Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754583AbaDRRyI (ORCPT ); Fri, 18 Apr 2014 13:54:08 -0400 Received: from merlin.infradead.org ([205.233.59.134]:59543 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752055AbaDRRyB (ORCPT ); Fri, 18 Apr 2014 13:54:01 -0400 Date: Fri, 18 Apr 2014 19:53:39 +0200 From: Peter Zijlstra To: Waiman Long 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 Message-ID: <20140418175339.GZ11182@twins.programming.kicks-ass.net> 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> <535161BF.90405@hp.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <535161BF.90405@hp.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 18, 2014 at 01:32:47PM -0400, Waiman Long wrote: > 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. Its not the lock cacheline, you just touched the per-cpu node cacheline for the first time, setting up the node. -- 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/