rcu: Accelerate RCU callbacks at grace-period end

Now that callback acceleration is idempotent, it is safe to accelerate
callbacks during grace-period cleanup on any CPUs that the kthread happens
to be running on.  This commit therefore propagates the completion
of the grace period to the per-CPU data structures, and also adds an
rcu_advance_cbs() just before the cpu_needs_another_gp() check in order
to reduce false-positive grace periods.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcutree.c

@@ -1396,6 +1396,9 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	rcu_for_each_node_breadth_first(rsp, rnp) {
 		raw_spin_lock_irq(&rnp->lock);
 		rnp->completed = rsp->gpnum;
+		rdp = this_cpu_ptr(rsp->rda);
+		if (rnp == rdp->mynode)
+			__rcu_process_gp_end(rsp, rnp, rdp);
 		nocb += rcu_nocb_gp_cleanup(rsp, rnp);
 		raw_spin_unlock_irq(&rnp->lock);
 		cond_resched();
@@ -1408,6 +1411,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	trace_rcu_grace_period(rsp->name, rsp->completed, "end");
 	rsp->fqs_state = RCU_GP_IDLE;
 	rdp = this_cpu_ptr(rsp->rda);
+	rcu_advance_cbs(rsp, rnp, rdp);  /* Reduce false positives below. */
 	if (cpu_needs_another_gp(rsp, rdp))
 		rsp->gp_flags = 1;
 	raw_spin_unlock_irq(&rnp->lock);
@@ -1497,6 +1501,15 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
 	struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
 	struct rcu_node *rnp = rcu_get_root(rsp);
 
+	/*
+	 * If there is no grace period in progress right now, any
+	 * callbacks we have up to this point will be satisfied by the
+	 * next grace period.  Also, advancing the callbacks reduces the
+	 * probability of false positives from cpu_needs_another_gp()
+	 * resulting in pointless grace periods.  So, advance callbacks!
+	 */
+	rcu_advance_cbs(rsp, rnp, rdp);
+
 	if (!rsp->gp_kthread ||
 	    !cpu_needs_another_gp(rsp, rdp)) {
 		/*
@@ -1509,14 +1522,6 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
 		return;
 	}
 
-	/*
-	 * Because there is no grace period in progress right now,
-	 * any callbacks we have up to this point will be satisfied
-	 * by the next grace period.  So this is a good place to
-	 * assign a grace period number to recently posted callbacks.
-	 */
-	rcu_accelerate_cbs(rsp, rnp, rdp);
-
 	rsp->gp_flags = RCU_GP_FLAG_INIT;
 	raw_spin_unlock(&rnp->lock); /* Interrupts remain disabled. */