Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752279AbbFZLOW (ORCPT ); Fri, 26 Jun 2015 07:14:22 -0400 Received: from foss.arm.com ([217.140.101.70]:59849 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751680AbbFZLOQ (ORCPT ); Fri, 26 Jun 2015 07:14:16 -0400 Date: Fri, 26 Jun 2015 12:14:11 +0100 From: Will Deacon To: Waiman Long Cc: Peter Zijlstra , Ingo Molnar , Arnd Bergmann , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Scott J Norton , Douglas Hatch Subject: Re: [PATCH v5 3/3] locking/qrwlock: Don't contend with readers when setting _QW_WAITING Message-ID: <20150626111411.GB9791@arm.com> References: <1434729002-57724-1-git-send-email-Waiman.Long@hp.com> <1434729002-57724-4-git-send-email-Waiman.Long@hp.com> <20150625183521.GV19282@twins.programming.kicks-ass.net> <558C65A0.5040005@hp.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <558C65A0.5040005@hp.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2442 Lines: 59 On Thu, Jun 25, 2015 at 09:33:36PM +0100, Waiman Long wrote: > On 06/25/2015 02:35 PM, Peter Zijlstra wrote: > > On Fri, Jun 19, 2015 at 11:50:02AM -0400, Waiman Long wrote: > >> The current cmpxchg() loop in setting the _QW_WAITING flag for writers > >> in queue_write_lock_slowpath() will contend with incoming readers > >> causing possibly extra cmpxchg() operations that are wasteful. This > >> patch changes the code to do a byte cmpxchg() to eliminate contention > >> with new readers. > >> > >> A multithreaded microbenchmark running 5M read_lock/write_lock loop > >> on a 8-socket 80-core Westmere-EX machine running 4.0 based kernel > >> with the qspinlock patch have the following execution times (in ms) > >> with and without the patch: > >> > >> With R:W ratio = 5:1 > >> > >> Threads w/o patch with patch % change > >> ------- --------- ---------- -------- > >> 2 990 895 -9.6% > >> 3 2136 1912 -10.5% > >> 4 3166 2830 -10.6% > >> 5 3953 3629 -8.2% > >> 6 4628 4405 -4.8% > >> 7 5344 5197 -2.8% > >> 8 6065 6004 -1.0% > >> 9 6826 6811 -0.2% > >> 10 7599 7599 0.0% > >> 15 9757 9766 +0.1% > >> 20 13767 13817 +0.4% > >> > >> With small number of contending threads, this patch can improve > >> locking performance by up to 10%. With more contending threads, > >> however, the gain diminishes. > >> > >> With the extended qrwlock structure defined in asm-generic/qrwlock, > >> the queue_write_unlock() function is also simplified to a > >> smp_store_release() call. > >> > >> Signed-off-by: Waiman Long > > This one does not in fact apply, seeing how I applied a previous > > version. > > > > Please send an incremental patch if you still want to change things to > > this form. > > I saw that Ingo has merged a previous version of the patch. I am fine > with that version. As Will is working on a qrwlock patch to enable ARM > to use it, I will let him make the structure move to qrwlock.h if he > choose to do so. Sure, I'll rework my series when -rc1 lands and take this change into account. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/