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

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Message-ID: <20160120194006.GW6357@twins.programming.kicks-ass.net>
Date:	Wed, 20 Jan 2016 20:40:06 +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:
> +static inline u32 stats_mean(struct stats *s)
> +{
> +	/*
> +	 * gcc is smart enough to convert to a bits shift when the
> +	 * divisor is constant and multiple of 2^x.
> +	 *
> +	 * The number of values could have not reached STATS_NR_VALUES
> +	 * yet, but we can consider it acceptable as the situation is
> +	 * only at the beginning of the burst of irqs.
> +	 */
> +	return s->sum / STATS_NR_VALUES;
> +}

Note that ->sum is u64, so you're very prone to truncation.