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>

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 35c1355..29cf096 100644 (file)
--- a/kernel/rcu/tree_stall.h
+++ b/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);
                }