From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753233AbbJFRmP (ORCPT ); Tue, 6 Oct 2015 13:42:15 -0400 Received: from e37.co.us.ibm.com ([32.97.110.158]:47496 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752652AbbJFRmL (ORCPT ); Tue, 6 Oct 2015 13:42:11 -0400 X-IBM-Helo: d03dlp01.boulder.ibm.com X-IBM-MailFrom: paulmck@linux.vnet.ibm.com X-IBM-RcptTo: linux-kernel@vger.kernel.org Date: Tue, 6 Oct 2015 10:42:04 -0700 From: "Paul E. McKenney" To: Josh Triplett Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, 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 Subject: Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for Tiny and Tree RCU readers Message-ID: <20151006174204.GL3910@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20151006161305.GA9799@linux.vnet.ibm.com> <1444148028-11551-1-git-send-email-paulmck@linux.vnet.ibm.com> <1444148028-11551-4-git-send-email-paulmck@linux.vnet.ibm.com> <20151006164445.GA9600@cloud> <20151006170101.GG3910@linux.vnet.ibm.com> <20151006171630.GC9600@cloud> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151006171630.GC9600@cloud> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15100617-0025-0000-0000-00001DAAC31E Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 06, 2015 at 10:16:30AM -0700, Josh Triplett wrote: > On Tue, Oct 06, 2015 at 10:01:01AM -0700, Paul E. McKenney wrote: > > On Tue, Oct 06, 2015 at 09:44:45AM -0700, Josh Triplett wrote: > > > On Tue, Oct 06, 2015 at 09:13:39AM -0700, Paul E. McKenney wrote: > > > > 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. > > > > > > This also adds explicit barrier() calls to several internal RCU > > > functions, but the commit message doesn't explain those at all. > > > > To compensate for them being removed from rcu_read_lock() and > > rcu_read_unlock(), but yes, I will update. > > That much seemed clear from the comments, but that doesn't explain *why* > those functions need barriers of their own even though rcu_read_lock() > and rcu_read_unlock() don't. Ah. The reason is that Tiny RCU and Tree RCU (the !PREEMPT ones) act by implicitly extending (and, if need be, merging) the RCU read-side critical sections to include all the code between successive quiescent states, for example, all the code between a pair of calls to schedule(). Therefore, there need to be barrier() calls in the quiescent-state functions. Some could be argued to be implicitly present due to translation-unit boundaries, but paranoia and all that. Would adding that sort of explanation help? Thanx, Paul > > > > 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 > > > > > > > > > >