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

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Date:	Fri, 14 Jun 2013 15:44:28 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Davidlohr Bueso <davidlohr.bueso@...com>
Cc:	Ingo Molnar <mingo@...e.hu>, Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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>,
	"Wilcox, Matthew R" <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.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


> 
> Unfortunately this patch didn't make any difference, in fact it hurt
> several of the workloads even more. I also tried disabling preemption
> when spinning on owner to actually resemble spinlocks, which was my
> original plan, yet not much difference. 
> 

That's also similar to the performance I got.  There are things about
optimistic spinning that I missed that results in the better mutex
performance.  So I'm scratching my head.

> A few ideas that come to mind are avoiding taking the ->wait_lock and
> avoid dealing with waiters when doing the optimistic spinning (just like
> mutexes do).
> 

For my patch, we actually spin without the wait_lock.

> I agree that we should first deal with the optimistic spinning before
> adding the MCS complexity.
> 
> > Matthew and I have also discussed possibly introducing some 
> > limited spinning for writer when semaphore is held by read.  
> > His idea was to have readers as well as writers set ->owner.  
> > Writers, as now, unconditionally clear owner.  Readers clear 
> > owner if sem->owner == current.  Writers spin on ->owner if ->owner 
> > is non-NULL and still active. That gives us a reasonable chance 
> > to spin since we'll be spinning on
> > the most recent acquirer of the lock.
> 
> I also tried implementing this concept on top of your patch, didn't make
> much of a difference with or without it. 
> 

It also didn't make a difference for me.

Tim

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