From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753014AbbJFQPL (ORCPT ); Tue, 6 Oct 2015 12:15:11 -0400 Received: from e36.co.us.ibm.com ([32.97.110.154]:58706 "EHLO e36.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752727AbbJFQOA (ORCPT ); Tue, 6 Oct 2015 12:14:00 -0400 X-IBM-Helo: d03dlp02.boulder.ibm.com X-IBM-MailFrom: paulmck@linux.vnet.ibm.com X-IBM-RcptTo: linux-kernel@vger.kernel.org From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com, Boqun Feng , "Paul E. McKenney" Subject: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for Tiny and Tree RCU readers Date: Tue, 6 Oct 2015 09:13:39 -0700 Message-Id: <1444148028-11551-4-git-send-email-paulmck@linux.vnet.ibm.com> X-Mailer: git-send-email 2.5.2 In-Reply-To: <1444148028-11551-1-git-send-email-paulmck@linux.vnet.ibm.com> References: <20151006161305.GA9799@linux.vnet.ibm.com> <1444148028-11551-1-git-send-email-paulmck@linux.vnet.ibm.com> X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15100616-0021-0000-0000-0000136EACE0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Boqun Feng Because preempt_disable() maps to barrier() for non-debug builds, it forces the compiler to spill and reload registers. Because Tree RCU and Tiny RCU now only appear in CONFIG_PREEMPT=n builds, these barrier() instances generate needless extra code for each instance of rcu_read_lock() and rcu_read_unlock(). This extra code slows down Tree RCU and bloats Tiny RCU. This commit therefore removes the preempt_disable() and preempt_enable() from the non-preemptible implementations of __rcu_read_lock() and __rcu_read_unlock(), respectively. However, for debug purposes, preempt_disable() and preempt_enable() are still invoked if CONFIG_PREEMPT_COUNT=y, because this allows detection of sleeping inside atomic sections in non-preemptible kernels. This is based on an earlier patch by Paul E. McKenney, fixing a bug encountered in kernels built with CONFIG_PREEMPT=n and CONFIG_PREEMPT_COUNT=y. Signed-off-by: Boqun Feng Signed-off-by: Paul E. McKenney --- include/linux/rcupdate.h | 6 ++++-- include/linux/rcutiny.h | 1 + kernel/rcu/tree.c | 9 +++++++++ 3 files changed, 14 insertions(+), 2 deletions(-) diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index d63bb77dab35..6c3ceceb6148 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -297,12 +297,14 @@ void synchronize_rcu(void); static inline void __rcu_read_lock(void) { - preempt_disable(); + if (IS_ENABLED(CONFIG_PREEMPT_COUNT)) + preempt_disable(); } static inline void __rcu_read_unlock(void) { - preempt_enable(); + if (IS_ENABLED(CONFIG_PREEMPT_COUNT)) + preempt_enable(); } static inline void synchronize_rcu(void) diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h index c8a0722f77ea..4c1aaf9cce7b 100644 --- a/include/linux/rcutiny.h +++ b/include/linux/rcutiny.h @@ -216,6 +216,7 @@ static inline bool rcu_is_watching(void) static inline void rcu_all_qs(void) { + barrier(); /* Avoid RCU read-side critical sections leaking across. */ } #endif /* __LINUX_RCUTINY_H */ diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index b9d9e0249e2f..93c0f23c3e45 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -337,12 +337,14 @@ static void rcu_momentary_dyntick_idle(void) */ void rcu_note_context_switch(void) { + barrier(); /* Avoid RCU read-side critical sections leaking down. */ trace_rcu_utilization(TPS("Start context switch")); rcu_sched_qs(); rcu_preempt_note_context_switch(); if (unlikely(raw_cpu_read(rcu_sched_qs_mask))) rcu_momentary_dyntick_idle(); trace_rcu_utilization(TPS("End context switch")); + barrier(); /* Avoid RCU read-side critical sections leaking up. */ } EXPORT_SYMBOL_GPL(rcu_note_context_switch); @@ -353,12 +355,19 @@ EXPORT_SYMBOL_GPL(rcu_note_context_switch); * RCU flavors in desperate need of a quiescent state, which will normally * be none of them). Either way, do a lightweight quiescent state for * all RCU flavors. + * + * The barrier() calls are redundant in the common case when this is + * called externally, but just in case this is called from within this + * file. + * */ void rcu_all_qs(void) { + barrier(); /* Avoid RCU read-side critical sections leaking down. */ if (unlikely(raw_cpu_read(rcu_sched_qs_mask))) rcu_momentary_dyntick_idle(); this_cpu_inc(rcu_qs_ctr); + barrier(); /* Avoid RCU read-side critical sections leaking up. */ } EXPORT_SYMBOL_GPL(rcu_all_qs); -- 2.5.2