rcu: Provide a boot time parameter to control lazy RCU

To allow more flexible arrangements while still provide a single kernel
for distros, provide a boot time parameter to enable/disable lazy RCU.

Specify:

	rcutree.enable_rcu_lazy=[y|1|n|0]

Which also requires

	rcu_nocbs=all

at boot time to enable/disable lazy RCU.

To disable it by default at build time when CONFIG_RCU_LAZY=y, the new
CONFIG_RCU_LAZY_DEFAULT_OFF can be used.
Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
Tested-by: Andrea Righi <andrea.righi@canonical.com>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -5034,6 +5034,11 @@
 			this kernel boot parameter, forcibly setting it
 			to zero.
 
+	rcutree.enable_rcu_lazy= [KNL]
+			To save power, batch RCU callbacks and flush after
+			delay, memory pressure or callback list growing too
+			big.
+
 	rcuscale.gp_async= [KNL]
 			Measure performance of asynchronous
 			grace-period primitives such as call_rcu().

kernel/rcu/Kconfig

@@ -314,6 +314,19 @@ config RCU_LAZY
 	  To save power, batch RCU callbacks and flush after delay, memory
 	  pressure, or callback list growing too big.
 
+	  Requires rcu_nocbs=all to be set.
+
+	  Use rcutree.enable_rcu_lazy=0 to turn it off at boot time.
+
+config RCU_LAZY_DEFAULT_OFF
+	bool "Turn RCU lazy invocation off by default"
+	depends on RCU_LAZY
+	default n
+	help
+	  Allows building the kernel with CONFIG_RCU_LAZY=y yet keep it default
+	  off. Boot time param rcutree.enable_rcu_lazy=1 can be used to switch
+	  it back on.
+
 config RCU_DOUBLE_CHECK_CB_TIME
 	bool "RCU callback-batch backup time check"
 	depends on RCU_EXPERT

kernel/rcu/tree.c

@@ -2753,6 +2753,9 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in)
 }
 
 #ifdef CONFIG_RCU_LAZY
+static bool enable_rcu_lazy __read_mostly = !IS_ENABLED(CONFIG_RCU_LAZY_DEFAULT_OFF);
+module_param(enable_rcu_lazy, bool, 0444);
+
 /**
  * call_rcu_hurry() - Queue RCU callback for invocation after grace period, and
  * flush all lazy callbacks (including the new one) to the main ->cblist while
@@ -2778,6 +2781,8 @@ void call_rcu_hurry(struct rcu_head *head, rcu_callback_t func)
 	__call_rcu_common(head, func, false);
 }
 EXPORT_SYMBOL_GPL(call_rcu_hurry);
+#else
+#define enable_rcu_lazy		false
 #endif
 
 /**
@@ -2826,7 +2831,7 @@ EXPORT_SYMBOL_GPL(call_rcu_hurry);
  */
 void call_rcu(struct rcu_head *head, rcu_callback_t func)
 {
-	__call_rcu_common(head, func, IS_ENABLED(CONFIG_RCU_LAZY));
+	__call_rcu_common(head, func, enable_rcu_lazy);
 }
 EXPORT_SYMBOL_GPL(call_rcu);