Message-ID: <1380263214.5774.12.camel@j-VirtualBox>
Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and
 locking code into its own file
From: Jason Low <jason.low2@hp.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>, Davidlohr Bueso <davidlohr@hp.com>,
        Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>, Alex Shi <alex.shi@linaro.org>,
        Andi Kleen <andi@firstfloor.org>,
        Michel Lespinasse <walken@google.com>,
        Davidlohr Bueso <davidlohr.bueso@hp.com>,
        Matthew R Wilcox <matthew.r.wilcox@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Rik van Riel <riel@redhat.com>,
        Peter Hurley <peter@hurleysoftware.com>, linux-kernel@vger.kernel.org,
        linux-mm <linux-mm@kvack.org>
Date: Thu, 26 Sep 2013 23:26:54 -0700
In-Reply-To: <20130927060213.GA6673@gmail.com>
References: <cover.1380144003.git.tim.c.chen@linux.intel.com>
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On Fri, 2013-09-27 at 08:02 +0200, Ingo Molnar wrote:
> * Tim Chen <tim.c.chen@linux.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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