[PATCH] 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.

Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
---
 kernel/rcu/tree_nocb.h | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index fa8e4f82e60c..c25c3e3dd22d 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -698,10 +698,14 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
 				   TPS("WakeBypassIsDeferred"));
 	}
 	if (rcu_nocb_poll) {
-		/* 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)) {
+			rcu_wait(READ_ONCE(my_rdp->nocb_toggling_rdp));
+		} else {
+			/* 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);
+		}
 	} else if (!needwait_gp) {
 		/* Wait for callbacks to appear. */
 		trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Sleep"));
@@ -980,7 +984,7 @@ static int rdp_offload_toggle(struct rcu_data *rdp,
 	raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
 	// Queue this rdp for add/del to/from the list to iterate on rcuog
 	WRITE_ONCE(rdp_gp->nocb_toggling_rdp, rdp);
-	if (rdp_gp->nocb_gp_sleep) {
+	if (rcu_nocb_poll || rdp_gp->nocb_gp_sleep) {
 		rdp_gp->nocb_gp_sleep = false;
 		wake_gp = true;
 	}
-- 
2.25.1