rcu: Do not NMI offline CPUs

Currently, RCU CPU stall warning messages will NMI whatever CPU looks
like it is blocking either the current grace period or the grace-period
kthread.  This can produce confusing output if the target CPU is offline.
This commit therefore checks for offline CPUs.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree_stall.h

@@ -333,9 +333,12 @@ static void rcu_dump_cpu_stacks(void)
 	rcu_for_each_leaf_node(rnp) {
 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
 		for_each_leaf_node_possible_cpu(rnp, cpu)
-			if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu))
-				if (!trigger_single_cpu_backtrace(cpu))
+			if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
+				if (cpu_is_offline(cpu))
+					pr_err("Offline CPU %d blocking current GP.\n", cpu);
+				else if (!trigger_single_cpu_backtrace(cpu))
 					dump_cpu_task(cpu);
+			}
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	}
 }
@@ -466,9 +469,13 @@ static void rcu_check_gp_kthread_starvation(void)
 			pr_err("RCU grace-period kthread stack dump:\n");
 			sched_show_task(gpk);
 			if (cpu >= 0) {
-				pr_err("Stack dump where RCU grace-period kthread last ran:\n");
-				if (!trigger_single_cpu_backtrace(cpu))
-					dump_cpu_task(cpu);
+				if (cpu_is_offline(cpu)) {
+					pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
+				} else  {
+					pr_err("Stack dump where RCU GP kthread last ran:\n");
+					if (!trigger_single_cpu_backtrace(cpu))
+						dump_cpu_task(cpu);
+				}
 			}
 			wake_up_process(gpk);
 		}