From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753104Ab0AYDsX (ORCPT ); Sun, 24 Jan 2010 22:48:23 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751842Ab0AYDsW (ORCPT ); Sun, 24 Jan 2010 22:48:22 -0500 Received: from e2.ny.us.ibm.com ([32.97.182.142]:40993 "EHLO e2.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751883Ab0AYDsW (ORCPT ); Sun, 24 Jan 2010 22:48:22 -0500 Date: Sun, 24 Jan 2010 19:48:16 -0800 From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: 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, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com Subject: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU Message-ID: <20100125034816.GA14043@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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 [Experimental RFC, not for inclusion.] I recently received a complaint that RCU was refusing to let a system go into low-power state immediately, instead waiting a few ticks after the system had gone idle before letting go of the last CPU. Of course, the reason for this was that there were a couple of RCU callbacks on the last CPU. Currently, rcu_needs_cpu() simply checks whether the current CPU has an outstanding RCU callback, which means that the last CPU to go into dyntick-idle mode might wait a few ticks for the relevant grace periods to complete. However, if all the other CPUs are in dyntick-idle mode, and if this CPU is in a quiescent state (which it is for RCU-bh and RCU-sched any time that we are considering going into dyntick-idle mode), then the grace period is instantly complete. This patch therefore repeatedly invokes the RCU grace-period machinery in order to force any needed grace periods to complete quickly. It does so a limited number of times in order to prevent starvation by an RCU callback function that might pass itself to call_rcu(). Thoughts? Signed-off-by: Paul E. McKenney diff --git a/init/Kconfig b/init/Kconfig index d95ca7c..42bf914 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -396,6 +396,22 @@ config RCU_FANOUT_EXACT Say N if unsure. +config RCU_FAST_NO_HZ + bool "Accelerate last non-dyntick-idle CPU's grace periods" + depends on TREE_RCU && NO_HZ && SMP + default n + help + This option causes RCU to attempt to accelerate grace periods + in order to allow the final CPU to enter dynticks-idle state + more quickly. On the other hand, this option increases the + overhead of the dynticks-idle checking, particularly on systems + with large numbers of CPUs. + + Say Y if energy efficiency is critically important, particularly + if you have relatively few CPUs. + + Say N if you are unsure. + config TREE_RCU_TRACE def_bool RCU_TRACE && ( TREE_RCU || TREE_PREEMPT_RCU ) select DEBUG_FS diff --git a/kernel/rcutree.c b/kernel/rcutree.c index 099a255..29d88c0 100644 --- a/kernel/rcutree.c +++ b/kernel/rcutree.c @@ -1550,10 +1550,9 @@ static int rcu_pending(int cpu) /* * 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. + * 1 if so. */ -int rcu_needs_cpu(int cpu) +static int rcu_needs_cpu_quick_check(int cpu) { /* RCU callbacks either ready or pending? */ return per_cpu(rcu_sched_data, cpu).nxtlist || diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h index e77cdf3..d6170a9 100644 --- a/kernel/rcutree_plugin.h +++ b/kernel/rcutree_plugin.h @@ -906,3 +906,72 @@ static void __init __rcu_init_preempt(void) } #endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */ + +#if defined(CONFIG_TREE_PREEMPT_RCU) || !defined(CONFIG_RCU_FAST_NO_HZ) + +/* + * 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 have preemptible RCU, just check whether this CPU needs + * any flavor of RCU. Do not chew up lots of CPU cycles with preemption + * disabled in a most-likely vain attempt to cause RCU not to need this CPU. + */ +int rcu_needs_cpu(int cpu) +{ + return rcu_needs_cpu_quick_check(cpu); +} + +#else + +#define RCU_NEEDS_CPU_FLUSHES 5 + +/* + * 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; +} + +#endif