rcu: Finer-grained grace-period-end checks in rcu_dump_cpu_stacks()

This commit pushes the grace-period-end checks further down into
rcu_dump_cpu_stacks(), and also uses lockless checks coupled with
finer-grained locking.

The result is that the current leaf rcu_node structure's ->lock is
acquired only if a stack backtrace might be needed from the current CPU,
and is held across only that CPU's backtrace.  As a result, if there are
no stalled CPUs associated with a given rcu_node structure, then its
->lock will not be acquired at all.  On large systems, it is usually
(though not always) the case that a small number of CPUs are stalling
the current grace period, which means that the ->lock need be acquired
only for a small fraction of the rcu_node structures.

[ paulmck: Apply Dan Carpenter feedback. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Author: paulmckrcu

kernel/rcu/tree_stall.h

@@ -342,20 +342,24 @@ static void rcu_dump_cpu_stacks(unsigned long gp_seq)
 	struct rcu_node *rnp;
 
 	rcu_for_each_leaf_node(rnp) {
-		if (gp_seq != data_race(rcu_state.gp_seq)) {
-			pr_err("INFO: Stall ended during stack backtracing.\n");
-			return;
-		}
 		printk_deferred_enter();
-		raw_spin_lock_irqsave_rcu_node(rnp, flags);
-		for_each_leaf_node_possible_cpu(rnp, cpu)
+		for_each_leaf_node_possible_cpu(rnp, cpu) {
+			if (gp_seq != data_race(rcu_state.gp_seq)) {
+				printk_deferred_exit();
+				pr_err("INFO: Stall ended during stack backtracing.\n");
+				return;
+			}
+			if (!(data_race(rnp->qsmask) & leaf_node_cpu_bit(rnp, cpu)))
+				continue;
+			raw_spin_lock_irqsave_rcu_node(rnp, flags);
 			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
 					dump_cpu_task(cpu);
 			}
-		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+		}
 		printk_deferred_exit();
 	}
 }