rcu: Print number of online CPUs in RCU CPU stall-warning messages

RCU's synchronous grace periods act quite differently when there is
only one online CPU, especially in the no-op case in kernels built with
CONFIG_PREEMPTION=n.  This change in behavior can be important debugging
information, so this commit adds the number of online CPUs to the RCU
CPU stall warning messages.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree_stall.h

@@ -565,9 +565,9 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
 
 	for_each_possible_cpu(cpu)
 		totqlen += rcu_get_n_cbs_cpu(cpu);
-	pr_cont("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu)\n",
+	pr_cont("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu ncpus=%d)\n",
 	       smp_processor_id(), (long)(jiffies - gps),
-	       (long)rcu_seq_current(&rcu_state.gp_seq), totqlen);
+	       (long)rcu_seq_current(&rcu_state.gp_seq), totqlen, rcu_state.n_online_cpus);
 	if (ndetected) {
 		rcu_dump_cpu_stacks();
 
@@ -626,9 +626,9 @@ static void print_cpu_stall(unsigned long gps)
 	raw_spin_unlock_irqrestore_rcu_node(rdp->mynode, flags);
 	for_each_possible_cpu(cpu)
 		totqlen += rcu_get_n_cbs_cpu(cpu);
-	pr_cont("\t(t=%lu jiffies g=%ld q=%lu)\n",
+	pr_cont("\t(t=%lu jiffies g=%ld q=%lu ncpus=%d)\n",
 		jiffies - gps,
-		(long)rcu_seq_current(&rcu_state.gp_seq), totqlen);
+		(long)rcu_seq_current(&rcu_state.gp_seq), totqlen, rcu_state.n_online_cpus);
 
 	rcu_check_gp_kthread_expired_fqs_timer();
 	rcu_check_gp_kthread_starvation();