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

[Daniel Lezcano]

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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