Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755056AbdDMRqn (ORCPT ); Thu, 13 Apr 2017 13:46:43 -0400 Received: from bombadil.infradead.org ([65.50.211.133]:59735 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753005AbdDMRql (ORCPT ); Thu, 13 Apr 2017 13:46:41 -0400 Date: Thu, 13 Apr 2017 19:46:31 +0200 From: Peter Zijlstra To: "Paul E. McKenney" 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 Message-ID: <20170413174631.56ycg545gwbsb4q2@hirez.programming.kicks-ass.net> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170413173100.GL3956@linux.vnet.ibm.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 928 Lines: 17 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: > > On Thu, Apr 13, 2017 at 09:55:16AM -0700, Paul E. McKenney wrote: > > > > > To avoid people tuning huge machines having to wait for me to give > > > > > them an answer as to why they are suffering lock contention after > > > > > cranking up the value of RCU_FANOUT_LEAF. > > > > So is there a good reason to increase FANOUT_LEAF ? > > Increasing it reduces the number of rcu_node structures, and thus the > number of cache misses during grace-period initialization and cleanup. > This has proven necessary in the past on large machines having long > memory latencies. And there are starting to be some pretty big machines > running in production, and even for typical commerical workloads. Is that perhaps a good moment to look at aligning the cpus in said nodes with the cache topology?