rcuscale: Move shutdown from wait_event() to wait_event_idle()

The rcu_scale_shutdown() and kfree_scale_shutdown() kthreads/functions
use wait_event() to wait for the rcuscale test to complete.  However,
each updater thread in such a test waits for at least 100 grace periods.
If each grace period takes more than 1.2 seconds, which is long, but
not insanely so, this can trigger the hung-task timeout.

This commit therefore replaces those wait_event() calls with calls to
wait_event_idle(), which do not trigger the hung-task timeout.

Reported-by: kernel test robot <yujie.liu@intel.com>
Reported-by: Liam Howlett <liam.howlett@oracle.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Yujie Liu <yujie.liu@intel.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

kernel/rcu/rcuscale.c

@@ -631,8 +631,7 @@ static int compute_real(int n)
 static int
 rcu_scale_shutdown(void *arg)
 {
-	wait_event(shutdown_wq,
-		   atomic_read(&n_rcu_scale_writer_finished) >= nrealwriters);
+	wait_event_idle(shutdown_wq, atomic_read(&n_rcu_scale_writer_finished) >= nrealwriters);
 	smp_mb(); /* Wake before output. */
 	rcu_scale_cleanup();
 	kernel_power_off();
@@ -771,8 +770,8 @@ kfree_scale_cleanup(void)
 static int
 kfree_scale_shutdown(void *arg)
 {
-	wait_event(shutdown_wq,
-		   atomic_read(&n_kfree_scale_thread_ended) >= kfree_nrealthreads);
+	wait_event_idle(shutdown_wq,
+			atomic_read(&n_kfree_scale_thread_ended) >= kfree_nrealthreads);
 
 	smp_mb(); /* Wake before output. */