rcu: Make priority of grace-period thread consistent

The priority of RCU grace period threads is set to kthread_prio when
they are launched from rcu_spawn_gp_kthread().  The same is not true
of rcu_spawn_one_nocb_kthread().  Accordingly, add priority elevation
to rcu_spawn_one_nocb_kthread().

Signed-off-by: Alison Chaiken <achaiken@aurora.tech>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Author: chaiken

kernel/rcu/tree_nocb.h

@@ -1239,6 +1239,7 @@ static void rcu_spawn_cpu_nocb_kthread(int cpu)
 	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 	struct rcu_data *rdp_gp;
 	struct task_struct *t;
+	struct sched_param sp;
 
 	if (!rcu_scheduler_fully_active || !rcu_nocb_is_setup)
 		return;
@@ -1248,6 +1249,7 @@ static void rcu_spawn_cpu_nocb_kthread(int cpu)
 		return;
 
 	/* If we didn't spawn the GP kthread first, reorganize! */
+	sp.sched_priority = kthread_prio;
 	rdp_gp = rdp->nocb_gp_rdp;
 	mutex_lock(&rdp_gp->nocb_gp_kthread_mutex);
 	if (!rdp_gp->nocb_gp_kthread) {
@@ -1258,6 +1260,8 @@ static void rcu_spawn_cpu_nocb_kthread(int cpu)
 			return;
 		}
 		WRITE_ONCE(rdp_gp->nocb_gp_kthread, t);
+		if (kthread_prio)
+			sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
 	}
 	mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);