Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753962AbbKBWyS (ORCPT ); Mon, 2 Nov 2015 17:54:18 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:44476 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751297AbbKBWyN (ORCPT ); Mon, 2 Nov 2015 17:54:13 -0500 Date: Mon, 2 Nov 2015 23:54:04 +0100 From: Peter Zijlstra To: Waiman Long Cc: Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , x86@kernel.org, linux-kernel@vger.kernel.org, Scott J Norton , Douglas Hatch , Davidlohr Bueso Subject: Re: [PATCH tip/locking/core v9 2/6] locking/qspinlock: prefetch next node cacheline Message-ID: <20151102225404.GY11639@twins.programming.kicks-ass.net> References: <1446247597-61863-1-git-send-email-Waiman.Long@hpe.com> <1446247597-61863-3-git-send-email-Waiman.Long@hpe.com> <20151102163626.GU3604@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151102163626.GU3604@twins.programming.kicks-ass.net> 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 Content-Length: 1383 Lines: 41 On Mon, Nov 02, 2015 at 05:36:26PM +0100, Peter Zijlstra wrote: > On Fri, Oct 30, 2015 at 07:26:33PM -0400, Waiman Long wrote: > > @@ -426,6 +437,15 @@ queue: > > cpu_relax(); > > > > /* > > + * If the next pointer is defined, we are not tail anymore. > > + * In this case, claim the spinlock & release the MCS lock. > > + */ > > + if (next) { > > + set_locked(lock); > > + goto mcs_unlock; > > + } > > + > > + /* > > * claim the lock: > > * > > * n,0,0 -> 0,0,1 : lock, uncontended > > @@ -458,6 +478,7 @@ queue: > > while (!(next = READ_ONCE(node->next))) > > cpu_relax(); > > > > +mcs_unlock: > > arch_mcs_spin_unlock_contended(&next->locked); > > pv_kick_node(lock, next); > > > > This however appears an independent optimization. Is it worth it? Would > we not already have observed a val != tail in this case? At which point > we're just adding extra code for no gain. > > That is, if we observe @next, must we then not also observe val != tail? Not quite; the ordering is the other way around. If we observe next we must also observe val != tail. But its a narrow thing. Is it really worth it? -- 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/