From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758013Ab2IEAqe (ORCPT ); Tue, 4 Sep 2012 20:46:34 -0400 Received: from relay3-d.mail.gandi.net ([217.70.183.195]:57453 "EHLO relay3-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752889Ab2IEAqc (ORCPT ); Tue, 4 Sep 2012 20:46:32 -0400 X-Originating-IP: 217.70.178.129 X-Originating-IP: 173.246.103.110 Date: Tue, 4 Sep 2012 17:46:19 -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, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu, patches@linaro.org, Alessio Igor Bogani , Avi Kivity , Chris Metcalf , Christoph Lameter , Geoff Levand , Gilad Ben Yossef , Hakan Akkan , "H. Peter Anvin" , Ingo Molnar , Kevin Hilman , Max Krasnyansky , Stephen Hemminger , Sven-Thorsten Dietrich Subject: Re: [PATCH tip/core/rcu 04/26] rcu: Settle config for userspace extended quiescent state Message-ID: <20120905004619.GB12373@jtriplet-mobl1> References: <20120830210520.GA2824@linux.vnet.ibm.com> <1346360743-3628-1-git-send-email-paulmck@linux.vnet.ibm.com> <1346360743-3628-4-git-send-email-paulmck@linux.vnet.ibm.com> <20120831234401.GB11771@jtriplet-mobl1> <20120905003459.GA2593@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120905003459.GA2593@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 Tue, Sep 04, 2012 at 05:34:59PM -0700, Paul E. McKenney wrote: > On Fri, Aug 31, 2012 at 04:44:01PM -0700, Josh Triplett wrote: > > On Thu, Aug 30, 2012 at 02:05:21PM -0700, Paul E. McKenney wrote: > > > From: Frederic Weisbecker > > > > > > Create a new config option under the RCU menu that put > > > CPUs under RCU extended quiescent state (as in dynticks > > > idle mode) when they run in userspace. This require > > > some contribution from architectures to hook into kernel > > > and userspace boundaries. > > > > > > Signed-off-by: Frederic Weisbecker > > > Cc: Alessio Igor Bogani > > > Cc: Andrew Morton > > > Cc: Avi Kivity > > > Cc: Chris Metcalf > > > Cc: Christoph Lameter > > > Cc: Geoff Levand > > > Cc: Gilad Ben Yossef > > > Cc: Hakan Akkan > > > Cc: H. Peter Anvin > > > Cc: Ingo Molnar > > > Cc: Josh Triplett > > > Cc: Kevin Hilman > > > Cc: Max Krasnyansky > > > Cc: Peter Zijlstra > > > Cc: Stephen Hemminger > > > Cc: Steven Rostedt > > > Cc: Sven-Thorsten Dietrich > > > Cc: Thomas Gleixner > > > Signed-off-by: Paul E. McKenney > > > > One question below, but nonethelesss: > > > > Reviewed-by: Josh Triplett > > > > > arch/Kconfig | 10 ++++++++++ > > > include/linux/rcupdate.h | 8 ++++++++ > > > init/Kconfig | 10 ++++++++++ > > > kernel/rcutree.c | 5 ++++- > > > 4 files changed, 32 insertions(+), 1 deletions(-) > > > > > > diff --git a/arch/Kconfig b/arch/Kconfig > > > index 72f2fa1..1401a75 100644 > > > --- a/arch/Kconfig > > > +++ b/arch/Kconfig > > > @@ -281,4 +281,14 @@ config SECCOMP_FILTER > > > > > > See Documentation/prctl/seccomp_filter.txt for details. > > > > > > +config HAVE_RCU_USER_QS > > > + bool > > > + help > > > + Provide kernel entry/exit hooks necessary for userspace > > > + RCU extended quiescent state. Syscalls need to be wrapped inside > > > + rcu_user_exit()-rcu_user_enter() through the slow path using > > > + TIF_NOHZ flag. Exceptions handlers must be wrapped as well. Irqs > > > + are already protected inside rcu_irq_enter/rcu_irq_exit() but > > > + preemption or signal handling on irq exit still need to be protected. > > > + > > > source "kernel/gcov/Kconfig" > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > > > index 81d3d5c..e411117 100644 > > > --- a/include/linux/rcupdate.h > > > +++ b/include/linux/rcupdate.h > > > @@ -191,10 +191,18 @@ extern void rcu_idle_enter(void); > > > extern void rcu_idle_exit(void); > > > extern void rcu_irq_enter(void); > > > extern void rcu_irq_exit(void); > > > + > > > +#ifdef CONFIG_RCU_USER_QS > > > extern void rcu_user_enter(void); > > > extern void rcu_user_exit(void); > > > extern void rcu_user_enter_irq(void); > > > extern void rcu_user_exit_irq(void); > > > +#else > > > +static inline void rcu_user_enter(void) { } > > > +static inline void rcu_user_exit(void) { } > > > +#endif /* CONFIG_RCU_USER_QS */ > > > + > > > + > > > extern void exit_rcu(void); > > > > > > /** > > > diff --git a/init/Kconfig b/init/Kconfig > > > index af6c7f8..f6a1830 100644 > > > --- a/init/Kconfig > > > +++ b/init/Kconfig > > > @@ -441,6 +441,16 @@ config PREEMPT_RCU > > > This option enables preemptible-RCU code that is common between > > > the TREE_PREEMPT_RCU and TINY_PREEMPT_RCU implementations. > > > > > > +config RCU_USER_QS > > > + bool "Consider userspace as in RCU extended quiescent state" > > > + depends on HAVE_RCU_USER_QS && SMP > > > > Does this actually depend on SMP, or does it depend on the non-TINY RCU > > implementation? If the latter, it should depend on that rather than > > SMP. > > > > (I assume that the tiny RCU implementation simply doesn't need all this > > machinery because it doesn't need coordinated quiescence at all? Or > > does tiny RCU still cause a periodic wakeup on UP?) > > It actually does depend on SMP. There has to be at least one CPU taking > scheduling-clock interrupts in order to keep time computation accurate, > so a de-facto UP system cannot adaptive-dynticks its sole CPU. Ah. That seems like a removable limitation, albeit a difficult one. Nonetheless, it makes sense to avoid providing the option when it won't help. However, once a config symbol for adaptive dynticks exists, perhaps that symbol should depend on SMP and RCU_USER_QS should depend on that instead, documenting the limitation in the right place and making it easier to find and change eventually. - Josh Triplett