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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
+ # Dynticks-idle tracking
+ select CONTEXT_TRACKING_IDLE
+ 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 TINY_SRCU
+ bool
+ default y if TINY_RCU
+ help
+ This option selects the single-CPU non-preemptible version of SRCU.
+
+config TREE_SRCU
+ bool
+ default y if !TINY_RCU
+ help
+ This option selects the full-fledged version of SRCU.
+
+config NEED_SRCU_NMI_SAFE
+ def_bool HAVE_NMI && !ARCH_HAS_NMI_SAFE_THIS_CPU_OPS && !TINY_SRCU
+
+config TASKS_RCU_GENERIC
+ def_bool TASKS_RCU || TASKS_RUDE_RCU || TASKS_TRACE_RCU
+ help
+ This option enables generic infrastructure code supporting
+ task-based RCU implementations. Not for manual selection.
+
+config FORCE_TASKS_RCU
+ bool "Force selection of TASKS_RCU"
+ depends on RCU_EXPERT
+ select TASKS_RCU
+ default n
+ help
+ This option force-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 in most cases.
+
+config TASKS_RCU
+ bool
+ default n
+ select IRQ_WORK
+
+config FORCE_TASKS_RUDE_RCU
+ bool "Force selection of Tasks Rude RCU"
+ depends on RCU_EXPERT
+ select TASKS_RUDE_RCU
+ default n
+ help
+ This option force-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. Not for manual selection in most cases.
+
+config TASKS_RUDE_RCU
+ bool
+ default n
+ select IRQ_WORK
+
+config FORCE_TASKS_TRACE_RCU
+ bool "Force selection of Tasks Trace RCU"
+ depends on RCU_EXPERT
+ select TASKS_TRACE_RCU
+ default n
+ 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. Not for manual
+ selection in most cases.
+
+config TASKS_TRACE_RCU
+ bool
+ default n
+ select IRQ_WORK
+
+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 || TASKS_RCU_GENERIC )
+
+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_BOOST
+ bool "Enable RCU priority boosting"
+ depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT
+ default y if PREEMPT_RT
+ 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_EXP_KTHREAD
+ bool "Perform RCU expedited work in a real-time kthread"
+ depends on RCU_BOOST && RCU_EXPERT
+ default !PREEMPT_RT && NR_CPUS <= 32
+ help
+ Use this option to further reduce the latencies of expedited
+ grace periods at the expense of being more disruptive.
+
+ This option is disabled by default on PREEMPT_RT=y kernels which
+ disable expedited grace periods after boot by unconditionally
+ setting rcupdate.rcu_normal_after_boot=1.
+
+ 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. The price of this reduced jitter
+ is that the overhead of call_rcu() increases and that some
+ workloads will incur significant increases in context-switch
+ rates.
+
+ 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 "x" is "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 need reduced OS jitter, despite added overhead.
+ Say N here if you are unsure.
+
+config RCU_NOCB_CPU_DEFAULT_ALL
+ bool "Offload RCU callback processing from all CPUs by default"
+ depends on RCU_NOCB_CPU
+ default n
+ help
+ Use this option to offload callback processing from all CPUs
+ by default, in the absence of the rcu_nocbs or nohz_full boot
+ parameter. This also avoids the need to use any boot parameters
+ to achieve the effect of offloading all CPUs on boot.
+
+ Say Y here if you want offload all CPUs by default on boot.
+ Say N here if you are unsure.
+
+config RCU_NOCB_CPU_CB_BOOST
+ bool "Offload RCU callback from real-time kthread"
+ depends on RCU_NOCB_CPU && RCU_BOOST
+ default y if PREEMPT_RT
+ help
+ Use this option to invoke offloaded callbacks as SCHED_FIFO
+ to avoid starvation by heavy SCHED_OTHER background load.
+ Of course, running as SCHED_FIFO during callback floods will
+ cause the rcuo[ps] kthreads to monopolize the CPU for hundreds
+ of milliseconds or more. Therefore, when enabling this option,
+ it is your responsibility to ensure that latency-sensitive
+ tasks either run with higher priority or run on some other CPU.
+
+ Say Y here if you want to set RT priority for offloading kthreads.
+ Say N here if you are building a !PREEMPT_RT kernel and are unsure.
+
+config TASKS_TRACE_RCU_READ_MB
+ bool "Tasks Trace RCU readers use memory barriers in user and idle"
+ depends on RCU_EXPERT && TASKS_TRACE_RCU
+ 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.
+
+config RCU_LAZY
+ bool "RCU callback lazy invocation functionality"
+ depends on RCU_NOCB_CPU
+ default n
+ help
+ To save power, batch RCU callbacks and flush after delay, memory
+ pressure, or callback list growing too big.
+
+config RCU_DOUBLE_CHECK_CB_TIME
+ bool "RCU callback-batch backup time check"
+ depends on RCU_EXPERT
+ default n
+ help
+ Use this option to provide more precise enforcement of the
+ rcutree.rcu_resched_ns module parameter in situations where
+ a single RCU callback might run for hundreds of microseconds,
+ thus defeating the 32-callback batching used to amortize the
+ cost of the fine-grained but expensive local_clock() function.
+
+ This option rounds rcutree.rcu_resched_ns up to the next
+ jiffy, and overrides the 32-callback batching if this limit
+ is exceeded.
+
+ Say Y here if you need tighter callback-limit enforcement.
+ Say N here if you are unsure.
+
+endmenu # "RCU Subsystem"