linux-kernel - Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file

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Date:	Fri, 27 Sep 2013 08:02:13 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Jason Low <jason.low2@...com>, Davidlohr Bueso <davidlohr@...com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	linux-kernel@...r.kernel.org, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and
 locking code into its own file


* Tim Chen <tim.c.chen@...ux.intel.com> wrote:

> > If we prefer to optimize this a bit though, perhaps we can first move 
> > the node->lock = 0 so that it gets executed after the "if (likely(prev 
> > == NULL)) {}" code block and then delete "node->lock = 1" inside the 
> > code block.
> 
> I suppose we can save one single assignment. The gain is probably not 
> noticeable as once we set node->next to NULL, node->locked is likely in 
> local cache line and the assignment operation is cheap.

Would be nice to have this as a separate, add-on patch. Every single 
instruction removal that has no downside is an upside!

You can add a comment that explains it.

Thanks,

	Ingo
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