rcu: Advise most users not to enable RCU user mode

Discourage distros from enabling CONFIG_RCU_USER_QS
because it brings overhead for no benefits yet.

It's not a useful feature on its own until we can
fully run an adaptive tickless kernel.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

init/Kconfig

@@ -451,6 +451,12 @@ config RCU_USER_QS
 	  excluded from the global RCU state machine and thus doesn't
 	  to keep the timer tick on for RCU.
 
+	  Unless you want to hack and help the development of the full
+	  tickless feature, you shouldn't enable this option. It adds
+	  unnecessary overhead.
+
+	  If unsure say N
+
 config RCU_USER_QS_FORCE
 	bool "Force userspace extended QS by default"
 	depends on RCU_USER_QS
@@ -459,6 +465,12 @@ config RCU_USER_QS_FORCE
 	  test this feature that treats userspace as an extended quiescent
 	  state until we have a real user like a full adaptive nohz option.
 
+	  Unless you want to hack and help the development of the full
+	  tickless feature, you shouldn't enable this option. It adds
+	  unnecessary overhead.
+
+	  If unsure say N
+
 config RCU_FANOUT
 	int "Tree-based hierarchical RCU fanout value"
 	range 2 64 if 64BIT