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.
 
          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
 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.
 
          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
 config RCU_FANOUT
        int "Tree-based hierarchical RCU fanout value"
        range 2 64 if 64BIT