From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755195AbaIBUOr (ORCPT ); Tue, 2 Sep 2014 16:14:47 -0400 Received: from qmta10.emeryville.ca.mail.comcast.net ([76.96.30.17]:44659 "EHLO qmta10.emeryville.ca.mail.comcast.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752455AbaIBUOq (ORCPT ); Tue, 2 Sep 2014 16:14:46 -0400 Date: Tue, 2 Sep 2014 15:14:43 -0500 (CDT) From: Christoph Lameter X-X-Sender: cl@gentwo.org To: "Paul E. McKenney" cc: Frederic Weisbecker , linux-kernel@vger.kernel.org Subject: [RFC] dynticks: dynticks_idle is only modified locally use this_cpu ops Message-ID: Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Since dynticks_idle is only ever modified by the local cpu we do not need to use an atomic there. The weak "atomicity" of this_cpu ops is sufficient since there is no other cpu modifying the variable. [This is a cautious patch that leaves the barriers in place] Signed-off-by: Christoph Lameter Index: linux/kernel/rcu/tree.c =================================================================== --- linux.orig/kernel/rcu/tree.c +++ linux/kernel/rcu/tree.c @@ -213,7 +213,7 @@ static DEFINE_PER_CPU(struct rcu_dyntick .dynticks = ATOMIC_INIT(1), #ifdef CONFIG_NO_HZ_FULL_SYSIDLE .dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE, - .dynticks_idle = ATOMIC_INIT(1), + .dynticks_idle = 1, #endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */ }; Index: linux/kernel/rcu/tree.h =================================================================== --- linux.orig/kernel/rcu/tree.h +++ linux/kernel/rcu/tree.h @@ -91,7 +91,7 @@ struct rcu_dynticks { #ifdef CONFIG_NO_HZ_FULL_SYSIDLE long long dynticks_idle_nesting; /* irq/process nesting level from idle. */ - atomic_t dynticks_idle; /* Even value for idle, else odd. */ + long dynticks_idle; /* Even value for idle, else odd. */ /* "Idle" excludes userspace execution. */ unsigned long dynticks_idle_jiffies; /* End of last non-NMI non-idle period. */ Index: linux/kernel/rcu/tree_plugin.h =================================================================== --- linux.orig/kernel/rcu/tree_plugin.h +++ linux/kernel/rcu/tree_plugin.h @@ -2644,9 +2644,9 @@ static void rcu_sysidle_enter(int irq) j = jiffies; ACCESS_ONCE(rdtp->dynticks_idle_jiffies) = j; smp_mb__before_atomic(); - atomic_inc(&rdtp->dynticks_idle); + this_cpu_inc(rcu_dynticks.dynticks_idle); smp_mb__after_atomic(); - WARN_ON_ONCE(atomic_read(&rdtp->dynticks_idle) & 0x1); + WARN_ON_ONCE(__this_cpu_read(rcu_dynticks.dynticks_idle) & 0x1); } /* @@ -2712,9 +2712,9 @@ static void rcu_sysidle_exit(int irq) /* Record end of idle period. */ smp_mb__before_atomic(); - atomic_inc(&rdtp->dynticks_idle); + this_cpu_inc(rcu_dynticks.dynticks_idle); smp_mb__after_atomic(); - WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks_idle) & 0x1)); + WARN_ON_ONCE(!(__this_cpu_read(rcu_dynticks.dynticks_idle) & 0x1)); /* * If we are the timekeeping CPU, we are permitted to be non-idle @@ -2755,7 +2755,7 @@ static void rcu_sysidle_check_cpu(struct WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu); /* Pick up current idle and NMI-nesting counter and check. */ - cur = atomic_read(&rdtp->dynticks_idle); + cur = rdtp->dynticks_idle; if (cur & 0x1) { *isidle = false; /* We are not idle! */ return;