From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754987Ab3HRDCo (ORCPT ); Sat, 17 Aug 2013 23:02:44 -0400 Received: from relay5-d.mail.gandi.net ([217.70.183.197]:51492 "EHLO relay5-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753299Ab3HRDCn (ORCPT ); Sat, 17 Aug 2013 23:02:43 -0400 X-Originating-IP: 50.43.39.152 Date: Sat, 17 Aug 2013 20:02:34 -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, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu Subject: Re: [PATCH tip/core/rcu 4/9] nohz_full: Add rcu_dyntick data for scalable detection of all-idle state Message-ID: <20130818030234.GG28923@leaf> References: <20130818014918.GA27827@linux.vnet.ibm.com> <1376790584-28120-1-git-send-email-paulmck@linux.vnet.ibm.com> <1376790584-28120-4-git-send-email-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1376790584-28120-4-git-send-email-paulmck@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, Aug 17, 2013 at 06:49:39PM -0700, Paul E. McKenney wrote: > From: "Paul E. McKenney" > > This commit adds fields to the rcu_dyntick structure that are used to > detect idle CPUs. These new fields differ from the existing ones in > that the existing ones consider a CPU executing in user mode to be idle, > where the new ones consider CPUs executing in user mode to be busy. > The handling of these new fields is otherwise quite similar to that for > the exiting fields. This commit also adds the initialization required > for these fields. > > So, why is usermode execution treated differently, with RCU considering > it a quiescent state equivalent to idle, while in contrast the new > full-system idle state detection considers usermode execution to be > non-idle? > > It turns out that although one of RCU's quiescent states is usermode > execution, it is not a full-system idle state. This is because the > purpose of the full-system idle state is not RCU, but rather determining > when accurate timekeeping can safely be disabled. Whenever accurate > timekeeping is required in a CONFIG_NO_HZ_FULL kernel, at least one > CPU must keep the scheduling-clock tick going. If even one CPU is > executing in user mode, accurate timekeeping is requires, particularly for > architectures where gettimeofday() and friends do not enter the kernel. > Only when all CPUs are really and truly idle can accurate timekeeping be > disabled, allowing all CPUs to turn off the scheduling clock interrupt, > thus greatly improving energy efficiency. > > This naturally raises the question "Why is this code in RCU rather than in > timekeeping?", and the answer is that RCU has the data and infrastructure > to efficiently make this determination. > > Signed-off-by: Paul E. McKenney > Acked-by: Frederic Weisbecker > Cc: Steven Rostedt One comment below. With that change: Reviewed-by: Josh Triplett > +#ifdef CONFIG_NO_HZ_FULL_SYSIDLE > + > +/* > + * Initialize dynticks sysidle state for CPUs coming online. > + */ > +static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp) > +{ > + rdtp->dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE; > +} > + > +#else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */ > + > +static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp) > +{ > +} > + > +#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */ Just move the ifdef around the function body: static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp) { #ifdef CONFIG_NO_HZ_FULL_SYSIDLE rdtp->dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE; #endif /* CONFIG_NO_HZ_FULL_SYSIDLE */ } - Josh Triplett