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

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Message-ID: <20081108190517.GA9806@elte.hu>
Date:	Sat, 8 Nov 2008 20:05:17 +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:

> 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.

no, we really had it in the vsyscall case: which uses vread, which 
uses __native_read_tsc(), which had the barriers.

And i think that's the _only_ valid place to have it.

So that's why my change moves it from the __native_read_tsc() over to 
_only_ the vget_cycles().

am i missing something on such a nice Saturday evening? :)

	Ingo
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