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.
80 config FORCE_TASKS_RCU
81 bool "Force selection of TASKS_RCU"
86 This option force-enables a task-based RCU implementation
87 that uses only voluntary context switch (not preemption!),
88 idle, and user-mode execution as quiescent states. Not for
89 manual selection in most cases.
96 config FORCE_TASKS_RUDE_RCU
97 bool "Force selection of Tasks Rude RCU"
102 This option force-enables a task-based RCU implementation
103 that uses only context switch (including preemption) and
104 user-mode execution as quiescent states. It forces IPIs and
105 context switches on all online CPUs, including idle ones,
106 so use with caution. Not for manual selection in most cases.
108 config TASKS_RUDE_RCU
113 config FORCE_TASKS_TRACE_RCU
114 bool "Force selection of Tasks Trace RCU"
115 depends on RCU_EXPERT
116 select TASKS_TRACE_RCU
119 This option enables a task-based RCU implementation that uses
120 explicit rcu_read_lock_trace() read-side markers, and allows
121 these readers to appear in the idle loop as well as on the
122 CPU hotplug code paths. It can force IPIs on online CPUs,
123 including idle ones, so use with caution. Not for manual
124 selection in most cases.
126 config TASKS_TRACE_RCU
131 config RCU_STALL_COMMON
134 This option enables RCU CPU stall code that is common between
135 the TINY and TREE variants of RCU. The purpose is to allow
136 the tiny variants to disable RCU CPU stall warnings, while
137 making these warnings mandatory for the tree variants.
139 config RCU_NEED_SEGCBLIST
140 def_bool ( TREE_RCU || TREE_SRCU || TASKS_RCU_GENERIC )
143 int "Tree-based hierarchical RCU fanout value"
146 depends on TREE_RCU && RCU_EXPERT
150 This option controls the fanout of hierarchical implementations
151 of RCU, allowing RCU to work efficiently on machines with
152 large numbers of CPUs. This value must be at least the fourth
153 root of NR_CPUS, which allows NR_CPUS to be insanely large.
154 The default value of RCU_FANOUT should be used for production
155 systems, but if you are stress-testing the RCU implementation
156 itself, small RCU_FANOUT values allow you to test large-system
157 code paths on small(er) systems.
159 Select a specific number if testing RCU itself.
160 Take the default if unsure.
162 config RCU_FANOUT_LEAF
163 int "Tree-based hierarchical RCU leaf-level fanout value"
164 range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD
165 range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD
166 range 2 3 if RCU_STRICT_GRACE_PERIOD
167 depends on TREE_RCU && RCU_EXPERT
168 default 16 if !RCU_STRICT_GRACE_PERIOD
169 default 2 if RCU_STRICT_GRACE_PERIOD
171 This option controls the leaf-level fanout of hierarchical
172 implementations of RCU, and allows trading off cache misses
173 against lock contention. Systems that synchronize their
174 scheduling-clock interrupts for energy-efficiency reasons will
175 want the default because the smaller leaf-level fanout keeps
176 lock contention levels acceptably low. Very large systems
177 (hundreds or thousands of CPUs) will instead want to set this
178 value to the maximum value possible in order to reduce the
179 number of cache misses incurred during RCU's grace-period
180 initialization. These systems tend to run CPU-bound, and thus
181 are not helped by synchronized interrupts, and thus tend to
182 skew them, which reduces lock contention enough that large
183 leaf-level fanouts work well. That said, setting leaf-level
184 fanout to a large number will likely cause problematic
185 lock contention on the leaf-level rcu_node structures unless
186 you boot with the skew_tick kernel parameter.
188 Select a specific number if testing RCU itself.
190 Select the maximum permissible value for large systems, but
191 please understand that you may also need to set the skew_tick
192 kernel boot parameter to avoid contention on the rcu_node
195 Take the default if unsure.
198 bool "Enable RCU priority boosting"
199 depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT
200 default y if PREEMPT_RT
202 This option boosts the priority of preempted RCU readers that
203 block the current preemptible RCU grace period for too long.
204 This option also prevents heavy loads from blocking RCU
207 Say Y here if you are working with real-time apps or heavy loads
208 Say N here if you are unsure.
210 config RCU_BOOST_DELAY
211 int "Milliseconds to delay boosting after RCU grace-period start"
216 This option specifies the time to wait after the beginning of
217 a given grace period before priority-boosting preempted RCU
218 readers blocking that grace period. Note that any RCU reader
219 blocking an expedited RCU grace period is boosted immediately.
221 Accept the default if unsure.
223 config RCU_EXP_KTHREAD
224 bool "Perform RCU expedited work in a real-time kthread"
225 depends on RCU_BOOST && RCU_EXPERT
226 default !PREEMPT_RT && NR_CPUS <= 32
228 Use this option to further reduce the latencies of expedited
229 grace periods at the expense of being more disruptive.
231 This option is disabled by default on PREEMPT_RT=y kernels which
232 disable expedited grace periods after boot by unconditionally
233 setting rcupdate.rcu_normal_after_boot=1.
235 Accept the default if unsure.
238 bool "Offload RCU callback processing from boot-selected CPUs"
240 depends on RCU_EXPERT || NO_HZ_FULL
243 Use this option to reduce OS jitter for aggressive HPC or
244 real-time workloads. It can also be used to offload RCU
245 callback invocation to energy-efficient CPUs in battery-powered
246 asymmetric multiprocessors. The price of this reduced jitter
247 is that the overhead of call_rcu() increases and that some
248 workloads will incur significant increases in context-switch
251 This option offloads callback invocation from the set of CPUs
252 specified at boot time by the rcu_nocbs parameter. For each
253 such CPU, a kthread ("rcuox/N") will be created to invoke
254 callbacks, where the "N" is the CPU being offloaded, and where
255 the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
256 RCU-sched (!PREEMPTION kernels). Nothing prevents this kthread
257 from running on the specified CPUs, but (1) the kthreads may be
258 preempted between each callback, and (2) affinity or cgroups can
259 be used to force the kthreads to run on whatever set of CPUs is
262 Say Y here if you need reduced OS jitter, despite added overhead.
263 Say N here if you are unsure.
265 config TASKS_TRACE_RCU_READ_MB
266 bool "Tasks Trace RCU readers use memory barriers in user and idle"
267 depends on RCU_EXPERT && TASKS_TRACE_RCU
268 default PREEMPT_RT || NR_CPUS < 8
270 Use this option to further reduce the number of IPIs sent
271 to CPUs executing in userspace or idle during tasks trace
272 RCU grace periods. Given that a reasonable setting of
273 the rcupdate.rcu_task_ipi_delay kernel boot parameter
274 eliminates such IPIs for many workloads, proper setting
275 of this Kconfig option is important mostly for aggressive
276 real-time installations and for battery-powered devices,
277 hence the default chosen above.
279 Say Y here if you hate IPIs.
280 Say N here if you hate read-side memory barriers.
281 Take the default if you are unsure.
283 endmenu # "RCU Subsystem"