linux-kernel - Re: [PATCH 1/2] rwsem: check the lock before cpmxchg in down_write_trylock and rwsem_do

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Date:	Fri, 21 Jun 2013 17:15:28 -0700
From:	Davidlohr Bueso <davidlohr.bueso@...com>
To:	Alex Shi <alex.shi@...el.com>
Cc:	Tim Chen <tim.c.chen@...ux.intel.com>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] rwsem: check the lock before cpmxchg in
 down_write_trylock and rwsem_do_wake

On Sat, 2013-06-22 at 08:10 +0800, Alex Shi wrote:
> On 06/22/2013 07:51 AM, Tim Chen wrote:
> > Doing cmpxchg will cause cache bouncing when checking
> > sem->count. This could cause scalability issue
> > in a large machine (e.g. a 80 cores box).
> > 
> > A pre-read of sem->count can mitigate this.
> > 
> > Signed-off-by: Alex Shi <alex.shi@...el.com>
> > Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> 
> Hi Tim,
> there is a technical error in this patch.
> the "From: " line should be 'Alex Shi', since he made the most input of
> this patch.
> 
> And I still think split this patch to 4 smaller will make it more simple
> to review, that I had sent you and Davidlohr.

Yep, and you had updated the changelog for 1/4: rwsem: check the lock
before cpmxchg in down_write_trylock to:

"cmpxchg will cause cache bouncing when do the value checking, that
cause scalability issue in a large machine (like a 80 cores box).

A lock status pre-read can relief this."

> 
> could you like to re-send with my 4 patch version? :)

For those 4 patches:
Acked-by: Davidlohr Bueso <davidlohr.bueso@...com>


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