315 lines
11 KiB
Plaintext
315 lines
11 KiB
Plaintext
# SPDX-License-Identifier: GPL-2.0-only
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#
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# RCU-related configuration options
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#
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menu "RCU Subsystem"
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config TREE_RCU
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bool
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default y if SMP
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# Dynticks-idle tracking
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select CONTEXT_TRACKING_IDLE
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help
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This option selects the RCU implementation that is
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designed for very large SMP system with hundreds or
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thousands of CPUs. It also scales down nicely to
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smaller systems.
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config PREEMPT_RCU
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bool
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default y if PREEMPTION
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select TREE_RCU
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help
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This option selects the RCU implementation that is
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designed for very large SMP systems with hundreds or
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thousands of CPUs, but for which real-time response
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is also required. It also scales down nicely to
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smaller systems.
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Select this option if you are unsure.
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config TINY_RCU
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bool
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default y if !PREEMPTION && !SMP
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help
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This option selects the RCU implementation that is
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designed for UP systems from which real-time response
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is not required. This option greatly reduces the
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memory footprint of RCU.
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config RCU_EXPERT
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bool "Make expert-level adjustments to RCU configuration"
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default n
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help
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This option needs to be enabled if you wish to make
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expert-level adjustments to RCU configuration. By default,
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no such adjustments can be made, which has the often-beneficial
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side-effect of preventing "make oldconfig" from asking you all
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sorts of detailed questions about how you would like numerous
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obscure RCU options to be set up.
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Say Y if you need to make expert-level adjustments to RCU.
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Say N if you are unsure.
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config SRCU
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bool
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help
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This option selects the sleepable version of RCU. This version
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permits arbitrary sleeping or blocking within RCU read-side critical
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sections.
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config TINY_SRCU
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bool
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default y if SRCU && TINY_RCU
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help
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This option selects the single-CPU non-preemptible version of SRCU.
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config TREE_SRCU
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bool
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default y if SRCU && !TINY_RCU
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help
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This option selects the full-fledged version of SRCU.
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config TASKS_RCU_GENERIC
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def_bool TASKS_RCU || TASKS_RUDE_RCU || TASKS_TRACE_RCU
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select SRCU
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help
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This option enables generic infrastructure code supporting
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task-based RCU implementations. Not for manual selection.
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config FORCE_TASKS_RCU
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bool "Force selection of TASKS_RCU"
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depends on RCU_EXPERT
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select TASKS_RCU
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default n
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help
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This option force-enables a task-based RCU implementation
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that uses only voluntary context switch (not preemption!),
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idle, and user-mode execution as quiescent states. Not for
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manual selection in most cases.
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config TASKS_RCU
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bool
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default n
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select IRQ_WORK
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config FORCE_TASKS_RUDE_RCU
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bool "Force selection of Tasks Rude RCU"
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depends on RCU_EXPERT
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select TASKS_RUDE_RCU
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default n
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help
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This option force-enables a task-based RCU implementation
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that uses only context switch (including preemption) and
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user-mode execution as quiescent states. It forces IPIs and
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context switches on all online CPUs, including idle ones,
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so use with caution. Not for manual selection in most cases.
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config TASKS_RUDE_RCU
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bool
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default n
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select IRQ_WORK
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config FORCE_TASKS_TRACE_RCU
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bool "Force selection of Tasks Trace RCU"
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depends on RCU_EXPERT
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select TASKS_TRACE_RCU
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default n
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help
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This option enables a task-based RCU implementation that uses
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explicit rcu_read_lock_trace() read-side markers, and allows
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these readers to appear in the idle loop as well as on the
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CPU hotplug code paths. It can force IPIs on online CPUs,
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including idle ones, so use with caution. Not for manual
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selection in most cases.
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config TASKS_TRACE_RCU
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bool
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default n
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select IRQ_WORK
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config RCU_STALL_COMMON
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def_bool TREE_RCU
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help
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This option enables RCU CPU stall code that is common between
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the TINY and TREE variants of RCU. The purpose is to allow
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the tiny variants to disable RCU CPU stall warnings, while
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making these warnings mandatory for the tree variants.
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config RCU_NEED_SEGCBLIST
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def_bool ( TREE_RCU || TREE_SRCU || TASKS_RCU_GENERIC )
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config RCU_FANOUT
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int "Tree-based hierarchical RCU fanout value"
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range 2 64 if 64BIT
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range 2 32 if !64BIT
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depends on TREE_RCU && RCU_EXPERT
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default 64 if 64BIT
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default 32 if !64BIT
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help
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This option controls the fanout of hierarchical implementations
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of RCU, allowing RCU to work efficiently on machines with
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large numbers of CPUs. This value must be at least the fourth
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root of NR_CPUS, which allows NR_CPUS to be insanely large.
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The default value of RCU_FANOUT should be used for production
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systems, but if you are stress-testing the RCU implementation
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itself, small RCU_FANOUT values allow you to test large-system
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code paths on small(er) systems.
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Select a specific number if testing RCU itself.
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Take the default if unsure.
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config RCU_FANOUT_LEAF
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int "Tree-based hierarchical RCU leaf-level fanout value"
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range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD
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range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD
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range 2 3 if RCU_STRICT_GRACE_PERIOD
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depends on TREE_RCU && RCU_EXPERT
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default 16 if !RCU_STRICT_GRACE_PERIOD
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default 2 if RCU_STRICT_GRACE_PERIOD
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help
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This option controls the leaf-level fanout of hierarchical
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implementations of RCU, and allows trading off cache misses
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against lock contention. Systems that synchronize their
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scheduling-clock interrupts for energy-efficiency reasons will
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want the default because the smaller leaf-level fanout keeps
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lock contention levels acceptably low. Very large systems
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(hundreds or thousands of CPUs) will instead want to set this
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value to the maximum value possible in order to reduce the
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number of cache misses incurred during RCU's grace-period
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initialization. These systems tend to run CPU-bound, and thus
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are not helped by synchronized interrupts, and thus tend to
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skew them, which reduces lock contention enough that large
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leaf-level fanouts work well. That said, setting leaf-level
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fanout to a large number will likely cause problematic
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lock contention on the leaf-level rcu_node structures unless
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you boot with the skew_tick kernel parameter.
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Select a specific number if testing RCU itself.
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Select the maximum permissible value for large systems, but
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please understand that you may also need to set the skew_tick
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kernel boot parameter to avoid contention on the rcu_node
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structure's locks.
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Take the default if unsure.
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config RCU_BOOST
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bool "Enable RCU priority boosting"
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depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT
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default y if PREEMPT_RT
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help
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This option boosts the priority of preempted RCU readers that
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block the current preemptible RCU grace period for too long.
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This option also prevents heavy loads from blocking RCU
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callback invocation.
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Say Y here if you are working with real-time apps or heavy loads
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Say N here if you are unsure.
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config RCU_BOOST_DELAY
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int "Milliseconds to delay boosting after RCU grace-period start"
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range 0 3000
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depends on RCU_BOOST
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default 500
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help
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This option specifies the time to wait after the beginning of
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a given grace period before priority-boosting preempted RCU
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readers blocking that grace period. Note that any RCU reader
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blocking an expedited RCU grace period is boosted immediately.
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Accept the default if unsure.
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config RCU_EXP_KTHREAD
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bool "Perform RCU expedited work in a real-time kthread"
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depends on RCU_BOOST && RCU_EXPERT
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default !PREEMPT_RT && NR_CPUS <= 32
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help
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Use this option to further reduce the latencies of expedited
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grace periods at the expense of being more disruptive.
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This option is disabled by default on PREEMPT_RT=y kernels which
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disable expedited grace periods after boot by unconditionally
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setting rcupdate.rcu_normal_after_boot=1.
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Accept the default if unsure.
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config RCU_NOCB_CPU
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bool "Offload RCU callback processing from boot-selected CPUs"
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depends on TREE_RCU
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depends on RCU_EXPERT || NO_HZ_FULL
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default n
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help
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Use this option to reduce OS jitter for aggressive HPC or
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real-time workloads. It can also be used to offload RCU
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callback invocation to energy-efficient CPUs in battery-powered
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asymmetric multiprocessors. The price of this reduced jitter
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is that the overhead of call_rcu() increases and that some
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workloads will incur significant increases in context-switch
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rates.
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This option offloads callback invocation from the set of CPUs
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specified at boot time by the rcu_nocbs parameter. For each
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such CPU, a kthread ("rcuox/N") will be created to invoke
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callbacks, where the "N" is the CPU being offloaded, and where
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the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
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RCU-sched (!PREEMPTION kernels). Nothing prevents this kthread
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from running on the specified CPUs, but (1) the kthreads may be
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preempted between each callback, and (2) affinity or cgroups can
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be used to force the kthreads to run on whatever set of CPUs is
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desired.
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Say Y here if you need reduced OS jitter, despite added overhead.
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Say N here if you are unsure.
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config RCU_NOCB_CPU_DEFAULT_ALL
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bool "Offload RCU callback processing from all CPUs by default"
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depends on RCU_NOCB_CPU
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default n
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help
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Use this option to offload callback processing from all CPUs
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by default, in the absence of the rcu_nocbs or nohz_full boot
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parameter. This also avoids the need to use any boot parameters
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to achieve the effect of offloading all CPUs on boot.
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Say Y here if you want offload all CPUs by default on boot.
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Say N here if you are unsure.
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config RCU_NOCB_CPU_CB_BOOST
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bool "Offload RCU callback from real-time kthread"
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depends on RCU_NOCB_CPU && RCU_BOOST
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default y if PREEMPT_RT
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help
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Use this option to invoke offloaded callbacks as SCHED_FIFO
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to avoid starvation by heavy SCHED_OTHER background load.
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Of course, running as SCHED_FIFO during callback floods will
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cause the rcuo[ps] kthreads to monopolize the CPU for hundreds
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of milliseconds or more. Therefore, when enabling this option,
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it is your responsibility to ensure that latency-sensitive
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tasks either run with higher priority or run on some other CPU.
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Say Y here if you want to set RT priority for offloading kthreads.
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Say N here if you are building a !PREEMPT_RT kernel and are unsure.
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config TASKS_TRACE_RCU_READ_MB
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bool "Tasks Trace RCU readers use memory barriers in user and idle"
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depends on RCU_EXPERT && TASKS_TRACE_RCU
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default PREEMPT_RT || NR_CPUS < 8
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help
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Use this option to further reduce the number of IPIs sent
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to CPUs executing in userspace or idle during tasks trace
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RCU grace periods. Given that a reasonable setting of
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the rcupdate.rcu_task_ipi_delay kernel boot parameter
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eliminates such IPIs for many workloads, proper setting
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of this Kconfig option is important mostly for aggressive
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real-time installations and for battery-powered devices,
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hence the default chosen above.
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Say Y here if you hate IPIs.
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Say N here if you hate read-side memory barriers.
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Take the default if you are unsure.
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endmenu # "RCU Subsystem"
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