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

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Message-ID: <20131001165957.GT3657@laptop.programming.kicks-ass.net>
Date:	Tue, 1 Oct 2013 18:59:57 +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 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.

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.. 


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