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

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Date:	Thu, 21 Jan 2016 14:04:05 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	tglx@...utronix.de, peterz@...radead.org, rafael@...nel.org,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	vincent.guittot@...aro.org
Subject: Re: [RFC V2 2/2] sched: idle: IRQ based next prediction for idle
 period

On 01/20/2016 09:14 PM, Nicolas Pitre wrote:
> On Wed, 20 Jan 2016, Daniel Lezcano wrote:
> [...]
>
> One more comment:
>
>> +		/*
>> +		 * If the mean value is null, just ignore this wakeup
>> +		 * source.
>> +		 */
>> +		mean = stats_mean(&w->stats);
>> +		if (!mean)
>> +			continue;
>> +
>> +		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;
>> +
>> +		/*
>> +		 * Let's compute the next event: the wakeup source is
>> +		 * considered predictable, we add the average interval
>> +		 * time added to the latest interruption event time.
>> +		 */
>> +		next = ktime_add_us(w->timestamp, stats_mean(&w->stats));
>
> You don't need to call stats_mean() again as you have it in the 'mean'
> variable already.

Good point.


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