rcu: Add tracing to irq/NMI dyntick-idle transitions

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/trace/events/rcu.h

@@ -421,16 +421,18 @@ TRACE_EVENT(rcu_fqs,
 
 /*
  * Tracepoint for dyntick-idle entry/exit events.  These take a string
- * as argument: "Start" for entering dyntick-idle mode, "End" for
- * leaving it, "--=" for events moving towards idle, and "++=" for events
- * moving away from idle.  "Error on entry: not idle task" and "Error on
- * exit: not idle task" indicate that a non-idle task is erroneously
+ * as argument: "Start" for entering dyntick-idle mode, "Startirq" for
+ * entering it from irq/NMI, "End" for leaving it, "Endirq" for leaving it
+ * to irq/NMI, "--=" for events moving towards idle, and "++=" for events
+ * moving away from idle.  "Error on entry: not idle task" and "Error
+ * on exit: not idle task" indicate that a non-idle task is erroneously
  * toying with the idle loop.
  *
  * These events also take a pair of numbers, which indicate the nesting
  * depth before and after the event of interest.  Note that task-related
- * events use the upper bits of each number, while interrupt-related
- * events use the lower bits.
+ * and interrupt-related events use two separate counters, and that the
+ * "++=" and "--=" events for irq/NMI will change the counter by two,
+ * otherwise by one.
  */
 TRACE_EVENT(rcu_dyntick,
 

kernel/rcu/tree.c

@@ -880,12 +880,15 @@ void rcu_nmi_exit(void)
 	 * leave it in non-RCU-idle state.
 	 */
 	if (rdtp->dynticks_nmi_nesting != 1) {
+		trace_rcu_dyntick(TPS("--="), rdtp->dynticks_nmi_nesting,
+				  rdtp->dynticks_nmi_nesting - 2);
 		WRITE_ONCE(rdtp->dynticks_nmi_nesting, /* No store tearing. */
 			   rdtp->dynticks_nmi_nesting - 2);
 		return;
 	}
 
 	/* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
+	trace_rcu_dyntick(TPS("Startirq"), rdtp->dynticks_nmi_nesting, 0);
 	WRITE_ONCE(rdtp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
 	rcu_dynticks_eqs_enter();
 }
@@ -1057,6 +1060,9 @@ void rcu_nmi_enter(void)
 		rcu_dynticks_eqs_exit();
 		incby = 1;
 	}
+	trace_rcu_dyntick(incby == 1 ? TPS("Endirq") : TPS("++="),
+			  rdtp->dynticks_nmi_nesting,
+			  rdtp->dynticks_nmi_nesting + incby);
 	WRITE_ONCE(rdtp->dynticks_nmi_nesting, /* Prevent store tearing. */
 		   rdtp->dynticks_nmi_nesting + incby);
 	barrier();