From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: Re: [PATCH v9 04/19] qspinlock: Extract out the exchange of tail code word Date: Thu, 17 Apr 2014 17:28:17 -0400 Message-ID: <53504771.9070102@hp.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20140417154902.GO11096@twins.programming.kicks-ass.net> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Peter Zijlstra Cc: linux-arch@vger.kernel.org, Rik van Riel , Raghavendra K T , Gleb Natapov , kvm@vger.kernel.org, Konrad Rzeszutek Wilk , Scott J Norton , x86@kernel.org, Paolo Bonzini , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Ingo Molnar , Chegu Vinod , David Vrabel , "H. Peter Anvin" , xen-devel@lists.xenproject.org, Thomas Gleixner , "Paul E. McKenney" , Linus Torvalds , Oleg Nesterov List-Id: linux-arch.vger.kernel.org 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. -Longman From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from g5t1625.atlanta.hp.com ([15.192.137.8]:46815 "EHLO g5t1625.atlanta.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751001AbaDQV2Y (ORCPT ); Thu, 17 Apr 2014 17:28:24 -0400 Message-ID: <53504771.9070102@hp.com> Date: Thu, 17 Apr 2014 17:28:17 -0400 From: Waiman Long MIME-Version: 1.0 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> In-Reply-To: <20140417154902.GO11096@twins.programming.kicks-ass.net> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-arch-owner@vger.kernel.org List-ID: 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 Message-ID: <20140417212817.SQP7vSX2EV140PLV45hkw2Pit8yGE5Y2Q7NwkK9i5Qo@z> 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. -Longman