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

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Date:	Mon, 7 May 2007 13:30:46 +0200 (CEST)
From:	Esben Nielsen <nielsen.esben@...glemail.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Ingo Molnar <mingo@...e.hu>,
	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,
	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 Sun, 6 May 2007, Linus Torvalds wrote:

>
>
> On Sun, 6 May 2007, 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.)
>
> Well, I'd like to just worry about that for a while.
>
> You say there is "no danger of overflow", and I mostly agree that once
> we're talking about 64-bit values, the overflow issue simply doesn't
> exist, and furthermore the difference between 63 and 64 bits is not really
> relevant, so there's no major reason to actively avoid signed entries.
>
> So in that sense, it all sounds perfectly sane. And I'm definitely not
> sure your "292 years after bootup" worry is really worth even considering.
>

I would hate to tell mission control for Mankind's first mission to another
star to reboot every 200 years because "there is no need to worry about it."

As a matter of principle an OS should never need a reboot (with exception 
for upgrading). If you say you have to reboot every 200 years, why not 
every 100? Every 50? .... Every 45 days (you know what I am referring 
to :-) ?

> When we're really so well off that we expect the hardware and software
> stack to be stable over a hundred years, I'd start to think about issues
> like that, in the meantime, to me worrying about those kinds of issues
> just means that you're worrying about the wrong things.
>
> BUT.
>
> There's a fundamental reason relative timestamps are difficult and almost
> always have overflow issues: the "long long in the future" case as an
> approximation of "infinite timeout" is almost always relevant.
>
> So rather than worry about the system staying up 292 years, I'd worry
> about whether people pass in big numbers (like some MAX_S64 approximation)
> as an approximation for "infinite", and once you have things like that,
> the "64 bits never overflows" argument is totally bogus.
>
> There's a damn good reason for using only *absolute* time. The whole
> "signed values of relative time" may _sound_ good, but it really sucks in
> subtle and horrible ways!
>

I think you are wrong here. The only place you need absolute time is a 
for the clock (CLOCK_REALTIME). You waste CPU using a 64 bit
representation when you could have used a 32 bit. With a 32 bit 
implementation you are forced to handle the corner cases with wrap 
around and too big arguments up front. With a 64 bit you hide those 
problems.

I think CFS would be best off using a 32 bit timer counting in micro 
seconds. That would wrap around in 72 minuttes. But as the timers are 
relative you will never be able to specify a timer larger than 36 minuttes 
in the future. But 36 minuttes is redicolously long for a scheduler and a 
simple test limiting time values to that value would not break anything.

Esben

> 			Linus
>
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