linux-kernel - Re: [git pull] scheduler updates

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Date:	Sat, 8 Nov 2008 20:10:10 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>
Subject: Re: [git pull] scheduler updates


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Sat, 8 Nov 2008, Arjan van de Ven wrote:
> > 
> > historically it was for early AMD cpus (K7, not sure if early K8 did
> > this) where 2 consecutive rdtsc's in the same codestream would get
> > reordered compared to eachother, so you could observe the tsc go
> > backwards...
> 
> .. but this only happens with two _consecutive_ ones.
> 
> The thing is, nobody sane does that in generic code. The scheduler wants 
> to have cycles, yes, but two consecutive scheduler invocations will have 
> spinlocks etc in between. That's true of _all_ sane uses of a TSC.
> 
> I don't see that there is ever any reason to do the barriers for any 
> normal case. And the cases where it does matter would actually be worth 
> pointing out (ie making the barriers explicit in those cases, and those 
> cases only).
> 
> Doing it in get_cycles() and "forgetting about it" may sound like a simple 
> solution, but it's likely wrong. For example, one of the few cases where 
> we realy care about time going backwards is gettimeofday() - which uses 
> tsc, but which also has tons of serializing instructions on its own. 
> EXCEPT WHEN IT IS a vsyscall!
> 
> But in that case, we don't even have the barrier, because we put it in the 
> wrong function and 'forgot about it'. Of course, we may not need it 
> (rdtscp maybe always serializes, I didn't check), but the point is, an 
> explicit barrier is actually better than one that is hidden.
> 
> So who _really_ needs it? And why not just do it there?

i think, the tree as offered to you, intends to do just that, unless i 
made some grave (and unintended) mistake somewhere.

The barrier is only present in the vread function: which is the 
vsyscall-read function, to be used from user-space.

Even in the past, no was actually forgotten or put in the wrong 
function as far as i can see because previously _everything_ 
(including the vread method) had the barrier.

The change from me simply removes the barrier from the places that 
dont need it - exactly for the reason you outlined: the scheduler is 
both imprecise and has a ton of natural serialization anyway, so it's 
a non-issue there.

Hm?

	Ingo
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