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 S1161864AbcFHI6q (ORCPT <rfc822;w@1wt.eu>);
	Wed, 8 Jun 2016 04:58:46 -0400
Received: from mail-wm0-f42.google.com ([74.125.82.42]:38663 "EHLO
	mail-wm0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S932551AbcFHI6n (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 8 Jun 2016 04:58:43 -0400
Date: Wed, 8 Jun 2016 10:58:37 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Waiman Long <waiman.long@hpe.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>,
        "Chen, Tim C" <tim.c.chen@intel.com>, Ingo Molnar <mingo@redhat.com>,
        Davidlohr Bueso <dbueso@suse.de>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Jason Low <jason.low2@hp.com>, Michel Lespinasse <walken@google.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Waiman Long <waiman.long@hp.com>, Al Viro <viro@zeniv.linux.org.uk>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: performance delta after VFS i_mutex=>i_rwsem conversion
Message-ID: <20160608085837.GA10792@gmail.com>
References: <5755D671.9070908@intel.com>
 <CA+55aFxH_7wjo_BgUPK5iomWedE2=DaUZVX-yruHOWEk7OTiHQ@mail.gmail.com>
 <5755E782.90800@hpe.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <5755E782.90800@hpe.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org


* Waiman Long <waiman.long@hpe.com> wrote:

> I do have a patchset that allow us to more accurately determine the state of
> the lock owner.
> 
> locking/rwsem: Add reader-owned state to the owner field
> http://www.spinics.net/lists/kernel/msg2258572.html
> 
> That should eliminate the performance gap between mutex and rwsem wrt
> spinning when only writers are present. I am hoping that that patchset can
> be queued for 4.8.

Yeah, so I actually had this series merged for testing last week, but a 
complication with a prereq patch made me unmerge it. But I have no fundamental 
objections, at all.

I also agree with Linus's general observation that we want to make 
down_write()/up_write() match mutex performance characteristics.

I think kernel developers should fundamentally be able to switch between 
mutex_lock()/unlock() and down_write()/up_write() and back, without noticing
any high level behavioral changes.

Any 'reader/writer mixing' artifacts are secondary concerns and we'll sort them 
out as they happen.

Thanks,

	Ingo