linux-kernel - Re: [regression, 3.16-rc] rwsem: optimistic spinning causing performance degradation

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Date:	Fri, 4 Jul 2014 09:52:47 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Low <jason.low2@...com>
Cc:	Dave Chinner <david@...morbit.com>,
	Davidlohr Bueso <davidlohr@...com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [regression, 3.16-rc] rwsem: optimistic spinning causing
 performance degradation

On Thu, Jul 03, 2014 at 06:54:50PM -0700, Jason Low wrote:
> Subject: [PATCH] rwsem: In rwsem_can_spin_on_owner(), return false if no owner
> 
> It was found that the rwsem optimistic spinning feature can potentially degrade
> performance when there are readers. Perf profiles indicate in some workloads
> that significant time can be spent spinning on !owner. This is because we don't
> set the lock owner when readers(s) obtain the rwsem.
> 
> In this patch, we'll modify rwsem_can_spin_on_owner() such that we'll return
> false if there is no lock owner. The rationale is that if we just entered the
> slowpath, yet there is no lock owner, then there is a possibility that a reader
> has the lock. To be conservative, we'll avoid spinning in these situations.
> 
> Dave Chinner found performance benefits with this patch in the xfs_repair
> workload, where the total run time went from approximately 4 minutes 24 seconds,
> down to approximately 1 minute 26 seconds with the patch.
> 
> Tested-by: Dave Chinner <dchinner@...hat.com>
> Signed-off-by: Jason Low <jason.low2@...com>

Davidlohr, you'll be running this through your AIM and other benchmarks,
I suppose?

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