From: Paul E. McKenney Date: Fri, 3 Feb 2017 17:27:00 +0000 (-0800) Subject: rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick X-Git-Tag: v4.12-rc1~38^2~5^2^2~7 X-Git-Url: https://asedeno.scripts.mit.edu/gitweb/?a=commitdiff_plain;h=024828800926dcb385b094e2438151647c3f8251;p=linux.git 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 this. Signed-off-by: Paul E. McKenney --- diff --git a/init/Kconfig b/init/Kconfig index a92f27da4a27..c549618c72f0 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.