Re: [PATCH 14/32] nohz/cpuset: Don't turn off the tick if rcu needs it

Paul E. McKenney
2011-08-16T20:13:50Z

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
>