rcu: further lower priority in rcu_yield()

Although rcu_yield() dropped from real-time to normal priority, there
is always the possibility that the competing tasks have been niced.
So nice to 19 in rcu_yield() to help ensure that other tasks have a
better chance of running.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/rcutree.c

@@ -1561,6 +1561,7 @@ static void rcu_yield(void (*f)(unsigned long), unsigned long arg)
 	mod_timer(&yield_timer, jiffies + 2);
 	sp.sched_priority = 0;
 	sched_setscheduler_nocheck(current, SCHED_NORMAL, &sp);
+	set_user_nice(current, 19);
 	schedule();
 	sp.sched_priority = RCU_KTHREAD_PRIO;
 	sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);