linux-kernel - Re: [PATCH v2] sched/fair: update scale invariance of PELT

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Date:   Sat, 29 Apr 2017 00:09:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        dietmar.eggemann@....com, Morten.Rasmussen@....com,
        yuyang.du@...el.com, pjt@...gle.com, bsegall@...gle.com
Subject: Re: [PATCH v2] sched/fair: update scale invariance of PELT

On Mon, Apr 10, 2017 at 11:18:29AM +0200, Vincent Guittot wrote:
> +++ b/include/linux/sched.h
> @@ -313,6 +313,7 @@ struct load_weight {
>   */
>  struct sched_avg {
>  	u64				last_update_time;
> +	u64				stolen_idle_time;
>  	u64				load_sum;
>  	u32				util_sum;
>  	u32				period_contrib;

> +		if (sa->util_sum < (LOAD_AVG_MAX * 1000)) {
> +			/*
> +			 * Add the idle time stolen by running at lower compute
> +			 * capacity
> +			 */
> +			delta += sa->stolen_idle_time;
> +		}
> +		sa->stolen_idle_time = 0;


So I was wondering if stolen_idle_time really needs to be a u64. Afaict
we'll be at LOAD_AVG_MAX after LOAD_AVG_MAX_N periods, or LOAD_AVG_MAX_N
* LOAD_AVG_PERIOD time, which ends up being 11040.

After that you'll truncate it anyway.. so there shouldn't be a need to
be much larger than that, no?