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

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Date:	Fri, 26 Oct 2012 11:35:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.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

On Fri, 2012-10-26 at 11:24 +0200, Ingo Molnar wrote:

> 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.

I _think_ something like: dl_runtime * dl_deadline < U64_MAX, might do
that. The question is, is this constraint usable? Simplified that boils
down to about 4 seconds each, which sounds pretty much ok for most
people -- but such statements usually come back to bite you (640kb
anybody...).

Hmm, patch 8 (which adds period support) changes this slightly again.
Would it then end up being something like:

  dl_period * dl_runtime < U64_MAX && dl_deadline * dl_runtime < U64_MAX

?

Juri, did I get that constraint right and do you know about use-cases
where this would be prohibitive?

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