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>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index cb4e2056ccf3cf799bb7c045aca346fedb2ed698..6232d2f9a84ef48098c03f546c8a1dcedcfe3477 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/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();