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

From: Nicolas Pitre
Date: Wed Jan 20 2016 - 15:14:37 EST

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'