rcu: Only dump stalled-tasks stacks if there was a real stall

The print_other_cpu_stall() function currently unconditionally invokes
rcu_print_detail_task_stall().  This is OK because if there was a stall
sufficient to cause print_other_cpu_stall() to be invoked, that stall
is very likely to persist through the entire print_other_cpu_stall()
execution.  However, if the stall did not persist, the variable ndetected
will be zero, and that variable is already tested in an "if" statement.
Therefore, this commit moves the call to rcu_print_detail_task_stall()
under that pre-existing "if" to improve readability, with a very rare
reduction in overhead.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
[ paulmck: Reworked commit log. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/rcu/tree.c

@@ -1379,6 +1379,9 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
 	       (long)rsp->gpnum, (long)rsp->completed, totqlen);
 	if (ndetected) {
 		rcu_dump_cpu_stacks(rsp);
+
+		/* Complain about tasks blocking the grace period. */
+		rcu_print_detail_task_stall(rsp);
 	} else {
 		if (READ_ONCE(rsp->gpnum) != gpnum ||
 		    READ_ONCE(rsp->completed) == gpnum) {
@@ -1395,9 +1398,6 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
 		}
 	}
 
-	/* Complain about tasks blocking the grace period. */
-	rcu_print_detail_task_stall(rsp);
-
 	rcu_check_gp_kthread_starvation(rsp);
 
 	panic_on_rcu_stall();