rcu: Force nohz_full tick on upon irq enter instead of exit

There is interrupt-exit code that forces on the tick for nohz_full CPUs
failing to respond to the current grace period in a timely fashion.
However, this code must compare ->dynticks_nmi_nesting to the value 2
in the interrupt-exit fastpath.  This commit therefore moves this code
to the interrupt-entry fastpath, where a lighter-weight comparison to
zero may be used.
Reported-by: Joel Fernandes <joel@joelfernandes.org>
[ paulmck: Apply Joel Fernandes TICK_DEP_MASK_RCU->TICK_DEP_BIT_RCU fix. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.c

@@ -651,12 +651,6 @@ static __always_inline void rcu_nmi_exit_common(bool irq)
 	 */
 	if (rdp->dynticks_nmi_nesting != 1) {
 		trace_rcu_dyntick(TPS("--="), rdp->dynticks_nmi_nesting, rdp->dynticks_nmi_nesting - 2, rdp->dynticks);
-		if (tick_nohz_full_cpu(rdp->cpu) &&
-		    rdp->dynticks_nmi_nesting == 2 &&
-		    rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
-			rdp->rcu_forced_tick = true;
-			tick_dep_set_cpu(rdp->cpu, TICK_DEP_MASK_RCU);
-		}
 		WRITE_ONCE(rdp->dynticks_nmi_nesting, /* No store tearing. */
 			   rdp->dynticks_nmi_nesting - 2);
 		return;
@@ -831,6 +825,11 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
 			rcu_cleanup_after_idle();
 
 		incby = 1;
+	} else if (tick_nohz_full_cpu(rdp->cpu) &&
+		   rdp->dynticks_nmi_nesting == DYNTICK_IRQ_NONIDLE &&
+		   rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
+		rdp->rcu_forced_tick = true;
+		tick_dep_set_cpu(rdp->cpu, TICK_DEP_BIT_RCU);
 	}
 	trace_rcu_dyntick(incby == 1 ? TPS("Endirq") : TPS("++="),
 			  rdp->dynticks_nmi_nesting,