From 024828800926dcb385b094e2438151647c3f8251 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Fri, 3 Feb 2017 09:27:00 -0800 Subject: 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 --- init/Kconfig | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) (limited to 'init') 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. -- cgit v1.2.3-70-g09d2