rcu: Add callbacks-invoked counters

This commit adds a count of the callbacks invoked to the per-CPU rcu_data
structure.  This count is printed by the show_rcu_gp_kthreads() that
is invoked by rcutorture and the RCU CPU stall-warning code.  It is also
intended for use by drgn.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.c

@@ -2443,6 +2443,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
 	local_irq_save(flags);
 	rcu_nocb_lock(rdp);
 	count = -rcl.len;
+	rdp->n_cbs_invoked += count;
 	trace_rcu_batch_end(rcu_state.name, count, !!rcl.head, need_resched(),
 			    is_idle_task(current), rcu_is_callbacks_kthread());
 

kernel/rcu/tree.h

@@ -171,6 +171,7 @@ struct rcu_data {
 					/* different grace periods. */
 	long		qlen_last_fqs_check;
 					/* qlen at last check for QS forcing */
+	unsigned long	n_cbs_invoked;	/* # callbacks invoked since boot. */
 	unsigned long	n_force_qs_snap;
 					/* did other CPU force QS recently? */
 	long		blimit;		/* Upper limit on a processed batch */

kernel/rcu/tree_stall.h

@@ -649,6 +649,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
  */
 void show_rcu_gp_kthreads(void)
 {
+	unsigned long cbs = 0;
 	int cpu;
 	unsigned long j;
 	unsigned long ja;
@@ -690,9 +691,11 @@ void show_rcu_gp_kthreads(void)
 	}
 	for_each_possible_cpu(cpu) {
 		rdp = per_cpu_ptr(&rcu_data, cpu);
+		cbs += data_race(rdp->n_cbs_invoked);
 		if (rcu_segcblist_is_offloaded(&rdp->cblist))
 			show_rcu_nocb_state(rdp);
 	}
+	pr_info("RCU callbacks invoked since boot: %lu\n", cbs);
 	show_rcu_tasks_gp_kthreads();
 }
 EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);