From dc1269186bed3afc5a2018527516be84fe55d3e0 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Wed, 5 Aug 2020 16:52:17 -0700 Subject: [PATCH] rcu: Reduce leaf fanout for strict RCU grace periods Because strict RCU grace periods will complete more quickly, they will experience greater lock contention on each leaf rcu_node structure's ->lock. This commit therefore reduces the leaf fanout in order to reduce this lock contention. Note that this also has the effect of reducing the number of CPUs supported to 16 in the case of CONFIG_RCU_FANOUT_LEAF=2 or 81 in the case of CONFIG_RCU_FANOUT_LEAF=3. However, greater numbers of CPUs are probably a bad idea when using CONFIG_RCU_STRICT_GRACE_PERIOD=y. Those wishing to live dangerously are free to edit their kernel/rcu/Kconfig files accordingly. Reported-by Jann Horn Signed-off-by: Paul E. McKenney --- kernel/rcu/Kconfig | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig index 0ebe15a849856..b71e21f73c403 100644 --- a/kernel/rcu/Kconfig +++ b/kernel/rcu/Kconfig @@ -135,10 +135,12 @@ config RCU_FANOUT config RCU_FANOUT_LEAF int "Tree-based hierarchical RCU leaf-level fanout value" - range 2 64 if 64BIT - range 2 32 if !64BIT + range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD + range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD + range 2 3 if RCU_STRICT_GRACE_PERIOD depends on TREE_RCU && RCU_EXPERT - default 16 + default 16 if !RCU_STRICT_GRACE_PERIOD + default 2 if RCU_STRICT_GRACE_PERIOD help This option controls the leaf-level fanout of hierarchical implementations of RCU, and allows trading off cache misses -- 2.30.2