# SPDX-License-Identifier: GPL-2.0-only
#
# RCU-related configuration options
#

menu "RCU Subsystem"

config TREE_RCU
	bool
	default y if !PREEMPTION && SMP
	help
	  This option selects the RCU implementation that is
	  designed for very large SMP system with hundreds or
	  thousands of CPUs.  It also scales down nicely to
	  smaller systems.

config PREEMPT_RCU
	bool
	default y if PREEMPTION
	help
	  This option selects the RCU implementation that is
	  designed for very large SMP systems with hundreds or
	  thousands of CPUs, but for which real-time response
	  is also required.  It also scales down nicely to
	  smaller systems.

	  Select this option if you are unsure.

config TINY_RCU
	bool
	default y if !PREEMPTION && !SMP
	help
	  This option selects the RCU implementation that is
	  designed for UP systems from which real-time response
	  is not required.  This option greatly reduces the
	  memory footprint of RCU.

config RCU_EXPERT
	bool "Make expert-level adjustments to RCU configuration"
	default n
	help
	  This option needs to be enabled if you wish to make
	  expert-level adjustments to RCU configuration.  By default,
	  no such adjustments can be made, which has the often-beneficial
	  side-effect of preventing "make oldconfig" from asking you all
	  sorts of detailed questions about how you would like numerous
	  obscure RCU options to be set up.

	  Say Y if you need to make expert-level adjustments to RCU.

	  Say N if you are unsure.

config SRCU
	bool
	help
	  This option selects the sleepable version of RCU. This version
	  permits arbitrary sleeping or blocking within RCU read-side critical
	  sections.

config TINY_SRCU
	bool
	default y if SRCU && TINY_RCU
	help
	  This option selects the single-CPU non-preemptible version of SRCU.

config TREE_SRCU
	bool
	default y if SRCU && !TINY_RCU
	help
	  This option selects the full-fledged version of SRCU.

config TASKS_RCU
	def_bool PREEMPTION
	select SRCU
	help
	  This option enables a task-based RCU implementation that uses
	  only voluntary context switch (not preemption!), idle, and
	  user-mode execution as quiescent states.

config RCU_STALL_COMMON
	def_bool ( TREE_RCU || PREEMPT_RCU )
	help
	  This option enables RCU CPU stall code that is common between
	  the TINY and TREE variants of RCU.  The purpose is to allow
	  the tiny variants to disable RCU CPU stall warnings, while
	  making these warnings mandatory for the tree variants.

config RCU_NEED_SEGCBLIST
	def_bool ( TREE_RCU || PREEMPT_RCU || TREE_SRCU )

config RCU_FANOUT
	int "Tree-based hierarchical RCU fanout value"
	range 2 64 if 64BIT
	range 2 32 if !64BIT
	depends on (TREE_RCU || PREEMPT_RCU) && RCU_EXPERT
	default 64 if 64BIT
	default 32 if !64BIT
	help
	  This option controls the fanout of hierarchical implementations
	  of RCU, allowing RCU to work efficiently on machines with
	  large numbers of CPUs.  This value must be at least the fourth
	  root of NR_CPUS, which allows NR_CPUS to be insanely large.
	  The default value of RCU_FANOUT should be used for production
	  systems, but if you are stress-testing the RCU implementation
	  itself, small RCU_FANOUT values allow you to test large-system
	  code paths on small(er) systems.

	  Select a specific number if testing RCU itself.
	  Take the default if unsure.

config RCU_FANOUT_LEAF
	int "Tree-based hierarchical RCU leaf-level fanout value"
	range 2 64 if 64BIT
	range 2 32 if !64BIT
	depends on (TREE_RCU || PREEMPT_RCU) && RCU_EXPERT
	default 16
	help
	  This option controls the leaf-level fanout of hierarchical
	  implementations of RCU, and allows trading off cache misses
	  against lock contention.  Systems that synchronize their
	  scheduling-clock interrupts for energy-efficiency reasons will
	  want the default because the smaller leaf-level fanout keeps
	  lock contention levels acceptably low.  Very large systems
	  (hundreds or thousands of CPUs) will instead want to set this
	  value to the maximum value possible in order to reduce the
	  number of cache misses incurred during RCU's grace-period
	  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.  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, 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.


config RCU_BOOST
	bool "Enable RCU priority boosting"
	depends on RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT
	default n
	help
	  This option boosts the priority of preempted RCU readers that
	  block the current preemptible RCU grace period for too long.
	  This option also prevents heavy loads from blocking RCU
	  callback invocation.

	  Say Y here if you are working with real-time apps or heavy loads
	  Say N here if you are unsure.

config RCU_BOOST_DELAY
	int "Milliseconds to delay boosting after RCU grace-period start"
	range 0 3000
	depends on RCU_BOOST
	default 500
	help
	  This option specifies the time to wait after the beginning of
	  a given grace period before priority-boosting preempted RCU
	  readers blocking that grace period.  Note that any RCU reader
	  blocking an expedited RCU grace period is boosted immediately.

	  Accept the default if unsure.


endmenu # "RCU Subsystem"
