From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753812Ab1HOOFM (ORCPT ); Mon, 15 Aug 2011 10:05:12 -0400 Received: from mx1.redhat.com ([209.132.183.28]:52213 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753606Ab1HOOFK (ORCPT ); Mon, 15 Aug 2011 10:05:10 -0400 Date: Mon, 15 Aug 2011 10:04:17 -0400 From: Josh Boyer To: "Paul E. McKenney" Cc: Frederic Weisbecker , linux-kernel@vger.kernel.org, Andrew Morton Subject: Re: 3.0-git15 Atomic scheduling in pidmap_init Message-ID: <20110815140416.GB2227@zod.bos.redhat.com> References: <20110805142245.GP2096@zod.bos.redhat.com> <20110805170805.GB22164@somewhere> <20110805222641.GB2245@linux.vnet.ibm.com> <20110805231216.GE22164@somewhere> <20110808020914.GC2385@linux.vnet.ibm.com> <20110808025505.GF29058@somewhere> <20110808031014.GE2385@linux.vnet.ibm.com> <20110809113514.GA27301@somewhere.redhat.com> <20110810124528.GC2566@zod.bos.redhat.com> <20110814230453.GP2650@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110814230453.GP2650@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 Sun, Aug 14, 2011 at 04:04:53PM -0700, Paul E. McKenney wrote: > > > > > Now rcu_init_percpu_data() still sets rdp->qs_pending to 1, and that > > > > > is going to stay as is as long as preemption is disabled. > > > > > > > > But setting rdp->qs_pending to 1 in rcu_init_percpu_data() has no effect > > > > until a grace period starts. So, if grace periods are prevented from > > > > Er... really? Because it gets set and __rcu_pending looks at it > > unconditionally in the case that is calling set_need_resched. It > > doesn't check if there is anything about a grace period going on or not. > > Frederic noted the condition that prevents this at boot time, but it > appears that newly onlined CPUs might send themselves needless resched > IPIs at runtime if RCU is idle. > > > > > starting, no need to mess with rcu_init_percpu_data(). Especially given > > > > that rcu_init_percpu_data() is also used at late boot and runtime for > > > > CPU hotplug. > > > > > > Ok. > > > > > > > > > > > So I believe that it is sufficient to change cpu_needs_another_gp() > > > > to check for boot being far enough along to allow grace periods. > > > > > > Yep, sounds good. > > > > I looked at doing this but got lost as to 1) how it would help the > > situtation I've reported, and 2) exactly how to do that. > > It would prevent control from reaching that point, and that might > well be needed for other reasons. (This bit about RCU needing to > work differently at boot time is, well, "interesting".) > > > I'd be happy to test, but at the moment the proposed solution is > > confusing to me. > > Please see the attached. Fixed it up quickly to apply on top of -rc2 and it seems to solve the problem nicely. Thanks for the patch. josh