From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753432Ab3AaMjf (ORCPT ); Thu, 31 Jan 2013 07:39:35 -0500 Received: from mga11.intel.com ([192.55.52.93]:51746 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751743Ab3AaMjd (ORCPT ); Thu, 31 Jan 2013 07:39:33 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.84,576,1355126400"; d="scan'208";a="280813725" Date: Thu, 31 Jan 2013 20:40:04 +0800 From: Yuanhan Liu To: Michel Lespinasse Cc: linux-kernel@vger.kernel.org, Ingo Molnar , David Howells Subject: Re: [PATCH] rwsem-spinlock: let rwsem write lock stealable Message-ID: <20130131124004.GA12678@yliu-dev.sh.intel.com> References: <1359537244-20588-1-git-send-email-yuanhan.liu@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 31, 2013 at 03:57:51AM -0800, Michel Lespinasse wrote: > On Wed, Jan 30, 2013 at 1:14 AM, Yuanhan Liu > wrote: > > We(Linux Kernel Performance project) found a regression introduced by > > commit 5a50508, which just convert all mutex lock to rwsem write lock. > > The semantics is same, but the results is quite huge in some cases. > > After investigation, we found the root cause: mutex support lock > > stealing. Here is the link for the detailed regression report: > > https://lkml.org/lkml/2013/1/29/84 > > > > Ingo suggests to add write lock stealing to rwsem as well: > > "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" > > > > I then tried it with rwsem-spinlock first as I found it much easier to > > implement it than lib/rwsem.c. And here I sent out this patch first for > > comments. I'd try lib/rwsem.c later once the change to rwsem-spinlock > > is OK to you guys. > > I noticed that you haven't modified __down_write_trylock() - for > consistency with __down_write() you should replace > if (sem->activity == 0 && list_empty(&sem->wait_list)) { > with > if (sem->activity == 0) { Yes, my bad for missing that. Thanks a lot for pointing it out. Will fix it. > > Other than that, I like the idea. I was originally uncomfortable with > doing lock stealing for the rwsem, but I think doing it for writers > only as you propose should be fine. Readers wait for any queued > writers, and in exchange they are guaranteed to get the lock once > they've blocked. > You *still* want to check for regressions that this > change might cause - not with anon_vma as this was a mutex not long > ago, but possibly with mmap_sem Yes. Well, at least it passed Fengguang's 0-DAY test, which did lots of tests on almost all ARCHs. Well, you reminds me that I just enabled RWSEM_GENERIC_SPINLOCK for x86 ARCH, thus I need to enable RWSEM_GENERIC_SPINLOCK to all ARCHs and do test again. BTW, mind to tell a nice test case for mmap_sem? > - but I'm crossing my fingers and > thinking that it'll most likely turn out fine. Thanks! > > I may be able to help with the non-spinlock version of this as I still > remember how this works. That would be great! Especially I will have vacation soon for Chinese New Year. --yliu