Re: [patch] CFS scheduler, -v8

Esben Nielsen
2007-05-07T11:30:56Z

On Sun, 6 May 2007, Linus Torvalds wrote:

>
>
> On Sun, 6 May 2007, Ingo Molnar wrote:
>>
>> * Linus Torvalds <torvalds@linux-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
>