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Date: Tue, 23 Jul 2013 11:51:24 +0200
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: Performance regression from switching lock to rw-sem for
 anon-vma tree
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To: Ingo Molnar <mingo@kernel.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>, Ingo Molnar <mingo@elte.hu>, Andrea Arcangeli <aarcange@redhat.com>, Mel Gorman <mgorman@suse.de>, "Shi, Alex" <alex.shi@intel.com>, Andi Kleen <andi@firstfloor.org>, Andrew Morton <akpm@linux-foundation.org>, Michel Lespinasse <walken@google.com>, Davidlohr Bueso <davidlohr.bueso@hp.com>, "Wilcox, Matthew R" <matthew.r.wilcox@intel.com>, Dave Hansen <dave.hansen@intel.com>, Rik van Riel <riel@redhat.com>, linux-kernel@vger.kernel.org, linux-mm <linux-mm@kvack.org>

On Tue, Jul 23, 2013 at 11:45:13AM +0200, Ingo Molnar wrote:
> Why not just try the delayed addition approach first? The spinning is time 
> limited AFAICS, so we don't _have to_ recognize those as writers per se, 
> only if the spinning fails and it wants to go on the waitlist. Am I 
> missing something?
> 
> It will change patterns, it might even change the fairness balance - but 
> is a legit change otherwise, especially if it helps performance.

Be very careful here. Some people (XFS) have very specific needs. Walken
and dchinner had a longish discussion on this a while back.

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