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>

diff --git a/init/Kconfig b/init/Kconfig
index c26b8a1d2b576f8b0286168bbed0647a7ded67a1..89093e178c27abb6538b82b99dbae6054d212d80 100644 (file)
--- a/init/Kconfig
+++ b/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