[PATCH v3] rcu/nocb: Avoid polling when myrdp->nocb_head_rdp list is empty

[Zqiang]

Currently, If the 'rcu_nocb_poll' bootargs is enable, all rcuog kthreads
enter polling mode. however, due to only insert CPU's rdp which belong to
rcu_nocb_mask to 'nocb_head_rdp' list or all CPU's rdp served by rcuog
kthread have been de-offloaded, these cause the 'nocb_head_rdp' list
served by rcuog kthread is empty, when the 'nocb_head_rdp' is empty,
the rcuog kthread in polling mode not actually do anything. fix it by
exiting polling mode when the 'nocb_head_rdp'list is empty, otherwise
entering polling mode.

Co-developed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
---
 v1->v2:
 Move rcu_nocb_poll flags check from rdp_offload_toggle() to 
 rcu_nocb_rdp_offload/deoffload(), avoid unnecessary setting of 
 rdp_gp->nocb_gp_sleep flags, because when rcu_nocb_poll is set  the 
 rdp_gp->nocb_gp_sleep is not used.
 
 v2->v3:
 When nocb_head_rdp list is empty. put rcuog kthreads in nocb_gp_wq
 waitqueue to wait offloading.

 kernel/rcu/tree_nocb.h | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index fa8e4f82e60c..a8f574d8850d 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -584,6 +584,14 @@ static int nocb_gp_toggle_rdp(struct rcu_data *rdp,
 	return ret;
 }
 
+static void nocb_gp_sleep(struct rcu_data *my_rdp, int cpu)
+{
+	trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Sleep"));
+	swait_event_interruptible_exclusive(my_rdp->nocb_gp_wq,
+					!READ_ONCE(my_rdp->nocb_gp_sleep));
+	trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("EndSleep"));
+}
+
 /*
  * No-CBs GP kthreads come here to wait for additional callbacks to show up
  * or for grace periods to end.
@@ -701,13 +709,19 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
 		/* Polling, so trace if first poll in the series. */
 		if (gotcbs)
 			trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Poll"));
-		schedule_timeout_idle(1);
+		if (list_empty(&my_rdp->nocb_head_rdp)) {
+			raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);
+			if (!my_rdp->nocb_toggling_rdp)
+				WRITE_ONCE(my_rdp->nocb_gp_sleep, true);
+			raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
+			/* Wait for any offloading rdp */
+			nocb_gp_sleep(my_rdp, cpu);
+		} else {
+			schedule_timeout_idle(1);
+		}
 	} else if (!needwait_gp) {
 		/* Wait for callbacks to appear. */
-		trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Sleep"));
-		swait_event_interruptible_exclusive(my_rdp->nocb_gp_wq,
-				!READ_ONCE(my_rdp->nocb_gp_sleep));
-		trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("EndSleep"));
+		nocb_gp_sleep(my_rdp, cpu);
 	} else {
 		rnp = my_rdp->mynode;
 		trace_rcu_this_gp(rnp, my_rdp, wait_gp_seq, TPS("StartWait"));
-- 
2.25.1