Messages in this thread | | | Date | Tue, 16 Aug 2011 13:13:42 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 14/32] nohz/cpuset: Don't turn off the tick if rcu needs it |
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On Mon, Aug 15, 2011 at 05:52:11PM +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
s/dynctik/dyntick/ ;-)
> 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.
One question below...
Thanx, Paul
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: Anton Blanchard <anton@au1.ibm.com> > Cc: Avi Kivity <avi@redhat.com> > Cc: Ingo Molnar <mingo@elte.hu> > Cc: Lai Jiangshan <laijs@cn.fujitsu.com> > Cc: Paul E . McKenney <paulmck@linux.vnet.ibm.com> > Cc: Paul Menage <menage@google.com> > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> > Cc: Stephen Hemminger <shemminger@vyatta.com> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Tim Pepper <lnxninja@linux.vnet.ibm.com> > --- > include/linux/rcupdate.h | 1 + > kernel/rcutree.c | 3 +-- > kernel/sched.c | 14 ++++++++++++++ > 3 files changed, 16 insertions(+), 2 deletions(-) > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > index 99f9aa7..55a482a 100644 > --- a/include/linux/rcupdate.h > +++ b/include/linux/rcupdate.h > @@ -133,6 +133,7 @@ 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; > > #ifdef CONFIG_NO_HZ > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > index ba06207..0009bfc 100644 > --- a/kernel/rcutree.c > +++ b/kernel/rcutree.c > @@ -205,7 +205,6 @@ 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. > @@ -1729,7 +1728,7 @@ 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/sched.c b/kernel/sched.c > index 0e1aa4e..353a66f 100644 > --- a/kernel/sched.c > +++ b/kernel/sched.c > @@ -2439,6 +2439,7 @@ DEFINE_PER_CPU(int, task_nohz_mode); > bool cpuset_nohz_can_stop_tick(void) > { > struct rq *rq; > + int cpu; > > rq = this_rq(); > > @@ -2446,6 +2447,19 @@ bool cpuset_nohz_can_stop_tick(void) > if (rq->nr_running > 1) > return false; > > + cpu = smp_processor_id(); > + > + /* > + * FIXME: will probably be removed soon as it's > + * already checked from tick_nohz_stop_sched_tick() > + */ > + if (rcu_needs_cpu(cpu)) > + return false; > + > + /* Is there a grace period to complete ? */ > + if (rcu_pending(cpu))
This is from a quiescent state for both RCU and RCU-bh, right? Or can their be RCU or RCU-bh read-side critical sections held across here? (It would be mildly bad if so.)
But force_quiescent_state() will catch cases where RCU needs quiescent states from CPUs, so is this check really needed?
> + return false; > + > return true; > } > > -- > 1.7.5.4 >
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