From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754100Ab0A0KEj (ORCPT ); Wed, 27 Jan 2010 05:04:39 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753748Ab0A0KEj (ORCPT ); Wed, 27 Jan 2010 05:04:39 -0500 Received: from e5.ny.us.ibm.com ([32.97.182.145]:41139 "EHLO e5.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751236Ab0A0KEi (ORCPT ); Wed, 27 Jan 2010 05:04:38 -0500 Date: Wed, 27 Jan 2010 02:04:34 -0800 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Andi Kleen , linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, josh@joshtriplett.org, dvhltc@us.ibm.com, niv@us.ibm.com, tglx@linutronix.de, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com Subject: Re: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU Message-ID: <20100127100434.GN6807@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100125034816.GA14043@linux.vnet.ibm.com> <873a1sft9q.fsf@basil.nowhere.org> <20100127052050.GC6807@linux.vnet.ibm.com> <20100127094336.GA12522@basil.fritz.box> <1264585850.4283.1992.camel@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1264585850.4283.1992.camel@laptop> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 27, 2010 at 10:50:50AM +0100, Peter Zijlstra wrote: > On Wed, 2010-01-27 at 10:43 +0100, Andi Kleen wrote: > > > > Can't you simply check that at runtime then? > > > > if (num_possible_cpus() > 20) > > ... > > > > BTW the new small is large. This years high end desktop PC will come with > > upto 12 CPU threads. It would likely be challenging to find a good > > number for 20 that holds up with the future. > > If only scalability were that easy :/ > > These massive core/thread count things are causing more problems as > well, the cpus/node ratios are constantly growing, giving grief in the > page allocator as well as other places that used to scale per node. > > As to the current problem, the call_rcu() interface doesn't make a hard > promise that the callback will be done on the same cpu, right? So why > not simply move the callback list over to a more active cpu? I could indeed do that. However, there is nothing stopping the more-active CPU from going into dynticks-idle mode between the time that I decide to push the callback to it and the time I actually do the pushing. :-( I considered pushing the callbacks to the orphanage, but that is a global lock that I would rather not acquire on each dyntick-idle transition. This conversation is having the effect of making me much more comfortable adding a kernel configuration parameter. Might not have been the intent, but there you have it! ;-) Thanx, Paul