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

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Date:	Fri, 26 Oct 2012 11:24:21 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 01/16] math128: Introduce various 128bit primitives


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, 2012-10-25 at 15:26 -0700, Linus Torvalds wrote:
> > it's the *rest* of the "u128" math I really object to. I also wonder
> > about the u64xu64 math case for SCHED_DEADLINE, because I assume that
> > it doesn't actually end up using the 128-bit result in that form, but
> > scales it down again some way? 
> 
> No, it does a compare on two u128, so it doesn't loose any 
> precision. If it were to scale down again and loose precision 
> I'd agree with you that introducing the u128 stuff is 
> pointless.
> 
> The point is (as mentioned in the comments below) overflowing 
> an actual u64 is rare, however since some of this 
> (specifically the dl_{runtime,deadline} parameters) is user 
> specified, we have to assume we will overflow.

So can we control this by restricting the users and avoiding the 
overflow?

A 2^64 result should be a *huge* amount of space already for 
just about anything.

Thanks,

	Ingo
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