rcu: Track rcu_nocb_kthread()'s sleeping and awakening

This commit adds event traces to track all of rcu_nocb_kthread()'s
blocking and awakening.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/trace/events/rcu.h

@@ -183,8 +183,12 @@ TRACE_EVENT(rcu_grace_period_init,
  *	"WakeOvf": Wake rcuo kthread, CB list is huge.
  *	"WakeNot": Don't wake rcuo kthread.
  *	"WakeNotPoll": Don't wake rcuo kthread because it is polling.
+ *	"Poll": Start of new polling cycle for rcu_nocb_poll.
+ *	"Sleep": Sleep waiting for CBs for !rcu_nocb_poll.
  *	"WokeEmpty": rcuo kthread woke to find empty list.
  *	"WokeNonEmpty": rcuo kthread woke to find non-empty list.
+ *	"WaitQueue": Enqueue partially done, timed wait for it to complete.
+ *	"WokeQueue": Partial enqueue now complete.
  */
 TRACE_EVENT(rcu_nocb_wake,
 

kernel/rcutree_plugin.h

@@ -2230,6 +2230,7 @@ static void rcu_nocb_wait_gp(struct rcu_data *rdp)
 static int rcu_nocb_kthread(void *arg)
 {
 	int c, cl;
+	bool firsttime = 1;
 	struct rcu_head *list;
 	struct rcu_head *next;
 	struct rcu_head **tail;
@@ -2238,8 +2239,15 @@ static int rcu_nocb_kthread(void *arg)
 	/* Each pass through this loop invokes one batch of callbacks */
 	for (;;) {
 		/* If not polling, wait for next batch of callbacks. */
-		if (!rcu_nocb_poll)
+		if (!rcu_nocb_poll) {
+			trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
+					    TPS("Sleep"));
 			wait_event_interruptible(rdp->nocb_wq, rdp->nocb_head);
+		} else if (firsttime) {
+			firsttime = 0;
+			trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
+					    TPS("Poll"));
+		}
 		list = ACCESS_ONCE(rdp->nocb_head);
 		if (!list) {
 			if (!rcu_nocb_poll)
@@ -2249,6 +2257,7 @@ static int rcu_nocb_kthread(void *arg)
 			flush_signals(current);
 			continue;
 		}
+		firsttime = 1;
 		trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
 				    TPS("WokeNonEmpty"));
 
@@ -2271,7 +2280,11 @@ static int rcu_nocb_kthread(void *arg)
 			next = list->next;
 			/* Wait for enqueuing to complete, if needed. */
 			while (next == NULL && &list->next != tail) {
+				trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
+						    TPS("WaitQueue"));
 				schedule_timeout_interruptible(1);
+				trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
+						    TPS("WokeQueue"));
 				next = list->next;
 			}
 			debug_rcu_head_unqueue(list);