rcu: Avoid redundant grace-period kthread wakeups

When setting up an in-the-future "advanced" grace period, the code needs
to wake up the relevant grace-period kthread, which it currently does
unconditionally.  However, this results in needless wakeups in the case
where the advanced grace period is being set up by the grace-period
kthread itself, which is a non-uncommon situation.  This commit therefore
checks to see if the running thread is the grace-period kthread, and
avoids doing the irq_work_queue()-mediated wakeup in that case.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/rcutree.c

@@ -1576,10 +1576,12 @@ rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
 
 	/*
 	 * We can't do wakeups while holding the rnp->lock, as that
-	 * could cause possible deadlocks with the rq->lock. Deter
-	 * the wakeup to interrupt context.
+	 * could cause possible deadlocks with the rq->lock. Defer
+	 * the wakeup to interrupt context.  And don't bother waking
+	 * up the running kthread.
 	 */
-	irq_work_queue(&rsp->wakeup_work);
+	if (current != rsp->gp_kthread)
+		irq_work_queue(&rsp->wakeup_work);
 }
 
 /*