1 # SPDX-License-Identifier: GPL-2.0-only
3 # RCU-related configuration options
12 This option selects the RCU implementation that is
13 designed for very large SMP system with hundreds or
14 thousands of CPUs. It also scales down nicely to
19 default y if PREEMPTION
22 This option selects the RCU implementation that is
23 designed for very large SMP systems with hundreds or
24 thousands of CPUs, but for which real-time response
25 is also required. It also scales down nicely to
28 Select this option if you are unsure.
32 default y if !PREEMPTION && !SMP
34 This option selects the RCU implementation that is
35 designed for UP systems from which real-time response
36 is not required. This option greatly reduces the
37 memory footprint of RCU.
40 bool "Make expert-level adjustments to RCU configuration"
43 This option needs to be enabled if you wish to make
44 expert-level adjustments to RCU configuration. By default,
45 no such adjustments can be made, which has the often-beneficial
46 side-effect of preventing "make oldconfig" from asking you all
47 sorts of detailed questions about how you would like numerous
48 obscure RCU options to be set up.
50 Say Y if you need to make expert-level adjustments to RCU.
52 Say N if you are unsure.
57 This option selects the sleepable version of RCU. This version
58 permits arbitrary sleeping or blocking within RCU read-side critical
63 default y if SRCU && TINY_RCU
65 This option selects the single-CPU non-preemptible version of SRCU.
69 default y if SRCU && !TINY_RCU
71 This option selects the full-fledged version of SRCU.
73 config TASKS_RCU_GENERIC
74 def_bool TASKS_RCU || TASKS_RUDE_RCU || TASKS_TRACE_RCU
77 This option enables generic infrastructure code supporting
78 task-based RCU implementations. Not for manual selection.
83 This option enables a task-based RCU implementation that uses
84 only voluntary context switch (not preemption!), idle, and
85 user-mode execution as quiescent states. Not for manual selection.
90 This option enables a task-based RCU implementation that uses
91 only context switch (including preemption) and user-mode
92 execution as quiescent states. It forces IPIs and context
93 switches on all online CPUs, including idle ones, so use
96 config TASKS_TRACE_RCU
100 This option enables a task-based RCU implementation that uses
101 explicit rcu_read_lock_trace() read-side markers, and allows
102 these readers to appear in the idle loop as well as on the CPU
103 hotplug code paths. It can force IPIs on online CPUs, including
104 idle ones, so use with caution.
106 config RCU_STALL_COMMON
109 This option enables RCU CPU stall code that is common between
110 the TINY and TREE variants of RCU. The purpose is to allow
111 the tiny variants to disable RCU CPU stall warnings, while
112 making these warnings mandatory for the tree variants.
114 config RCU_NEED_SEGCBLIST
115 def_bool ( TREE_RCU || TREE_SRCU || TASKS_RCU_GENERIC )
118 int "Tree-based hierarchical RCU fanout value"
121 depends on TREE_RCU && RCU_EXPERT
125 This option controls the fanout of hierarchical implementations
126 of RCU, allowing RCU to work efficiently on machines with
127 large numbers of CPUs. This value must be at least the fourth
128 root of NR_CPUS, which allows NR_CPUS to be insanely large.
129 The default value of RCU_FANOUT should be used for production
130 systems, but if you are stress-testing the RCU implementation
131 itself, small RCU_FANOUT values allow you to test large-system
132 code paths on small(er) systems.
134 Select a specific number if testing RCU itself.
135 Take the default if unsure.
137 config RCU_FANOUT_LEAF
138 int "Tree-based hierarchical RCU leaf-level fanout value"
139 range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD
140 range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD
141 range 2 3 if RCU_STRICT_GRACE_PERIOD
142 depends on TREE_RCU && RCU_EXPERT
143 default 16 if !RCU_STRICT_GRACE_PERIOD
144 default 2 if RCU_STRICT_GRACE_PERIOD
146 This option controls the leaf-level fanout of hierarchical
147 implementations of RCU, and allows trading off cache misses
148 against lock contention. Systems that synchronize their
149 scheduling-clock interrupts for energy-efficiency reasons will
150 want the default because the smaller leaf-level fanout keeps
151 lock contention levels acceptably low. Very large systems
152 (hundreds or thousands of CPUs) will instead want to set this
153 value to the maximum value possible in order to reduce the
154 number of cache misses incurred during RCU's grace-period
155 initialization. These systems tend to run CPU-bound, and thus
156 are not helped by synchronized interrupts, and thus tend to
157 skew them, which reduces lock contention enough that large
158 leaf-level fanouts work well. That said, setting leaf-level
159 fanout to a large number will likely cause problematic
160 lock contention on the leaf-level rcu_node structures unless
161 you boot with the skew_tick kernel parameter.
163 Select a specific number if testing RCU itself.
165 Select the maximum permissible value for large systems, but
166 please understand that you may also need to set the skew_tick
167 kernel boot parameter to avoid contention on the rcu_node
170 Take the default if unsure.
173 bool "Enable RCU priority boosting"
174 depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT
175 default y if PREEMPT_RT
177 This option boosts the priority of preempted RCU readers that
178 block the current preemptible RCU grace period for too long.
179 This option also prevents heavy loads from blocking RCU
182 Say Y here if you are working with real-time apps or heavy loads
183 Say N here if you are unsure.
185 config RCU_BOOST_DELAY
186 int "Milliseconds to delay boosting after RCU grace-period start"
191 This option specifies the time to wait after the beginning of
192 a given grace period before priority-boosting preempted RCU
193 readers blocking that grace period. Note that any RCU reader
194 blocking an expedited RCU grace period is boosted immediately.
196 Accept the default if unsure.
199 bool "Offload RCU callback processing from boot-selected CPUs"
201 depends on RCU_EXPERT || NO_HZ_FULL
204 Use this option to reduce OS jitter for aggressive HPC or
205 real-time workloads. It can also be used to offload RCU
206 callback invocation to energy-efficient CPUs in battery-powered
207 asymmetric multiprocessors. The price of this reduced jitter
208 is that the overhead of call_rcu() increases and that some
209 workloads will incur significant increases in context-switch
212 This option offloads callback invocation from the set of CPUs
213 specified at boot time by the rcu_nocbs parameter. For each
214 such CPU, a kthread ("rcuox/N") will be created to invoke
215 callbacks, where the "N" is the CPU being offloaded, and where
216 the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
217 RCU-sched (!PREEMPTION kernels). Nothing prevents this kthread
218 from running on the specified CPUs, but (1) the kthreads may be
219 preempted between each callback, and (2) affinity or cgroups can
220 be used to force the kthreads to run on whatever set of CPUs is
223 Say Y here if you need reduced OS jitter, despite added overhead.
224 Say N here if you are unsure.
226 config TASKS_TRACE_RCU_READ_MB
227 bool "Tasks Trace RCU readers use memory barriers in user and idle"
228 depends on RCU_EXPERT
229 default PREEMPT_RT || NR_CPUS < 8
231 Use this option to further reduce the number of IPIs sent
232 to CPUs executing in userspace or idle during tasks trace
233 RCU grace periods. Given that a reasonable setting of
234 the rcupdate.rcu_task_ipi_delay kernel boot parameter
235 eliminates such IPIs for many workloads, proper setting
236 of this Kconfig option is important mostly for aggressive
237 real-time installations and for battery-powered devices,
238 hence the default chosen above.
240 Say Y here if you hate IPIs.
241 Say N here if you hate read-side memory barriers.
242 Take the default if you are unsure.
244 endmenu # "RCU Subsystem"