From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965735AbdDSQOK (ORCPT ); Wed, 19 Apr 2017 12:14:10 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:51910 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965463AbdDSQOF (ORCPT ); Wed, 19 Apr 2017 12:14:05 -0400 Date: Wed, 19 Apr 2017 09:13:53 -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: <20170413170434.xk4zq3p75pu3ubxw@hirez.programming.kicks-ass.net> <20170413173100.GL3956@linux.vnet.ibm.com> <20170413174631.56ycg545gwbsb4q2@hirez.programming.kicks-ass.net> <20170413181926.GP3956@linux.vnet.ibm.com> <20170413182309.vmyivo3oqrtfhhxt@hirez.programming.kicks-ass.net> <20170413184232.GQ3956@linux.vnet.ibm.com> <20170419132226.yvo3jyweb3d2a632@hirez.programming.kicks-ass.net> <20170419134835.bpuhurle2jjr66hm@hirez.programming.kicks-ass.net> <20170419150809.GL3956@linux.vnet.ibm.com> <20170419154040.knkdg2j6awrp74ua@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170419154040.knkdg2j6awrp74ua@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17041916-0040-0000-0000-0000031CA5AB X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006939; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000208; SDB=6.00849668; UDB=6.00419574; IPR=6.00628301; BA=6.00005304; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00015095; XFM=3.00000013; UTC=2017-04-19 16:13:57 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17041916-0041-0000-0000-00000710B877 Message-Id: <20170419161353.GS3956@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-04-19_14:,, 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-1704190134 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 19, 2017 at 05:40:40PM +0200, Peter Zijlstra wrote: > On Wed, Apr 19, 2017 at 08:08:09AM -0700, Paul E. McKenney wrote: > > And even that would not be completely sufficient. After all, the state > > in the leaf rcu_node structure will be out of date during grace-period > > initialization and cleanup. So to -completely- synchronize state for > > the incoming CPU, I would have to acquire the root rcu_node structure's > > lock and look at the live state. Needless to say, the performance and > > scalability implications of acquiring a global lock on each and every > > idle exit event is not going to be at all pretty. > > Arguably you could use a seqlock to read the global state. Will still > ponder things a bit more, esp. those bugs you pointed me at from just > reading gpnum. Looking forward to hearing what you come up with! Thanx, Paul