Re: [PATCH 11/41] nohz/cpuset: Don't turn off the tick if rcu needs it

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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 <fweisbec@gmail.com>
> > > Cc: Alessio Igor Bogani <abogani@kernel.org>
> > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > Cc: Avi Kivity <avi@redhat.com>
> > > Cc: Chris Metcalf <cmetcalf@tilera.com>
> > > Cc: Christoph Lameter <cl@linux.com>
> > > Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
> > > Cc: Geoff Levand <geoff@infradead.org>
> > > Cc: Gilad Ben Yossef <gilad@benyossef.com>
> > > Cc: Hakan Akkan <hakanakkan@gmail.com>
> > > Cc: Ingo Molnar <mingo@kernel.org>
> > > Cc: Kevin Hilman <khilman@ti.com>
> > > Cc: Max Krasnyansky <maxk@qualcomm.com>
> > > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > Cc: Stephen Hemminger <shemminger@vyatta.com>
> > > Cc: Steven Rostedt <rostedt@goodmis.org>
> > > Cc: Sven-Thorsten Dietrich <thebigcorporation@gmail.com>
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > ---
> > >  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
> > > 
> > 
>