linux-kernel - Re: [PATCH 8/5] llist: Remove cpu

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Message-ID: <20110912142305.GN7761@one.firstfloor.org>
Date:	Mon, 12 Sep 2011 16:23:05 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Huang Ying <ying.huang@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH 8/5] llist: Remove cpu_relax() usage in cmpxchg loops

On Mon, Sep 12, 2011 at 04:05:58PM +0200, Peter Zijlstra wrote:
> Subject: llist: Remove cpu_relax() usage in cmpxchg loops
> From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Date: Mon Sep 12 15:50:49 CEST 2011
> 
> Initial benchmarks show they're a net loss (2 socket wsm):
> 

May still save power. however:

>  		entry = cmpxchg(&head->first, old_entry, new);
>  		if (entry == old_entry)
>  			break;
> -		cpu_relax();

You need at least a barrier() then, otherwise the compiler could
change the memory order in the loop.

-Andi
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