From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1763644AbdDSNWm (ORCPT ); Wed, 19 Apr 2017 09:22:42 -0400 Received: from bombadil.infradead.org ([65.50.211.133]:55221 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1763606AbdDSNWh (ORCPT ); Wed, 19 Apr 2017 09:22:37 -0400 Date: Wed, 19 Apr 2017 15:22:26 +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: <20170419132226.yvo3jyweb3d2a632@hirez.programming.kicks-ass.net> References: <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> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170413184232.GQ3956@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 On Thu, Apr 13, 2017 at 11:42:32AM -0700, Paul E. McKenney wrote: > I believe that you are missing the fact that RCU grace-period > initialization and cleanup walks through the rcu_node tree breadth > first, using rcu_for_each_node_breadth_first(). Indeed. That is the part I completely missed. > This macro (shown below) > implements this breadth-first walk using a simple sequential traversal of > the ->node[] array that provides the structures making up the rcu_node > tree. As you can see, this scan is completely independent of how CPU > numbers might be mapped to rcu_data slots in the leaf rcu_node structures. So this code is clearly not a hotpath, but still its performance matters? Seems like you cannot win here :/