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

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Date:	Mon, 7 May 2007 13:09:42 +0200 (CEST)
From:	Esben Nielsen <nielsen.esben@...glemail.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Esben Nielsen <nielsen.esben@...glemail.com>,
	Ingo Molnar <mingo@...e.hu>,
	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,
	Willy Tarreau <w@....eu>,
	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



On Sat, 5 May 2007, Linus Torvalds wrote:

>
>
> On Sat, 5 May 2007, Esben Nielsen wrote:
>>
>> I have been wondering why you use usigned for timers anyway. It is also like
>> that in hrtimers. Why not use signed and avoid (almost) all worries about wrap
>> around issues. The trick is that when all
>>   a < b
>> is be replaced by
>>   a - b < 0
>> the code will work on all 2-complement machines even if the (signed!) integers
>> a and b wrap around.
>
> No. BOTH of the above are buggy.
>
> The C language definition doesn't allow signed integers to wrap (ie it's
> undefined behaviour), so "a-b < 0" can be rewritten by the compiler as a
> simple signed "a < b".
>
> And the unsigned (or signed) "a < b" is just broken wrt any kind of
> wrap-around (whether wrapping around zero or the sign bit).
>
> 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".
>
> Notice? The signed variant is basically _never_ correct.
>

What is (long)(a-b) ? I have tried to look it up in the C99 standeard but 
I can't find it. Maybe it is in the referred LIA-1 standeard, which I 
can't find with google.

I think the best would be to use "a-b > ULONG_MAX/2" when you mean "a<b" 
as that should be completely portable.

According to C99 Appendix H2.2 
(http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf) an 
implementation can choose to do modulo signed integers as it is 
mandatory for unsigned integers. If an implementation have choosen 
to do that it must be a bug to to do the "a-b < 0" -> "a<b" optimization.

I have never experienced a compiler/architecture combination _not_ doing 
wrapped signed integers.

Esben


> 		Linus
>
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