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

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Message-ID: <20131002124539.GF7941@localhost.localdomain>
Date:	Wed, 2 Oct 2013 14:45:41 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 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?

> 
> 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.
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