From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754641Ab3AaNKj (ORCPT ); Thu, 31 Jan 2013 08:10:39 -0500 Received: from mail-ea0-f169.google.com ([209.85.215.169]:59311 "EHLO mail-ea0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752424Ab3AaNKh (ORCPT ); Thu, 31 Jan 2013 08:10:37 -0500 Date: Thu, 31 Jan 2013 14:10:32 +0100 From: Ingo Molnar To: Michel Lespinasse Cc: Yuanhan Liu , linux-kernel@vger.kernel.org, David Howells Subject: Re: [PATCH] rwsem-spinlock: let rwsem write lock stealable Message-ID: <20130131131032.GA6627@gmail.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 * 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) { > > 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 - but I'm crossing my fingers and > thinking that it'll most likely turn out fine. My gut feeling, from having implemented lock stealing in a number of locking primitives in the past, is that writer lock-stealing will be a measurable win for mmap_sem as well. Thanks, Ingo