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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
select IRQ_WORK
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 || 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_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 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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