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

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Message-ID: <20081108104116.48bd26e6@infradead.org>
Date:	Sat, 8 Nov 2008 10:41:16 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, 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

On Sat, 8 Nov 2008 10:28:21 -0800 (PST)
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 
> 
> On Sat, 8 Nov 2008, Ingo Molnar wrote:
> >
> > Ingo Molnar (2):
> >       sched: improve sched_clock() performance
> >       sched: optimize sched_clock() a bit
> 
> Btw, why do we do that _idiotic_ rdtsc_barrier() AT ALL?
> 
> No sane user can possibly want it. If you do 'rdtsc', there's nothing
> you can do about a few cycles difference due to OoO _anyway_. Adding
> barriers is entirely meaningless - it's not going to make the return
> value mean anything else.
> 
> Can we please just remove that idiocy? Or can somebody give a _sane_ 
> argument for it?

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...



-- 
Arjan van de Ven 	Intel Open Source Technology Centre
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