rcu/nocb: Add option to force all call_rcu() to lazy

This will be used in the rcu scale test, to ensure that fly-by call_rcu()s do
no cause call_rcu_lazy() CBs to be flushed to the rdp ->cblist.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

kernel/rcu/rcu.h

@@ -472,6 +472,7 @@ void do_trace_rcu_torture_read(const char *rcutorturename,
 			       unsigned long c_old,
 			       unsigned long c);
 void rcu_gp_set_torture_wait(int duration);
+void rcu_force_call_rcu_to_lazy(bool force);
 #else
 static inline void rcutorture_get_gp_data(enum rcutorture_type test_type,
 					  int *flags, unsigned long *gp_seq)
@@ -490,6 +491,7 @@ void do_trace_rcu_torture_read(const char *rcutorturename,
 	do { } while (0)
 #endif
 static inline void rcu_gp_set_torture_wait(int duration) { }
+static inline void rcu_force_call_rcu_to_lazy(bool force) { }
 #endif
 
 #if IS_ENABLED(CONFIG_RCU_TORTURE_TEST) || IS_MODULE(CONFIG_RCU_TORTURE_TEST)

kernel/rcu/tree.c

@@ -3171,9 +3171,18 @@ void call_rcu_lazy(struct rcu_head *head, rcu_callback_t func)
 EXPORT_SYMBOL_GPL(call_rcu_lazy);
 #endif
 
+static bool force_call_rcu_to_lazy;
+
+void rcu_force_call_rcu_to_lazy(bool force)
+{
+	if (IS_ENABLED(CONFIG_RCU_SCALE_TEST))
+		WRITE_ONCE(force_call_rcu_to_lazy, force);
+}
+EXPORT_SYMBOL_GPL(rcu_force_call_rcu_to_lazy);
+
 void call_rcu(struct rcu_head *head, rcu_callback_t func)
 {
-	return __call_rcu_common(head, func, false);
+	return __call_rcu_common(head, func, force_call_rcu_to_lazy);
 
 }
 EXPORT_SYMBOL_GPL(call_rcu);