linux-kernel - Re: [RFC PATCH] sched: Reduce overestimating avg

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Date:	Wed, 31 Jul 2013 12:00:26 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Jason Low <jason.low2@...com>
CC:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	KML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>, Alex Shi <alex.shi@...el.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Namhyung Kim <namhyung@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Mel Gorman <mgorman@...e.de>, aswin@...com,
	scott.norton@...com, chegu_vinod@...com,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH] sched: Reduce overestimating avg_idle

On 07/31/2013 05:37 AM, Jason Low wrote:

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index e8b3350..62b484b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1341,12 +1341,12 @@ ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags)
>
>   	if (rq->idle_stamp) {
>   		u64 delta = rq->clock - rq->idle_stamp;
> -		u64 max = 2*sysctl_sched_migration_cost;
> +		u64 max = (sysctl_sched_migration_cost * 3) / 2;
>
> -		if (delta > max)
> +		update_avg(&rq->avg_idle, delta);
> +
> +		if (rq->avg_idle > max)
>   			rq->avg_idle = max;
> -		else
> -			update_avg(&rq->avg_idle, delta);
>   		rq->idle_stamp = 0;
>   	}

I wonder if we could get even more conservative values
of avg_idle by clamping delta to max, before calling
update_avg...

Or rather, I wonder if that would matter enough to make
a difference, and in what direction that difference would
be.

In other words:

	if (rq->idle_stamp) {
		u64 delta = rq->clock - rq->idle_stamp;
		u64 max = (sysctl_sched_migration_cost * 3) / 2;

		if (delta > max)
			delta = max;

		update_avg(&rq->avg_idle, delta);
		rq->idle_stamp = 0;
	}

-- 
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