rcu/nocb: Check for deferred nocb wakeups before nohz_full early exit

In theory, a timer is used to defer wakeups of no-CBs grace-period
kthreads when the wakeup cannot be done safely directly from the
call_rcu().  In practice, the one-jiffy delay is not always consistent
with timely callback invocation under heavy call_rcu() loads.  Therefore,
there are a number of checks for a pending deferred wakeup, including
from the scheduling-clock interrupt.  Unfortunately, this check follows
the rcu_nohz_full_cpu() early exit, which renders it useless on such CPUs.

This commit therefore moves the check for the pending deferred no-CB
wakeup to precede the rcu_nohz_full_cpu() early exit.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

kernel/rcu/tree.c

@@ -2739,6 +2739,10 @@ static int rcu_pending(void)
 	/* Check for CPU stalls, if enabled. */
 	check_cpu_stall(rdp);
 
+	/* Does this CPU need a deferred NOCB wakeup? */
+	if (rcu_nocb_need_deferred_wakeup(rdp))
+		return 1;
+
 	/* Is this CPU a NO_HZ_FULL CPU that should ignore RCU? */
 	if (rcu_nohz_full_cpu())
 		return 0;
@@ -2763,10 +2767,6 @@ static int rcu_pending(void)
 	    unlikely(READ_ONCE(rdp->gpwrap))) /* outside lock */
 		return 1;
 
-	/* Does this CPU need a deferred NOCB wakeup? */
-	if (rcu_nocb_need_deferred_wakeup(rdp))
-		return 1;
-
 	/* nothing to do */
 	return 0;
 }