 at 05:41:16PM -0700, subhra mazumdar wrote:
> Lower the lower limit of idle cpu search in select_idle_cpu() and also put
> an upper limit. This helps in scalability of the search by restricting the
> search window. 

> @@ -6297,15 +6297,24 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  
>  	if (sched_feat(SIS_PROP)) {
>  		u64 span_avg = sd->span_weight * avg_idle;
> -		if (span_avg > 4*avg_cost)
> +		if (span_avg > 2*avg_cost) {
>  			nr = div_u64(span_avg, avg_cost);
> -		else
> -			nr = 4;
> +			if (nr > 4)
> +				nr = 4;
> +		} else {
> +			nr = 2;
> +		}
>  	}

Why do you need to put a max on? Why isn't the proportional thing
working as is? (is the average no good because of big variance or what)

Again, why do you need to lower the min; what's wrong with 4?

The reason I picked 4 is that many laptops have 4 CPUs and desktops
really want to avoid queueing if at all possible.