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Date: Tue, 18 Jun 2013 07:18:56 +0800
From: Alex Shi <alex.shi@intel.com>
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Subject: Re: Performance regression from switching lock to rw-sem for anon-vma
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To: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Davidlohr Bueso <davidlohr.bueso@hp.com>, Ingo Molnar <mingo@elte.hu>, Rik van Riel <riel@redhat.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>, Andrea Arcangeli <aarcange@redhat.com>, Mel Gorman <mgorman@suse.de>, Andi Kleen <andi@firstfloor.org>, Andrew Morton <akpm@linux-foundation.org>, Michel Lespinasse <walken@google.com>, "Wilcox, Matthew R" <matthew.r.wilcox@intel.com>, Dave Hansen <dave.hansen@intel.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>

On 06/18/2013 02:45 AM, Tim Chen wrote:
>>> +			if (unlikely(sem->count < RWSEM_WAITING_BIAS)) {
>>> > > +				cpu_relax();
>>> > > +				continue;
>>> > > +			}
> The above two if statements could be cleaned up as a single check:
> 		
> 			if (unlikely(sem->count < RWSEM_WAITING_BIAS))
> 				return sem;
> 	 
> This one statement is sufficient to check that we don't have a writer
> stolen the lock before we attempt to acquire the read lock by modifying
> sem->count.  
> 
> 

Thanks. I will send out the patchset base your suggestion.


-- 
Thanks
    Alex

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