From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752526AbbG3Pc6 (ORCPT ); Thu, 30 Jul 2015 11:32:58 -0400 Received: from casper.infradead.org ([85.118.1.10]:55671 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752488AbbG3Pc5 (ORCPT ); Thu, 30 Jul 2015 11:32:57 -0400 Date: Thu, 30 Jul 2015 17:32:51 +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, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com, Alexander Gordeev Subject: Re: [PATCH tip/core/rcu 02/12] rcu: Panic if RCU tree can not accommodate all CPUs Message-ID: <20150730153251.GL25159@twins.programming.kicks-ass.net> References: <20150717223041.GA14464@linux.vnet.ibm.com> <1437172263-15466-1-git-send-email-paulmck@linux.vnet.ibm.com> <1437172263-15466-2-git-send-email-paulmck@linux.vnet.ibm.com> <20150730122835.GX19282@twins.programming.kicks-ass.net> <20150730152517.GE27280@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150730152517.GE27280@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 30, 2015 at 08:25:17AM -0700, Paul E. McKenney wrote: > On Thu, Jul 30, 2015 at 02:28:35PM +0200, Peter Zijlstra wrote: > > On Fri, Jul 17, 2015 at 03:30:53PM -0700, Paul E. McKenney wrote: > > > + if (n > rcu_capacity[MAX_RCU_LVLS]) > > > + panic("rcu_init_geometry: rcu_capacity[] is too small"); > > > > How can this ever happen? We _know_ NR_CPUS at compile time, there's no > > way we can get more CPUs than that -- even if the hardware has more, > > we'll stop enumerating. > > You can make this happen by building with CONFIG_RCU_FANOUT=2 and > CONFIG_RCU_FANOUT_LEAF=2, then running on a system with more than 16 CPUs. > The kernel boot parameter rcutree.rcu_fanout_leaf=2 can be substituted for > CONFIG_RCU_FANOUT_LEAF=2, hence the need for a runtime test. I do this > sort of thing for my rcutorture testing in order to test a four-level > rcu_node tree with only 16 CPUs. How about we make the build fail if NR_CPUS exceeds that maximum fanout?