Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753215AbaDRRdJ (ORCPT ); Fri, 18 Apr 2014 13:33:09 -0400 Received: from g4t3426.houston.hp.com ([15.201.208.54]:50881 "EHLO g4t3426.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751139AbaDRRdF (ORCPT ); Fri, 18 Apr 2014 13:33:05 -0400 Message-ID: <535161BF.90405@hp.com> Date: Fri, 18 Apr 2014 13:32:47 -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: Peter Zijlstra CC: Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , linux-arch@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, xen-devel@lists.xenproject.org, kvm@vger.kernel.org, Paolo Bonzini , Konrad Rzeszutek Wilk , "Paul E. McKenney" , Rik van Riel , Linus Torvalds , Raghavendra K T , David Vrabel , Oleg Nesterov , Gleb Natapov , Scott J Norton , Chegu Vinod Subject: Re: [PATCH v9 04/19] qspinlock: Extract out the exchange of tail code word References: <1397747051-15401-1-git-send-email-Waiman.Long@hp.com> <1397747051-15401-5-git-send-email-Waiman.Long@hp.com> <20140417154902.GO11096@twins.programming.kicks-ass.net> <53504771.9070102@hp.com> <20140418081517.GY11096@twins.programming.kicks-ass.net> In-Reply-To: <20140418081517.GY11096@twins.programming.kicks-ass.net> 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 04/18/2014 04:15 AM, Peter Zijlstra wrote: > On Thu, Apr 17, 2014 at 05:28:17PM -0400, Waiman Long wrote: >> On 04/17/2014 11:49 AM, Peter Zijlstra wrote: >>> On Thu, Apr 17, 2014 at 11:03:56AM -0400, Waiman Long wrote: >>>> @@ -192,36 +220,25 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val) >>>> node->next = NULL; >>>> >>>> /* >>>> + * We touched a (possibly) cold cacheline; attempt the trylock once >>>> + * more in the hope someone let go while we weren't watching as long >>>> + * as no one was queuing. >>>> */ >>>> + if (!(val& _Q_TAIL_MASK)&& queue_spin_trylock(lock)) >>>> + goto release; >>> But you just did a potentially very expensive op; @val isn't >>> representative anymore! >> That is not true. I pass in a pointer to val to trylock_pending() (the >> pointer thing) so that it will store the latest value that it reads from the >> lock back into val. I did miss one in the PV qspinlock exit loop. I will add >> it back when I do the next version. > But you did that read _before_ you touched a cold cacheline, that's 100s > of cycles. Whatever value you read back then is now complete nonsense. For spin_lock(), the lock cacheline is touched by a cmpxchg(). It can takes 100s of cycles whether it is hot or cold. I will take the precheck out, it is not such a big deal anyway. -Longman -- 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/