[PATCH] rcu: Only pin GP kthread when full dynticks is actually used

[Frederic Weisbecker <fweisbec@gmail.com>]

CONFIG_NO_HZ_FULL may be enabled widely on distros nowadays but actual
users should be a tiny minority, if actually any.

Also there is a risk that affining the GP kthread to a single CPU could
end up noticeably reducing RCU performances and increasing energy
consumption.

So lets affine the GP kthread only when nohz full is actually used
(ie: when the nohz_full= parameter is filled or CONFIG_NO_HZ_FULL_ALL=y)

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
 kernel/rcu/tree_plugin.h | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index cbc2c45..726f52c 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2843,12 +2843,16 @@ static bool rcu_nohz_full_cpu(struct rcu_state *rsp)
  */
 static void rcu_bind_gp_kthread(void)
 {
-#ifdef CONFIG_NO_HZ_FULL
-	int cpu = ACCESS_ONCE(tick_do_timer_cpu);
+	int cpu;
+
+	if (!tick_nohz_full_enabled())
+		return;
+
+	cpu = ACCESS_ONCE(tick_do_timer_cpu);
 
 	if (cpu < 0 || cpu >= nr_cpu_ids)
 		return;
+
 	if (raw_smp_processor_id() != cpu)
 		set_cpus_allowed_ptr(current, cpumask_of(cpu));
-#endif /* #ifdef CONFIG_NO_HZ_FULL */
 }
-- 
1.8.3.1