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

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Message-ID: <524C2F45.4040401@linux.intel.com>
Date:	Wed, 02 Oct 2013 07:35:49 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.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 10/2/2013 5:45 AM, 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 is not actually an atomic.

and on some modern machines, neither is the lock, for the uncontended case ;-)

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