rcu: Add tracing for rcuo no-CBs CPU wakeup handshake

Lost wakeups from call_rcu() to the rcuo kthreads can result in hangs
that are difficult to diagnose.  This commit therefore adds tracing to
help pin down the cause of these hangs.
Reported-by: Clark Williams <williams@redhat.com>
Reported-by: Carsten Emde <C.Emde@osadl.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Add const per kbuild test robot's advice. ]

include/trace/events/rcu.h

@@ -171,6 +171,42 @@ TRACE_EVENT(rcu_grace_period_init,
 		  __entry->grplo, __entry->grphi, __entry->qsmask)
 );
 
+/*
+ * Tracepoint for RCU no-CBs CPU callback handoffs.  This event is intended
+ * to assist debugging of these handoffs.
+ *
+ * The first argument is the name of the RCU flavor, and the second is
+ * the number of the offloaded CPU are extracted.  The third and final
+ * argument is a string as follows:
+ *
+ *	"WakeEmpty": Wake rcuo kthread, first CB to empty list.
+ *	"WakeOvf": Wake rcuo kthread, CB list is huge.
+ *	"WakeNot": Don't wake rcuo kthread.
+ *	"WakeNotPoll": Don't wake rcuo kthread because it is polling.
+ *	"WokeEmpty": rcuo kthread woke to find empty list.
+ *	"WokeNonEmpty": rcuo kthread woke to find non-empty list.
+ */
+TRACE_EVENT(rcu_nocb_wake,
+
+	TP_PROTO(const char *rcuname, int cpu, const char *reason),
+
+	TP_ARGS(rcuname, cpu, reason),
+
+	TP_STRUCT__entry(
+		__field(const char *, rcuname)
+		__field(int, cpu)
+		__field(const char *, reason)
+	),
+
+	TP_fast_assign(
+		__entry->rcuname = rcuname;
+		__entry->cpu = cpu;
+		__entry->reason = reason;
+	),
+
+	TP_printk("%s %d %s", __entry->rcuname, __entry->cpu, __entry->reason)
+);
+
 /*
  * Tracepoint for tasks blocking within preemptible-RCU read-side
  * critical sections.  Track the type of RCU (which one day might
@@ -667,6 +703,7 @@ TRACE_EVENT(rcu_barrier,
 #define trace_rcu_future_grace_period(rcuname, gpnum, completed, c, \
 				      level, grplo, grphi, event) \
 				      do { } while (0)
+#define trace_rcu_nocb_wake(rcuname, cpu, reason) do { } while (0)
 #define trace_rcu_preempt_task(rcuname, pid, gpnum) do { } while (0)
 #define trace_rcu_unlock_preempted_task(rcuname, gpnum, pid) do { } while (0)
 #define trace_rcu_quiescent_state_report(rcuname, gpnum, mask, qsmask, level, \

kernel/rcutree_plugin.h

@@ -2108,15 +2108,22 @@ static void __call_rcu_nocb_enqueue(struct rcu_data *rdp,
 
 	/* If we are not being polled and there is a kthread, awaken it ... */
 	t = ACCESS_ONCE(rdp->nocb_kthread);
-	if (rcu_nocb_poll | !t)
+	if (rcu_nocb_poll | !t) {
+		trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
+				    TPS("WakeNotPoll"));
 		return;
+	}
 	len = atomic_long_read(&rdp->nocb_q_count);
 	if (old_rhpp == &rdp->nocb_head) {
 		wake_up(&rdp->nocb_wq); /* ... only if queue was empty ... */
 		rdp->qlen_last_fqs_check = 0;
+		trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("WakeEmpty"));
 	} else if (len > rdp->qlen_last_fqs_check + qhimark) {
 		wake_up_process(t); /* ... or if many callbacks queued. */
 		rdp->qlen_last_fqs_check = LONG_MAX / 2;
+		trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("WakeOvf"));
+	} else {
+		trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("WakeNot"));
 	}
 	return;
 }
@@ -2233,10 +2240,15 @@ static int rcu_nocb_kthread(void *arg)
 			wait_event_interruptible(rdp->nocb_wq, rdp->nocb_head);
 		list = ACCESS_ONCE(rdp->nocb_head);
 		if (!list) {
+			if (!rcu_nocb_poll)
+				trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
+						    TPS("WokeEmpty"));
 			schedule_timeout_interruptible(1);
 			flush_signals(current);
 			continue;
 		}
+		trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
+				    TPS("WokeNonEmpty"));
 
 		/*
 		 * Extract queued callbacks, update counts, and wait