linux-kernel - Re: [PATCH] optimize ktime_divns for constant divisors

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Message-ID: <2110446.Lbe6oKjkNj@wuerfel>
Date:	Fri, 05 Dec 2014 22:03:31 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] optimize ktime_divns for constant divisors

On Friday 05 December 2014 13:00:22 Nicolas Pitre wrote:
> 
> BTW this is worth applying despite the on-going discussion with Arnd 
> on a separate optimization.

Agreed

> On Wed, 3 Dec 2014, Nicolas Pitre wrote:
> 
> > At least on ARM, do_div() is optimized to turn constant divisors into
> > an inline multiplication by the reciprocal value at compile time. 
> > However this optimization is missed entirely whenever ktime_divns() is
> > used and the slow out-of-line division code is used all the time.
> > 
> > Let ktime_divns() use do_div() inline whenever the divisor is constant
> > and small enough.  This will make things like ktime_to_us() and 
> > ktime_to_ms() much faster.
> > 
> > Signed-off-by: Nicolas Pitre <nico@...aro.org>

Acked-by: Arnd Bergmann <arnd@...db.de>
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