From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756968Ab2FPUbZ (ORCPT ); Sat, 16 Jun 2012 16:31:25 -0400 Received: from relay3-d.mail.gandi.net ([217.70.183.195]:58564 "EHLO relay3-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756564Ab2FPUbY (ORCPT ); Sat, 16 Jun 2012 16:31:24 -0400 X-Originating-IP: 217.70.178.132 X-Originating-IP: 50.43.46.74 Date: Sat, 16 Jun 2012 13:31:06 -0700 From: Josh Triplett To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, fweisbec@gmail.com, patches@linaro.org Subject: Re: [PATCH tip/core/rcu 02/15] rcu: Size rcu_node tree from nr_cpu_ids rather than NR_CPUS Message-ID: <20120616203106.GA12293@leaf> References: <20120615210550.GA27506@linux.vnet.ibm.com> <1339794370-28119-1-git-send-email-paulmck@linux.vnet.ibm.com> <1339794370-28119-2-git-send-email-paulmck@linux.vnet.ibm.com> <20120615214726.GT31184@leaf> <20120616003714.GS2389@linux.vnet.ibm.com> <20120616051712.GA8252@leaf> <20120616063848.GB2420@linux.vnet.ibm.com> <20120616091733.GD8252@leaf> <20120616144453.GE2420@linux.vnet.ibm.com> <20120616145154.GA10128@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120616145154.GA10128@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Jun 16, 2012 at 07:51:54AM -0700, Paul E. McKenney wrote: > On Sat, Jun 16, 2012 at 07:44:53AM -0700, Paul E. McKenney wrote: > > On Sat, Jun 16, 2012 at 02:17:33AM -0700, Josh Triplett wrote: > > > On Fri, Jun 15, 2012 at 11:38:48PM -0700, Paul E. McKenney wrote: > > > > On Fri, Jun 15, 2012 at 10:17:12PM -0700, Josh Triplett wrote: > > > > > On Fri, Jun 15, 2012 at 05:37:14PM -0700, Paul E. McKenney wrote: > > > > > > On Fri, Jun 15, 2012 at 02:47:26PM -0700, Josh Triplett wrote: > > > > > > > On Fri, Jun 15, 2012 at 02:05:57PM -0700, Paul E. McKenney wrote: > > > > > > > > From: "Paul E. McKenney" > > > > > > > > > > > > > > > > The rcu_node tree array is sized based on compile-time constants, > > > > > > > > including NR_CPUS. Although this approach has worked well in the past, > > > > > > > > the recent trend by many distros to define NR_CPUS=4096 results in > > > > > > > > excessive grace-period-initialization latencies. > > > > > > > > > > > > > > > > This commit therefore substitutes the run-time computed nr_cpu_ids for > > > > > > > > the compile-time NR_CPUS when building the tree. This can result in > > > > > > > > much of the compile-time-allocated rcu_node array being unused. If > > > > > > > > this is a major problem, you are in a specialized situation anyway, > > > > > > > > so you can manually adjust the NR_CPUS, RCU_FANOUT, and RCU_FANOUT_LEAF > > > > > > > > kernel config parameters. > > > > > > > > > > > > > > > > Signed-off-by: Paul E. McKenney > > > > > > > > --- > > > > > > > > kernel/rcutree.c | 2 +- > > > > > > > > kernel/rcutree_plugin.h | 2 ++ > > > > > > > > 2 files changed, 3 insertions(+), 1 deletions(-) > > > > > > > > > > > > > > > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > > > > > > > > index a151184..9098910 100644 > > > > > > > > --- a/kernel/rcutree.c > > > > > > > > +++ b/kernel/rcutree.c > > > > > > > > @@ -2672,7 +2672,7 @@ static void __init rcu_init_geometry(void) > > > > > > > > { > > > > > > > > int i; > > > > > > > > int j; > > > > > > > > - int n = NR_CPUS; > > > > > > > > + int n = nr_cpu_ids; > > > > > > > > > > > > > > Same question as before: why have this as a variable when it never > > > > > > > changes? > > > > > > > > > > > > Ah, that explains why. This prevented me from forgetting the random > > > > > > NR_CPUS. > > > > > > > > > > Does that mean it can go away now that you've written the patches? > > > > > > > > If I don't have to change from nr_cpu_ids to yet another thing over > > > > the next while, then it might be worth changing. > > > > > > That sounds like an argument for a #define or a static const, rather > > > than a local variable. :) > > > > OK, static const it is! > > Except that the compiler doesn't like the run-time initialization of > a static const variable. > > Can't have everything, I guess. ;-) Ah, right, nr_cpu_ids is not a static const. In that case, just waiting until you think this definition won't churn anymore and substituting nr_cpu_ids for n everywhere seems like the right solution. - Josh Triplett