From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH v2 0/9] Remove spin_unlock_wait() Date: Thu, 6 Jul 2017 10:03:02 -0700 Message-ID: <20170706170302.GG2393@linux.vnet.ibm.com> References: <20170629235918.GA6445@linux.vnet.ibm.com> <20170705232955.GA15992@linux.vnet.ibm.com> <063D6719AE5E284EB5DD2968C1650D6DD0033F01@AcuExch.aculab.com> <20170706152110.GZ2393@linux.vnet.ibm.com> <20170706161047.nse2s4gquljv5bni@hirez.programming.kicks-ass.net> <20170706162412.GE2393@linux.vnet.ibm.com> <20170706164134.6a54adwcdvjx6ouc@hirez.programming.kicks-ass.net> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: David Laight , "linux-kernel@vger.kernel.org" , "netfilter-devel@vger.kernel.org" , "netdev@vger.kernel.org" , "oleg@redhat.com" , "akpm@linux-foundation.org" , "mingo@redhat.com" , "dave@stgolabs.net" , "manfred@colorfullife.com" , "tj@kernel.org" , "arnd@arndb.de" , "linux-arch@vger.kernel.org" , "will.deacon@arm.com" , "stern@rowland.harvard.edu" , "parri.andrea@gmail.com" , "torvalds@linux-foundation.org" Return-path: Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:56890 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751862AbdGFRDK (ORCPT ); Thu, 6 Jul 2017 13:03:10 -0400 Received: from pps.filterd (m0098419.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.20/8.16.0.20) with SMTP id v66GxBHK015186 for ; Thu, 6 Jul 2017 13:03:09 -0400 Received: from e13.ny.us.ibm.com (e13.ny.us.ibm.com [129.33.205.203]) by mx0b-001b2d01.pphosted.com with ESMTP id 2bhqu8bvax-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Thu, 06 Jul 2017 13:03:09 -0400 Received: from localhost by e13.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 6 Jul 2017 13:03:09 -0400 Content-Disposition: inline In-Reply-To: <20170706164134.6a54adwcdvjx6ouc@hirez.programming.kicks-ass.net> Sender: netdev-owner@vger.kernel.org List-ID: On Thu, Jul 06, 2017 at 06:41:34PM +0200, Peter Zijlstra wrote: > On Thu, Jul 06, 2017 at 09:24:12AM -0700, Paul E. McKenney wrote: > > On Thu, Jul 06, 2017 at 06:10:47PM +0200, Peter Zijlstra wrote: > > > On Thu, Jul 06, 2017 at 08:21:10AM -0700, Paul E. McKenney wrote: > > > > And yes, there are architecture-specific optimizations for an > > > > empty spin_lock()/spin_unlock() critical section, and the current > > > > arch_spin_unlock_wait() implementations show some of these optimizations. > > > > But I expect that performance benefits would need to be demonstrated at > > > > the system level. > > > > > > I do in fact contended there are any optimizations for the exact > > > lock+unlock semantics. > > > > You lost me on this one. > > For the exact semantics you'd have to fully participate in the fairness > protocol. You have to in fact acquire the lock in order to have the > other contending CPUs wait (otherwise my earlier case 3 will fail). > > At that point I'm not sure there is much actual code you can leave out. > > What actual optimization is there left at that point? Got it. It was just that I was having a hard time parsing your sentence. You were contending that there are no optimizations for all implementations for the full semantics. Thanx, Paul