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

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Message-ID: <20160120184414.GQ6357@twins.programming.kicks-ass.net>
Date:	Wed, 20 Jan 2016 19:44:14 +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 12:46:48PM -0500, Nicolas Pitre wrote:
> > +struct stats {
> > +	u64           sum;                     /* sum of values */
> > +	u32           values[STATS_NR_VALUES]; /* array of values */
> > +	unsigned char w_ptr;                   /* current window pointer */
> 
> Why did you change this from an unsigned int?
> 
> This won't provide any memory space saving given that the structure has 
> to be padded up to the next 64-bit boundary.

Not to mention that loading bytes is more expensive on many archs
compared to full words.

Also, its not a pointer, its an index.

So:	unsigned int w_idx;