Commit dc126918 authored Aug 05, 2020 by Paul E. McKenney

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>

parent 8cbd0e38

kernel/rcu/Kconfig

+5 −3

Original line number	Diff line number	Diff line
		@@ -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

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