From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1755143Ab3A2JFd (ORCPT <rfc822;w@1wt.eu>);
	Tue, 29 Jan 2013 04:05:33 -0500
Received: from mga03.intel.com ([143.182.124.21]:23063 "EHLO mga03.intel.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1754749Ab3A2JF3 (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Tue, 29 Jan 2013 04:05:29 -0500
X-ExtLoop1: 1
X-IronPort-AV: E=Sophos;i="4.84,559,1355126400"; 
   d="scan'208";a="249616825"
Date: Tue, 29 Jan 2013 17:06:20 +0800
From: Yuanhan Liu <yuanhan.liu@linux.intel.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Huang Ying <ying.huang@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        David Howells <dhowells@redhat.com>, linux-kernel@vger.kernel.org,
        lkp@linux.intel.com
Subject: Re: aim7 performance regression by commit 5a50508 report from LKP
Message-ID: <20130129090620.GT12678@yliu-dev.sh.intel.com>
References: <20130129082537.GS12678@yliu-dev.sh.intel.com>
 <20130129084400.GA5350@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20130129084400.GA5350@gmail.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Tue, Jan 29, 2013 at 09:44:00AM +0100, Ingo Molnar wrote:
> 
> * Yuanhan Liu <yuanhan.liu@linux.intel.com> wrote:
> 
> > [...]
> 
> Very nice measurements and analysis, thanks!
> 
> > As stated above, anybody can have a chance to own the lock in 
> > mutex once somebody release the lock. Well, there is only one 
> > to own the lock in rwsem write lock, and the one is known 
> > already: the one in the head of wait list. That would result 
> > to more contention in rwsem write lock case, especially if the 
> > one _will_ own the lock is not running now.
> 
> I think we should allow lock-steal between rwsem writers - that 
> will not hurt fairness as most rwsem fairness concerns relate to 
> reader vs. writer fairness.

Agreed, and I'm sure this will improve performance and may make this
performance regression go away. 

David, is that Ok to you? If so, I may have a try.

> 
> Am I correct to assume that all relevant users in this workload 
> are down_write() users?

Yes, as commit 5a50508 just convert all mutex to down_write.

Thanks.

	--yliu
> 
> You can see the type of lock use in:
> 
>    perf record -g
>    perf report
> 
> I bet that allowing rwsem writer lock-steal would improve other 
> workloads as well.