From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755079Ab0A0OLR (ORCPT ); Wed, 27 Jan 2010 09:11:17 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754865Ab0A0OLQ (ORCPT ); Wed, 27 Jan 2010 09:11:16 -0500 Received: from e2.ny.us.ibm.com ([32.97.182.142]:56651 "EHLO e2.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751968Ab0A0OLQ (ORCPT ); Wed, 27 Jan 2010 09:11:16 -0500 Date: Wed, 27 Jan 2010 06:11:10 -0800 From: "Paul E. McKenney" To: Steven Rostedt 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 Subject: Re: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU Message-ID: <20100127141109.GA21087@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100125034816.GA14043@linux.vnet.ibm.com> <1264432323.31321.412.camel@gandalf.stny.rr.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1264432323.31321.412.camel@gandalf.stny.rr.com> 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 Mon, Jan 25, 2010 at 10:12:03AM -0500, Steven Rostedt wrote: > 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; Good point!!! Or I can move the assignment to "c" to the end of each branch of the "if" statement, and do something like the following: c = !!per_cpu(rcu_sched_data, cpu).nxtlist; But either way, you are right, it does not make sense to go to all the trouble of forcing a grace period and then failing to take advantage of it. Thanx, Paul