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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Message-ID: <1380263214.5774.12.camel@j-VirtualBox>
Date:	Thu, 26 Sep 2013 23:26:54 -0700
From:	Jason Low <jason.low2@...com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Tim Chen <tim.c.chen@...ux.intel.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

On Fri, 2013-09-27 at 08:02 +0200, Ingo Molnar wrote:
> * 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.

Yup, especially a spin lock (and one that I have found to be be used
very frequently when running workloads on big machines).

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