rcu: Add long-term CPU kicking

This commit prepares for the removal of short-term CPU kicking (in a
subsequent commit).  It does so by starting to invoke resched_cpu()
for each holdout at each force-quiescent-state interval that is more
than halfway through the stall-warning interval.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/rcu/tree.c

@@ -1225,6 +1225,12 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
 			 rdp->rsp->gp_start + 2 * jiffies_till_sched_qs) ||
 	    ULONG_CMP_GE(jiffies, rdp->rsp->gp_start + jiffies_till_sched_qs))
 		resched_cpu(rdp->cpu);  /* Force CPU into scheduler. */
+	/*
+	 * If more than halfway to RCU CPU stall-warning time, do
+	 * a resched_cpu() to try to loosen things up a bit.
+	 */
+	if (jiffies - rdp->rsp->gp_start > rcu_jiffies_till_stall_check() / 2)
+		resched_cpu(rdp->cpu);
 
 	return 0;
 }