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

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Message-ID: <20160120192952.GU6357@twins.programming.kicks-ass.net>
Date:	Wed, 20 Jan 2016 20:29:52 +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 02:17:57PM -0500, Nicolas Pitre wrote:
> > It is also a friggin pointless /1000. The cpuidle code also loves to do
> > this, and its silly, u64 add/sub are _way_ cheaper than u64 / 1000.
> 
> For the purpose of this code, nanoseconds simply provides too many bits 
> for what we care.  Computing the variance implies squared values.
> 
> *However* we can simply do diff = (timestamp - w->timestamp) >> 10 
> instead.  No need to have an exact microsecs base.

Right, you could also reduce bits at the variance computation, but yes.