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authorPaul E. McKenney <paulmck@kernel.org>2020-08-05 16:52:17 -0700
committerPaul E. McKenney <paulmck@kernel.org>2020-08-24 18:40:23 -0700
commitdc1269186bed3afc5a2018527516be84fe55d3e0 (patch)
tree60ef8b9f467c3cbdda1655f840c96346527080ab /kernel/rcu/Kconfig
parent8cbd0e38a9f2de38e8991c5c1c6f9024b2731d17 (diff)
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 <jannh@google.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Diffstat (limited to 'kernel/rcu/Kconfig')
-rw-r--r--kernel/rcu/Kconfig8
1 files changed, 5 insertions, 3 deletions
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 0ebe15a84985..b71e21f73c40 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