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

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Message-ID: <20131002160116.GG7941@localhost.localdomain>
Date:	Wed, 2 Oct 2013 18:01:18 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Arjan van de Ven <arjan@...ux.intel.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 Wed, Oct 02, 2013 at 07:35:49AM -0700, Arjan van de Ven wrote:
> 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 ;-)
> 

Yeah, we are never getting short on black magic as the time goes :)
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