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

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Message-ID: <20160120202242.GL6356@twins.programming.kicks-ass.net>
Date:	Wed, 20 Jan 2016 21:22:42 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Daniel Lezcano <daniel.lezcano@...aro.org>, tglx@...utronix.de,
	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 Wed, Jan 20, 2016 at 02:57:07PM -0500, Nicolas Pitre wrote:
> On Wed, 20 Jan 2016, Peter Zijlstra wrote:
> 
> > 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.
> 
> It won't.  It is the sum of u32 values, so the mean of those values 
> can't exceed 32 bits.

Ah indeed!