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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
- 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"
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