Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755272AbdDMS3w (ORCPT ); Thu, 13 Apr 2017 14:29:52 -0400 Received: from bombadil.infradead.org ([65.50.211.133]:46137 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752457AbdDMS3t (ORCPT ); Thu, 13 Apr 2017 14:29:49 -0400 Date: Thu, 13 Apr 2017 20:29:39 +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: <20170413182939.jobaxup4k7n3qbhf@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: 644 Lines: 14 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.