From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760495Ab2EWP5I (ORCPT ); Wed, 23 May 2012 11:57:08 -0400 Received: from e9.ny.us.ibm.com ([32.97.182.139]:36214 "EHLO e9.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756439Ab2EWP5F (ORCPT ); Wed, 23 May 2012 11:57:05 -0400 Date: Wed, 23 May 2012 08:15:42 -0700 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: LKML , linaro-sched-sig@lists.linaro.org, Alessio Igor Bogani , Andrew Morton , Avi Kivity , Chris Metcalf , Christoph Lameter , Daniel Lezcano , Geoff Levand , Gilad Ben Yossef , Hakan Akkan , Ingo Molnar , Kevin Hilman , Max Krasnyansky , Peter Zijlstra , Stephen Hemminger , Steven Rostedt , Sven-Thorsten Dietrich , Thomas Gleixner Subject: Re: [PATCH 11/41] nohz/cpuset: Don't turn off the tick if rcu needs it Message-ID: <20120523151537.GA2402@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1335830115-14335-1-git-send-email-fweisbec@gmail.com> <1335830115-14335-12-git-send-email-fweisbec@gmail.com> <20120522171658.GA8087@linux.vnet.ibm.com> <20120523135205.GB1663@somewhere> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120523135205.GB1663@somewhere> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12052315-7182-0000-0000-00000195BDAD Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 23, 2012 at 03:52:09PM +0200, Frederic Weisbecker wrote: > On Tue, May 22, 2012 at 10:16:58AM -0700, Paul E. McKenney wrote: > > On Tue, May 01, 2012 at 01:54:45AM +0200, Frederic Weisbecker wrote: > > > If RCU is waiting for the current CPU to complete a grace > > > period, don't turn off the tick. Unlike dynctik-idle, we > > > are not necessarily going to enter into rcu extended quiescent > > > state, so we may need to keep the tick to note current CPU's > > > quiescent states. > > > > > > [added build fix from Zen Lin] > > > > Hello, Frederic, > > > > One question below -- why not rcu_needs_cpu() instead of rcu_pending()? > > > > Thanx, Paul > > > > > Signed-off-by: Frederic Weisbecker > > > Cc: Alessio Igor Bogani > > > Cc: Andrew Morton > > > Cc: Avi Kivity > > > Cc: Chris Metcalf > > > Cc: Christoph Lameter > > > Cc: Daniel Lezcano > > > Cc: Geoff Levand > > > Cc: Gilad Ben Yossef > > > Cc: Hakan Akkan > > > Cc: Ingo Molnar > > > Cc: Kevin Hilman > > > Cc: Max Krasnyansky > > > Cc: Paul E. McKenney > > > Cc: Peter Zijlstra > > > Cc: Stephen Hemminger > > > Cc: Steven Rostedt > > > Cc: Sven-Thorsten Dietrich > > > Cc: Thomas Gleixner > > > --- > > > include/linux/rcupdate.h | 1 + > > > kernel/rcutree.c | 3 +-- > > > kernel/time/tick-sched.c | 22 ++++++++++++++++++---- > > > 3 files changed, 20 insertions(+), 6 deletions(-) > > > > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > > > index 81c04f4..e06639e 100644 > > > --- a/include/linux/rcupdate.h > > > +++ b/include/linux/rcupdate.h > > > @@ -184,6 +184,7 @@ static inline int rcu_preempt_depth(void) > > > extern void rcu_sched_qs(int cpu); > > > extern void rcu_bh_qs(int cpu); > > > extern void rcu_check_callbacks(int cpu, int user); > > > +extern int rcu_pending(int cpu); > > > struct notifier_block; > > > extern void rcu_idle_enter(void); > > > extern void rcu_idle_exit(void); > > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > > > index 6c4a672..e141c7e 100644 > > > --- a/kernel/rcutree.c > > > +++ b/kernel/rcutree.c > > > @@ -212,7 +212,6 @@ int rcu_cpu_stall_suppress __read_mostly; > > > module_param(rcu_cpu_stall_suppress, int, 0644); > > > > > > static void force_quiescent_state(struct rcu_state *rsp, int relaxed); > > > -static int rcu_pending(int cpu); > > > > > > /* > > > * Return the number of RCU-sched batches processed thus far for debug & stats. > > > @@ -1915,7 +1914,7 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp) > > > * by the current CPU, returning 1 if so. This function is part of the > > > * RCU implementation; it is -not- an exported member of the RCU API. > > > */ > > > -static int rcu_pending(int cpu) > > > +int rcu_pending(int cpu) > > > { > > > return __rcu_pending(&rcu_sched_state, &per_cpu(rcu_sched_data, cpu)) || > > > __rcu_pending(&rcu_bh_state, &per_cpu(rcu_bh_data, cpu)) || > > > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > > > index 43fa7ac..4f99766 100644 > > > --- a/kernel/time/tick-sched.c > > > +++ b/kernel/time/tick-sched.c > > > @@ -506,9 +506,21 @@ void tick_nohz_idle_enter(void) > > > local_irq_enable(); > > > } > > > > > > +#ifdef CONFIG_CPUSETS_NO_HZ > > > +static bool can_stop_adaptive_tick(void) > > > +{ > > > + if (!sched_can_stop_tick()) > > > + return false; > > > + > > > + /* Is there a grace period to complete ? */ > > > + if (rcu_pending(smp_processor_id())) > > > > You lost me on this one. Why can't this be rcu_needs_cpu()? > > We already have an rcu_needs_cpu() check in tick_nohz_stop_sched_tick() > that prevents the tick to shut down if the CPU has local callbacks to handle. > > The rcu_pending() check is there in case some other CPU is waiting for the > current one to help completing a grace period, by reporting a quiescent state > for example. This happens because we may stop the tick in the kernel, not only > userspace. And if we are in the kernel, we still need to be part of the global > state machine. Ah! But RCU will notice that the CPU is in dyntick-idle mode, and will therefore take any needed quiescent-state action on that CPU's behalf. So there should be no need to call rcu_pending() anywhere outside of the RCU core code. Thanx, Paul > > > + return false; > > > + > > > + return true; > > > +} > > > + > > > static void tick_nohz_cpuset_stop_tick(struct tick_sched *ts) > > > { > > > -#ifdef CONFIG_CPUSETS_NO_HZ > > > int cpu = smp_processor_id(); > > > > > > if (!cpuset_adaptive_nohz() || is_idle_task(current)) > > > @@ -517,12 +529,14 @@ static void tick_nohz_cpuset_stop_tick(struct tick_sched *ts) > > > if (!ts->tick_stopped && ts->nohz_mode == NOHZ_MODE_INACTIVE) > > > return; > > > > > > - if (!sched_can_stop_tick()) > > > + if (!can_stop_adaptive_tick()) > > > return; > > > > > > tick_nohz_stop_sched_tick(ts, ktime_get(), cpu); > > > -#endif > > > } > > > +#else > > > +static void tick_nohz_cpuset_stop_tick(struct tick_sched *ts) { } > > > +#endif > > > > > > /** > > > * tick_nohz_irq_exit - update next tick event from interrupt exit > > > @@ -852,7 +866,7 @@ void tick_nohz_check_adaptive(void) > > > struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched); > > > > > > if (ts->tick_stopped && !is_idle_task(current)) { > > > - if (!sched_can_stop_tick()) > > > + if (!can_stop_adaptive_tick()) > > > tick_nohz_restart_sched_tick(); > > > } > > > } > > > -- > > > 1.7.5.4 > > > > > >