From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750853Ab0AYPML (ORCPT ); Mon, 25 Jan 2010 10:12:11 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751956Ab0AYPMH (ORCPT ); Mon, 25 Jan 2010 10:12:07 -0500 Received: from hrndva-omtalb.mail.rr.com ([71.74.56.124]:34779 "EHLO hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751079Ab0AYPMG (ORCPT ); Mon, 25 Jan 2010 10:12:06 -0500 X-Authority-Analysis: v=1.0 c=1 a=yYHeZbYeH2cA:10 a=7U3hwN5JcxgA:10 a=t7bN9XTwOkWIhFUyazwA:9 a=2irqs-MymA6yLsAaCicA:7 a=3aZJd6cbtwP7retiYS8c_Vv5n4YA:4 X-Cloudmark-Score: 0 X-Originating-IP: 74.67.89.75 Subject: Re: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU From: Steven Rostedt Reply-To: rostedt@goodmis.org To: paulmck@linux.vnet.ibm.com 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, josh@joshtriplett.org, dvhltc@us.ibm.com, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com In-Reply-To: <20100125034816.GA14043@linux.vnet.ibm.com> References: <20100125034816.GA14043@linux.vnet.ibm.com> Content-Type: text/plain; charset="ISO-8859-15" Organization: Kihon Technologies Inc. Date: Mon, 25 Jan 2010 10:12:03 -0500 Message-ID: <1264432323.31321.412.camel@gandalf.stny.rr.com> Mime-Version: 1.0 X-Mailer: Evolution 2.28.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 2010-01-24 at 19:48 -0800, Paul E. McKenney wrote: > +/* > + * Check to see if any future RCU-related work will need to be done > + * by the current CPU, even if none need be done immediately, returning > + * 1 if so. This function is part of the RCU implementation; it is -not- > + * an exported member of the RCU API. > + * > + * Because we are not supporting preemptible RCU, attempt to accelerate > + * any current grace periods so that RCU no longer needs this CPU, but > + * only if all other CPUs are already in dynticks-idle mode. This will > + * allow the CPU cores to be powered down immediately, as opposed to after > + * waiting many milliseconds for grace periods to elapse. > + */ > +int rcu_needs_cpu(int cpu) > +{ > + int c = 1; > + int i; > + int thatcpu; > + > + /* Don't bother unless we are the last non-dyntick-idle CPU. */ > + for_each_cpu(thatcpu, nohz_cpu_mask) > + if (thatcpu != cpu) > + return rcu_needs_cpu_quick_check(cpu); > + > + /* Try to push remaining RCU-sched and RCU-bh callbacks through. */ > + for (i = 0; i < RCU_NEEDS_CPU_FLUSHES && c; i++) { > + c = 0; > + if (per_cpu(rcu_sched_data, cpu).nxtlist) { > + c = 1; > + rcu_sched_qs(cpu); > + force_quiescent_state(&rcu_sched_state, 0); > + __rcu_process_callbacks(&rcu_sched_state, > + &per_cpu(rcu_sched_data, cpu)); > + } > + if (per_cpu(rcu_bh_data, cpu).nxtlist) { > + c = 1; > + rcu_bh_qs(cpu); > + force_quiescent_state(&rcu_bh_state, 0); > + __rcu_process_callbacks(&rcu_bh_state, > + &per_cpu(rcu_bh_data, cpu)); > + } > + } > + > + /* If RCU callbacks are still pending, RCU still needs this CPU. */ > + return c; What happens if the last loop pushes out all callbacks? Then we would be returning 1 when we could really be returning 0. Wouldn't a better answer be: return per_cpu(rcu_sched_data, cpu).nxtlist || per_cpu(rcu_bh_data, cpu).nxtlist; -- Steve > +} > + > +#endif