linux-kernel - Re: [PATCH v3 4/5] clocksource: Enable clocksource

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Date:	Wed, 06 Mar 2013 13:20:13 -0800
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Feng Tang <feng.tang@...el.com>,
	John Stultz <john.stultz@...aro.org>,
	Ingo Molnar <mingo@...e.hu>,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	x86@...nel.org, Len Brown <lenb@...nel.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	linux-kernel@...r.kernel.org, gong.chen@...ux.intel.com
Subject: Re: [PATCH v3 4/5] clocksource: Enable clocksource_cyc2ns() to cover
 big cycles

On 03/06/2013 01:15 PM, Thomas Gleixner wrote:
> On Wed, 6 Mar 2013, H. Peter Anvin wrote:
> 
>> On 03/06/2013 06:09 AM, Thomas Gleixner wrote:
>>>
>>> This breaks everything which does not have a 64/32bit divide
>>> instruction. And you can't replace it with do_div() as that would
>>> impose massive overhead on those architectures in the fast path.
>>>
>>
>> Could we do the same kind of scaling-by-multiplication that we do in
>> kernel/time.c for this?
> 
> Not sure what you are referring to. kernel/time.c contains a lot of stuff :)
> 

This stuff, specifically the third clause (which incidentally could be
extended to the fourth clause without much trouble... I have
experimented with it already.)

It uses a N*N->2N multiply and a shift to do overflowless scaling; it is
±1 LSB in the upper half of the value range with can be remedied with an
additional 2N add.

	-hpa

/*
 * Convert jiffies to milliseconds and back.
 *
 * Avoid unnecessary multiplications/divisions in the
 * two most common HZ cases:
 */
unsigned int jiffies_to_msecs(const unsigned long j)
{
#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
        return (MSEC_PER_SEC / HZ) * j;
#elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)
        return (j + (HZ / MSEC_PER_SEC) - 1)/(HZ / MSEC_PER_SEC);
#else
# if BITS_PER_LONG == 32
        return (HZ_TO_MSEC_MUL32 * j) >> HZ_TO_MSEC_SHR32;	<---
# else
        return (j * HZ_TO_MSEC_NUM) / HZ_TO_MSEC_DEN;
# endif
#endif
}

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