# SPDX-License-Identifier: GPL-2.0-only## RCU-related configuration options# menu "RCU Subsystem" config TREE_RCU	bool	default y if 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	select TREE_RCU	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_GENERIC	def_bool TASKS_RCU || TASKS_RUDE_RCU || TASKS_TRACE_RCU	select SRCU	help	  This option enables generic infrastructure code supporting	  task-based RCU implementations.  Not for manual selection. config TASKS_RCU	def_bool PREEMPTION	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.  Not for manual selection. config TASKS_RUDE_RCU	def_bool 0	help	  This option enables a task-based RCU implementation that uses	  only context switch (including preemption) and user-mode	  execution as quiescent states.  It forces IPIs and context	  switches on all online CPUs, including idle ones, so use	  with caution. config TASKS_TRACE_RCU	def_bool 0	help	  This option enables a task-based RCU implementation that uses	  explicit rcu_read_lock_trace() read-side markers, and allows	  these readers to appear in the idle loop as well as on the CPU	  hotplug code paths.  It can force IPIs on online CPUs, including	  idle ones, so use with caution. config RCU_STALL_COMMON	def_bool TREE_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 || 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 && 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 && 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_FAST_NO_HZ	bool "Accelerate last non-dyntick-idle CPU's grace periods"	depends on NO_HZ_COMMON && SMP && RCU_EXPERT	default n	help	  This option permits CPUs to enter dynticks-idle state even if	  they have RCU callbacks queued, and prevents RCU from waking	  these CPUs up more than roughly once every four jiffies (by	  default, you can adjust this using the rcutree.rcu_idle_gp_delay	  parameter), thus improving energy efficiency.  On the other	  hand, this option increases the duration of RCU grace periods,	  for example, slowing down synchronize_rcu(). 	  Say Y if energy efficiency is critically important, and you	  	don't care about increased grace-period durations. 	  Say N if you are 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. config RCU_NOCB_CPU	bool "Offload RCU callback processing from boot-selected CPUs"	depends on TREE_RCU	depends on RCU_EXPERT || NO_HZ_FULL	default n	help	  Use this option to reduce OS jitter for aggressive HPC or	  real-time workloads.	It can also be used to offload RCU	  callback invocation to energy-efficient CPUs in battery-powered	  asymmetric multiprocessors. 	  This option offloads callback invocation from the set of CPUs	  specified at boot time by the rcu_nocbs parameter.  For each	  such CPU, a kthread ("rcuox/N") will be created to invoke	  callbacks, where the "N" is the CPU being offloaded, and where	  the "p" for RCU-preempt (PREEMPTION kernels) and "s" for RCU-sched	  (!PREEMPTION kernels).  Nothing prevents this kthread from running	  on the specified CPUs, but (1) the kthreads may be preempted	  between each callback, and (2) affinity or cgroups can be used	  to force the kthreads to run on whatever set of CPUs is desired. 	  Say Y here if you want to help to debug reduced OS jitter.	  Say N here if you are unsure. config TASKS_TRACE_RCU_READ_MB	bool "Tasks Trace RCU readers use memory barriers in user and idle"	depends on RCU_EXPERT	default PREEMPT_RT || NR_CPUS < 8	help	  Use this option to further reduce the number of IPIs sent	  to CPUs executing in userspace or idle during tasks trace	  RCU grace periods.  Given that a reasonable setting of	  the rcupdate.rcu_task_ipi_delay kernel boot parameter	  eliminates such IPIs for many workloads, proper setting	  of this Kconfig option is important mostly for aggressive	  real-time installations and for battery-powered devices,	  hence the default chosen above. 	  Say Y here if you hate IPIs.	  Say N here if you hate read-side memory barriers.	  Take the default if you are unsure. endmenu # "RCU Subsystem"