From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753510AbaHMQlL (ORCPT ); Wed, 13 Aug 2014 12:41:11 -0400 Received: from g4t3426.houston.hp.com ([15.201.208.54]:2858 "EHLO g4t3426.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751518AbaHMQlJ (ORCPT ); Wed, 13 Aug 2014 12:41:09 -0400 Message-ID: <53EB9522.2070804@hp.com> Date: Wed, 13 Aug 2014 12:41:06 -0400 From: Waiman Long User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.12) Gecko/20130109 Thunderbird/10.0.12 MIME-Version: 1.0 To: Dave Chinner CC: Jason Low , Ingo Molnar , Peter Zijlstra , linux-kernel@vger.kernel.org, Davidlohr Bueso , Scott J Norton Subject: Re: [PATCH 2/7] locking/rwsem: more aggressive use of optimistic spinning References: <1407119782-41119-1-git-send-email-Waiman.Long@hp.com> <1407119782-41119-3-git-send-email-Waiman.Long@hp.com> <1407125450.4710.38.camel@j-VirtualBox> <53DFAA53.4010003@hp.com> <20140813055153.GD20518@dastard> In-Reply-To: <20140813055153.GD20518@dastard> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/13/2014 01:51 AM, Dave Chinner wrote: > On Mon, Aug 04, 2014 at 11:44:19AM -0400, Waiman Long wrote: >> On 08/04/2014 12:10 AM, Jason Low wrote: >>> On Sun, 2014-08-03 at 22:36 -0400, Waiman Long wrote: >>>> The rwsem_can_spin_on_owner() function currently allows optimistic >>>> spinning only if the owner field is defined and is running. That is >>>> too conservative as it will cause some tasks to miss the opportunity >>>> of doing spinning in case the owner hasn't been able to set the owner >>>> field in time or the lock has just become available. >>>> >>>> This patch enables more aggressive use of optimistic spinning by >>>> assuming that the lock is spinnable unless proved otherwise. >>>> >>>> Signed-off-by: Waiman Long >>>> --- >>>> kernel/locking/rwsem-xadd.c | 2 +- >>>> 1 files changed, 1 insertions(+), 1 deletions(-) >>>> >>>> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c >>>> index d058946..dce22b8 100644 >>>> --- a/kernel/locking/rwsem-xadd.c >>>> +++ b/kernel/locking/rwsem-xadd.c >>>> @@ -285,7 +285,7 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem) >>>> static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem) >>>> { >>>> struct task_struct *owner; >>>> - bool on_cpu = false; >>>> + bool on_cpu = true; /* Assume spinnable unless proved not to be */ >>> Hi, >>> >>> So "on_cpu = true" was recently converted to "on_cpu = false" in order >>> to address issues such as a 5x performance regression in the xfs_repair >>> workload that was caused by the original rwsem optimistic spinning code. >>> >>> However, patch 4 in this patchset does address some of the problems with >>> spinning when there are readers. CC'ing Dave Chinner, who did the >>> testing with the xfs_repair workload. >>> >> This patch set enables proper reader spinning and so the problem >> that we see with xfs_repair workload should go away. I should have >> this patch after patch 4 to make it less confusing. BTW, patch 3 can >> significantly reduce spinlock contention in rwsem. So I believe the >> xfs_repair workload should run faster with this patch than both 3.15 >> and 3.16. > I see lots of handwaving. I documented the test I ran when I > reported the problem so anyone with a 16p system and an SSD can > reproduce it. I don't have the bandwidth to keep track of the lunacy > of making locks scale these days - that's what you guys are doing. > > I gave you a simple, reliable workload that is extremely sensitive > to rwsem perturbations, so you should be adding it to your > regression tests rather than leaving it for others to notice you > screwed up.... > > Cheers, > > Dave. If you can send me a rwsem workload that I can use for testing purpose, it will be highly appreciated. -Longman