linux-kernel - Re: [RFC V2 2/2] sched: idle: IRQ based next prediction for idle period

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Message-ID: <20160120193449.GV6357@twins.programming.kicks-ass.net>
Date:	Wed, 20 Jan 2016 20:34:49 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	tglx@...utronix.de, rafael@...nel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, nicolas.pitre@...aro.org,
	vincent.guittot@...aro.org
Subject: Re: [RFC V2 2/2] sched: idle: IRQ based next prediction for idle
 period

On Wed, Jan 20, 2016 at 05:00:33PM +0100, Daniel Lezcano wrote:
> +		variance = stats_variance(&w->stats, mean);
> +		/*
> +		 * We want to check the last interval is:
> +		 *
> +		 *  mean - stddev < interval < mean + stddev
> +		 *
> +		 * That simplifies to:
> +		 *
> +		 * -stddev < interval - mean < stddev
> +		 *
> +		 * abs(interval - mean) < stddev
> +		 *
> +		 * The standard deviation is the sqrt of the variance:
> +		 *
> +		 * abs(interval - mean) < sqrt(variance)
> +		 *
> +		 * and we want to prevent to do an sqrt, so we square
> +		 * the equation:
> +		 *
> +		 * (interval - mean)^2 < variance
> +		 *
> +		 * So if the latest value of the stats complies with
> +		 * this condition, then the wakeup source is
> +		 * considered predictable and can be used to predict
> +		 * the next event.
> +		 */
> +		interval = w->stats.values[w->stats.w_ptr];
> +		if ((u64)((interval - mean) * (interval - mean)) > variance)
> +			continue;
> +

Cute :-)

You could consider putting a simple 3 point median filter in front of
stat_add() to get rid of the worst input noise.

(I know, improving all this was a point for later)