rcu: Control grace-period delays directly from value

In a misguided attempt to avoid an #ifdef, the use of the
gp_init_delay module parameter was conditioned on the corresponding
RCU_TORTURE_TEST_SLOW_INIT Kconfig variable, using IS_ENABLED() at
the point of use in the code.  This meant that the compiler always saw
the delay, which meant that RCU_TORTURE_TEST_SLOW_INIT_DELAY had to be
unconditionally defined.  This in turn caused "make oldconfig" to ask
pointless questions about the value of RCU_TORTURE_TEST_SLOW_INIT_DELAY
in cases where it was not even used.

This commit avoids these pointless questions by defining gp_init_delay
under #ifdef.  In one branch, gp_init_delay is initialized to
RCU_TORTURE_TEST_SLOW_INIT_DELAY and is also a module parameter (thus
allowing boot-time modification), and in the other branch gp_init_delay
is a const variable initialized by default to zero.

This approach also simplifies the code at the delay point by eliminating
the IS_DEFINED().  Because gp_init_delay is constant zero in the no-delay
case intended for production use, the "gp_init_delay > 0" check causes
the delay to become dead code, as desired in this case.  In addition,
this commit replaces magic constant "10" with the preprocessor variable
PER_RCU_NODE_PERIOD, which controls the number of grace periods that
are allowed to elapse at full speed before a delay is inserted.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by:
Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -162,11 +162,14 @@ static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp);
 static int kthread_prio = CONFIG_RCU_KTHREAD_PRIO;
 module_param(kthread_prio, int, 0644);
 
-/* Delay in jiffies for grace-period initialization delays. */
-static int gp_init_delay = IS_ENABLED(CONFIG_RCU_TORTURE_TEST_SLOW_INIT)
-				? CONFIG_RCU_TORTURE_TEST_SLOW_INIT_DELAY
-				: 0;
+/* Delay in jiffies for grace-period initialization delays, debug only. */
+#ifdef CONFIG_RCU_TORTURE_TEST_SLOW_INIT
+static int gp_init_delay = CONFIG_RCU_TORTURE_TEST_SLOW_INIT_DELAY;
 module_param(gp_init_delay, int, 0644);
+#else /* #ifdef CONFIG_RCU_TORTURE_TEST_SLOW_INIT */
+static const int gp_init_delay;
+#endif /* #else #ifdef CONFIG_RCU_TORTURE_TEST_SLOW_INIT */
+#define PER_RCU_NODE_PERIOD 10	/* Number of grace periods between delays. */
 
 /*
  * Track the rcutorture test sequence number and the update version
@@ -1843,9 +1846,8 @@ static int rcu_gp_init(struct rcu_state *rsp)
 		raw_spin_unlock_irq(&rnp->lock);
 		cond_resched_rcu_qs();
 		ACCESS_ONCE(rsp->gp_activity) = jiffies;
-		if (IS_ENABLED(CONFIG_RCU_TORTURE_TEST_SLOW_INIT) &&
-		    gp_init_delay > 0 &&
-		    !(rsp->gpnum % (rcu_num_nodes * 10)))
+		if (gp_init_delay > 0 &&
+		    !(rsp->gpnum % (rcu_num_nodes * PER_RCU_NODE_PERIOD)))
 			schedule_timeout_uninterruptible(gp_init_delay);
 	}
 

lib/Kconfig.debug

@@ -1268,6 +1268,7 @@ config RCU_TORTURE_TEST_SLOW_INIT_DELAY
 	int "How much to slow down RCU grace-period initialization"
 	range 0 5
 	default 3
+	depends on RCU_TORTURE_TEST_SLOW_INIT
 	help
 	  This option specifies the number of jiffies to wait between
 	  each rcu_node structure initialization.