linux-kernel - Re: [PATCH 01/16] math128: Introduce various 128bit primitives

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Date:	Thu, 25 Oct 2012 15:47:29 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Juri Lelli <juri.lelli@...il.com>, tglx@...utronix.de,
	mingo@...hat.com, rostedt@...dmis.org, oleg@...hat.com,
	fweisbec@...il.com, darren@...art.com, johan.eker@...csson.com,
	p.faure@...tech.ch, linux-kernel@...r.kernel.org,
	claudio@...dence.eu.com, michael@...rulasolutions.com,
	fchecconi@...il.com, tommaso.cucinotta@...up.it,
	nicola.manica@...i.unitn.it, luca.abeni@...tn.it,
	dhaval.giani@...il.com, hgu1972@...il.com,
	paulmck@...ux.vnet.ibm.com, raistlin@...ux.it,
	insop.song@...csson.com, liming.wang@...driver.com,
	jkacur@...hat.com, harald.gustafsson@...csson.com,
	vincent.guittot@...aro.org, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 01/16] math128: Introduce various 128bit primitives

On Wed, 2012-10-24 at 16:18 -0700, Linus Torvalds wrote:
> On Wed, Oct 24, 2012 at 2:53 PM, Juri Lelli <juri.lelli@...il.com> wrote:
> > From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> >
> > Grow rudimentary u128 support without relying on gcc/libgcc.
> 
> I missed the part where somebody explains why and what needs this?
> It's going to be very expensive indeed on some platforms, so the fact
> that it is *sometimes* cheap doesn't necessarily imply it should ever
> be used.
> 
> So please, explain what the pressing need is that is so worthwhile
> that this is worth it. Maybe it was in a 00/16 cover letter, but not
> only was that not sent out to the people who got 01, you'd still want
> it in the commit message. 

There's two use cases:

 1) the proposed SCHED_DEADLINE needs to do some u64xu64 math, it
    ends up having to multiply a deadline (in usec) with runtime (also
    in usec).

 2) the infrastructure adds mul_u64_u32_shr(), which is something we
    do a lot of with all the time manipulation, apply a multiplier to
    some u64 clock value.

    We can do better on some archs than we can in generic, so this
    interface could give a win there.


But yes, in general people should be very very reluctant to use this.
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