Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752113AbdGFRDN (ORCPT ); Thu, 6 Jul 2017 13:03:13 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:47048 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751936AbdGFRDL (ORCPT ); Thu, 6 Jul 2017 13:03:11 -0400 Date: Thu, 6 Jul 2017 10:03:02 -0700 From: "Paul E. McKenney" To: Peter Zijlstra 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" Subject: Re: [PATCH v2 0/9] Remove spin_unlock_wait() Reply-To: paulmck@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170706164134.6a54adwcdvjx6ouc@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17070617-0008-0000-0000-00000257AA50 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007331; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000214; SDB=6.00883751; UDB=6.00440907; IPR=6.00663939; BA=6.00005455; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00016115; XFM=3.00000015; UTC=2017-07-06 17:03:07 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17070617-0009-0000-0000-000035EEEC6E Message-Id: <20170706170302.GG2393@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-07-06_11:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1703280000 definitions=main-1707060292 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1293 Lines: 28 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