Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755691AbdDMTmh (ORCPT ); Thu, 13 Apr 2017 15:42:37 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:59000 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1755320AbdDMTme (ORCPT ); Thu, 13 Apr 2017 15:42:34 -0400 Date: Thu, 13 Apr 2017 12:42:28 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com Subject: Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick Reply-To: paulmck@linux.vnet.ibm.com References: <20170412165441.GA17149@linux.vnet.ibm.com> <1492016149-18834-4-git-send-email-paulmck@linux.vnet.ibm.com> <20170413091535.r6iw7s3pc2znvl6b@hirez.programming.kicks-ass.net> <20170413160332.GZ3956@linux.vnet.ibm.com> <20170413161948.ymvzlzhporgmldvn@hirez.programming.kicks-ass.net> <20170413165516.GI3956@linux.vnet.ibm.com> <20170413170434.xk4zq3p75pu3ubxw@hirez.programming.kicks-ass.net> <20170413173100.GL3956@linux.vnet.ibm.com> <20170413182939.jobaxup4k7n3qbhf@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170413182939.jobaxup4k7n3qbhf@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17041319-0052-0000-0000-000001D65FF5 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006930; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000208; SDB=6.00847113; UDB=6.00417920; IPR=6.00625558; BA=6.00005288; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00015035; XFM=3.00000013; UTC=2017-04-13 19:42:32 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17041319-0053-0000-0000-00004FEDED69 Message-Id: <20170413194228.GS3956@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-04-13_15:,, 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=8.0.1-1702020001 definitions=main-1704130164 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 849 Lines: 20 On Thu, Apr 13, 2017 at 08:29:39PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 10:31:00AM -0700, Paul E. McKenney wrote: > > On Thu, Apr 13, 2017 at 07:04:34PM +0200, Peter Zijlstra wrote: > > > > And I have vague memories of it actually causing lock contention, but > > > I've forgotten how that worked. > > > > That is a new one on me. I can easily see how not skewing ticks could > > cause serious lock contention, but am missing how skewed ticks would > > do so. > > It could've been something like cacheline bouncing. Where with a > synchronized tick, the (global) cacheline would get used by all CPUs on > a node before heading out to the next node etc.. Where with a skewed > tick, it would forever bounce around. In other words, motivating the order of the skewed ticks to be guided by hardware locality? Thanx, Paul