rcu: Separate accounting of callbacks from callback-free CPUs

Currently, callback invocations from callback-free CPUs are accounted to
the CPU that registered the callback, but using the same field that is
used for normal callbacks.  This makes it impossible to determine from
debugfs output whether callbacks are in fact being diverted.  This commit
therefore adds a separate ->n_nocbs_invoked field in the rcu_data structure
in which diverted callback invocations are counted.  RCU's debugfs tracing
still displays normal callback invocations using ci=, but displayed
diverted callbacks with nci=.

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

kernel/rcutree.h

@@ -287,6 +287,7 @@ struct rcu_data {
 	long		qlen_last_fqs_check;
 					/* qlen at last check for QS forcing */
 	unsigned long	n_cbs_invoked;	/* count of RCU cbs invoked. */
+	unsigned long	n_nocbs_invoked; /* count of no-CBs RCU cbs invoked. */
 	unsigned long   n_cbs_orphaned; /* RCU cbs orphaned by dying CPU */
 	unsigned long   n_cbs_adopted;  /* RCU cbs adopted from dying CPU */
 	unsigned long	n_force_qs_snap;

kernel/rcutree_plugin.h

@@ -2406,7 +2406,7 @@ static int rcu_nocb_kthread(void *arg)
 		trace_rcu_batch_end(rdp->rsp->name, c, !!list, 0, 0, 1);
 		ACCESS_ONCE(rdp->nocb_p_count) -= c;
 		ACCESS_ONCE(rdp->nocb_p_count_lazy) -= cl;
-		rdp->n_cbs_invoked += c;
+		rdp->n_nocbs_invoked += c;
 	}
 	return 0;
 }

kernel/rcutree_trace.c

@@ -148,8 +148,9 @@ static void print_one_rcu_data(struct seq_file *m, struct rcu_data *rdp)
 		   per_cpu(rcu_cpu_kthread_loops, rdp->cpu) & 0xffff);
 #endif /* #ifdef CONFIG_RCU_BOOST */
 	seq_printf(m, " b=%ld", rdp->blimit);
-	seq_printf(m, " ci=%lu co=%lu ca=%lu\n",
-		   rdp->n_cbs_invoked, rdp->n_cbs_orphaned, rdp->n_cbs_adopted);
+	seq_printf(m, " ci=%lu nci=%lu co=%lu ca=%lu\n",
+		   rdp->n_cbs_invoked, rdp->n_nocbs_invoked,
+		   rdp->n_cbs_orphaned, rdp->n_cbs_adopted);
 }
 
 static int show_rcudata(struct seq_file *m, void *v)