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

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Message-ID: <CALAqxLVzOAeOj8Rdo9=owEQ8Jy=-J1jQbaPoqxZqYJBdbwFTwA@mail.gmail.com>
Date:	Thu, 18 Dec 2014 13:21:54 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] optimize ktime_divns for constant divisors

On Fri, Dec 5, 2014 at 1:03 PM, Arnd Bergmann <arnd@...db.de> wrote:
> 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>


Ok, I've queued the original patch up for testing.

thanks
-john
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