linux-kernel - Re: Performance regression from switching lock to rw-sem for anon-vma tree

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Message-ID: <20130723095124.GW27075@twins.programming.kicks-ass.net>
Date:	Tue, 23 Jul 2013 11:51:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Tim Chen <tim.c.chen@...ux.intel.com>, Ingo Molnar <mingo@...e.hu>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mgorman@...e.de>, "Shi, Alex" <alex.shi@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	"Wilcox, Matthew R" <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm <linux-mm@...ck.org>
Subject: Re: Performance regression from switching lock to rw-sem for
 anon-vma tree

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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