linux-kernel - Re: [patch] CFS scheduler, -v8

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Message-ID: <20070506083600.GA16043@1wt.eu>
Date:	Sun, 6 May 2007 10:36:00 +0200
From:	Willy Tarreau <w@....eu>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Esben Nielsen <nielsen.esben@...glemail.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Con Kolivas <kernel@...ivas.org>,
	Nick Piggin <npiggin@...e.de>, Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Williams <pwil3058@...pond.net.au>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Gene Heskett <gene.heskett@...il.com>, Mark Lord <lkml@....ca>,
	Zach Carter <linux@...hcarter.com>,
	buddabrod <buddabrod@...il.com>
Subject: Re: [patch] CFS scheduler, -v8

Hi Ingo,

On Sun, May 06, 2007 at 10:29:11AM +0200, Ingo Molnar wrote:
> 
> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > So the _only_ valid way to handle timers is to
> >  - either not allow wrapping at all (in which case "unsigned" is better, 
> >    since it is bigger)
> >  - or use wrapping explicitly, and use unsigned arithmetic (which is 
> >    well-defined in C) and do something like "(long)(a-b) > 0".
> 
> hm, there is a corner-case in CFS where a fix like this is necessary.
> 
> CFS uses 64-bit values for almost everything, and the majority of values 
> are of 'relative' nature with no danger of overflow. (They are signed 
> because they are relative values that center around zero and can be 
> negative or positive.)

(...)

> -		if (key < entry->fair_key) {
> +		if ((s64)(entry->fair_key - key) > 0) {

Just a hint: while your code here is correct, it is a good practise to
check against < 0 instead, so that if for any reason you sometimes forget
to cast to signed, the compiler will emit a warning stating that the
condition is always false. This would simply become :

-		if (key < entry->fair_key) {
+		if ((s64)(key - entry->fair_key) < 0) {

Just my .02 euros :-)
Willy

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