[PATCH RFC tip/core/rcu 09/14] rcu/nocb: Don't wake no-CBs GP kthread if timer posted under overload

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

When under overload conditions, __call_rcu_nocb_wake() will wake the
no-CBs GP kthread any time the no-CBs CB kthread is asleep or there
are no ready-to-invoke callbacks, but only after a timer delay.  If the
no-CBs GP kthread has a ->nocb_bypass_timer pending, the deferred wakeup
from __call_rcu_nocb_wake() is redundant.  This commit therefore makes
__call_rcu_nocb_wake() avoid posting the redundant deferred wakeup if
->nocb_bypass_timer is pending.  This requires adding a bit of ordering
of timer actions.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
---
 kernel/rcu/tree_plugin.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index efd7f6fa2790..379cb7e50a62 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1906,8 +1906,10 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 			rcu_advance_cbs_nowake(rdp->mynode, rdp);
 			rdp->nocb_gp_adv_time = j;
 		}
-		if (rdp->nocb_cb_sleep ||
-		    !rcu_segcblist_ready_cbs(&rdp->cblist))
+		smp_mb(); /* Enqueue before timer_pending(). */
+		if ((rdp->nocb_cb_sleep ||
+		     !rcu_segcblist_ready_cbs(&rdp->cblist)) &&
+		    !timer_pending(&rdp->nocb_bypass_timer))
 			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE,
 					   TPS("WakeOvfIsDeferred"));
 		rcu_nocb_unlock_irqrestore(rdp, flags);
@@ -1926,6 +1928,7 @@ static void do_nocb_bypass_wakeup_timer(struct timer_list *t)
 
 	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Timer"));
 	rcu_nocb_lock_irqsave(rdp, flags);
+	smp_mb__after_spinlock(); /* Timer expire before wakeup. */
 	__call_rcu_nocb_wake(rdp, true, flags);
 }
 
-- 
2.17.1