linux-kernel - Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock

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Message-ID: <20131002125052.GE28601@twins.programming.kicks-ass.net>
Date:	Wed, 2 Oct 2013 14:50:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Fernando Luis Vázquez Cao 
	<fernando_b1@....ntt.co.jp>, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock

On Wed, Oct 02, 2013 at 02:45:41PM +0200, Frederic Weisbecker wrote:
> On Tue, Oct 01, 2013 at 06:59:57PM +0200, Peter Zijlstra wrote:
> > On Tue, Oct 01, 2013 at 06:47:10PM +0200, Frederic Weisbecker wrote:
> > > Yeah thinking more about it, the preempt disable was probably not
> > > necessary. Now that's trading 2 atomics + 1 Lock/Unlock with 2 Lock/Unlock.
> > 
> > It trades the current 2 atomics for 2 LOCK/UNLOCK. And on x86_64 that's
> > 2 atomics.
> 
> Do you mean 2 atomics for LOCK/UNLOCK? Or is that pair optimized somehow
> in x86?

Unlock isn't an atomic on x86_64; it can be on certain i386 builds. See
UNLOCK_LOCK_PREFIX.

> > 
> > So all we get is some extra branches; which we must hope for don't
> > actually mess things up too bad.
> > 
> > Ohh.. wait a sec.. we also call local_clock() which includes another
> > atomic :/ Bugger.. 
> 
> Yeah. Anyway, I'm going to try something on top of that. May be we'll get
> a fresher mind and ideas on how to optimize that all after.

Fair enough.
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