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Subject[PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick
If you set RCU_FANOUT_LEAF too high, you can get lock contention
on the leaf rcu_node, and you should boot with the skew_tick kernel
parameter set in order to avoid this lock contention. This commit
therefore upgrades the RCU_FANOUT_LEAF help text to explicitly state

Signed-off-by: Paul E. McKenney <>
init/Kconfig | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index a92f27da4a27..946e561e67b7 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -612,11 +612,17 @@ config RCU_FANOUT_LEAF
initialization. These systems tend to run CPU-bound, and thus
are not helped by synchronized interrupts, and thus tend to
skew them, which reduces lock contention enough that large
- leaf-level fanouts work well.
+ leaf-level fanouts work well. That said, setting leaf-level
+ fanout to a large number will likely cause problematic
+ lock contention on the leaf-level rcu_node structures unless
+ you boot with the skew_tick kernel parameter.

Select a specific number if testing RCU itself.

- Select the maximum permissible value for large systems.
+ Select the maximum permissible value for large systems, but
+ please understand that you may also need to set the
+ skew_tick kernel boot parameter to avoid contention
+ on the rcu_node structure's locks.

Take the default if unsure.

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