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>

1 files changed, 12 insertions, 0 deletions

diff --git a/init/Kconfig b/init/Kconfig
index c26b8a1d2b57..89093e178c27 100644
--- 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