[PATCH v2] rcu/nocb: Avoid polling when myrdp->nocb_head_rdp list is empty

[Zqiang <qiang1.zhang@intel.com>]

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@intel.com>
---
 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.

 kernel/rcu/tree_nocb.h | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index fa8e4f82e60c..2a52c9abc681 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"));
@@ -1030,7 +1034,7 @@ static long rcu_nocb_rdp_deoffload(void *arg)
 
 	mutex_lock(&rdp_gp->nocb_gp_kthread_mutex);
 	if (rdp_gp->nocb_gp_kthread) {
-		if (wake_gp)
+		if (wake_gp || rcu_nocb_poll)
 			wake_up_process(rdp_gp->nocb_gp_kthread);
 
 		/*
@@ -1152,7 +1156,7 @@ static long rcu_nocb_rdp_offload(void *arg)
 	 *      rcu_nocb_unlock()         rcu_nocb_unlock()
 	 */
 	wake_gp = rdp_offload_toggle(rdp, true, flags);
-	if (wake_gp)
+	if (wake_gp || rcu_nocb_poll)
 		wake_up_process(rdp_gp->nocb_gp_kthread);
 	swait_event_exclusive(rdp->nocb_state_wq,
 			      rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB) &&
-- 
2.25.1