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+# 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
+ select IRQ_WORK
+ 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 && !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 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
+ 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"