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1# SPDX-License-Identifier: GPL-2.0-only
2#
3# RCU-related configuration options
4#
5
6menu "RCU Subsystem"
7
8config TREE_RCU
9 bool
10 default y if SMP
11 # Dynticks-idle tracking
12 select CONTEXT_TRACKING_IDLE
13 help
14 This option selects the RCU implementation that is
15 designed for very large SMP system with hundreds or
16 thousands of CPUs. It also scales down nicely to
17 smaller systems.
18
19config PREEMPT_RCU
20 bool
21 default y if PREEMPTION
22 select TREE_RCU
23 help
24 This option selects the RCU implementation that is
25 designed for very large SMP systems with hundreds or
26 thousands of CPUs, but for which real-time response
27 is also required. It also scales down nicely to
28 smaller systems.
29
30 Select this option if you are unsure.
31
32config TINY_RCU
33 bool
34 default y if !PREEMPTION && !SMP
35 help
36 This option selects the RCU implementation that is
37 designed for UP systems from which real-time response
38 is not required. This option greatly reduces the
39 memory footprint of RCU.
40
41config RCU_EXPERT
42 bool "Make expert-level adjustments to RCU configuration"
43 default n
44 help
45 This option needs to be enabled if you wish to make
46 expert-level adjustments to RCU configuration. By default,
47 no such adjustments can be made, which has the often-beneficial
48 side-effect of preventing "make oldconfig" from asking you all
49 sorts of detailed questions about how you would like numerous
50 obscure RCU options to be set up.
51
52 Say Y if you need to make expert-level adjustments to RCU.
53
54 Say N if you are unsure.
55
56config SRCU
57 def_bool y
58
59config TINY_SRCU
60 bool
61 default y if TINY_RCU
62 help
63 This option selects the single-CPU non-preemptible version of SRCU.
64
65config TREE_SRCU
66 bool
67 default y if !TINY_RCU
68 help
69 This option selects the full-fledged version of SRCU.
70
71config NEED_SRCU_NMI_SAFE
72 def_bool HAVE_NMI && !ARCH_HAS_NMI_SAFE_THIS_CPU_OPS && !TINY_SRCU
73
74config TASKS_RCU_GENERIC
75 def_bool TASKS_RCU || TASKS_RUDE_RCU || TASKS_TRACE_RCU
76 help
77 This option enables generic infrastructure code supporting
78 task-based RCU implementations. Not for manual selection.
79
80config FORCE_TASKS_RCU
81 bool "Force selection of TASKS_RCU"
82 depends on RCU_EXPERT
83 select TASKS_RCU
84 default n
85 help
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.
90
91config TASKS_RCU
92 bool
93 default n
94 select IRQ_WORK
95
96config FORCE_TASKS_RUDE_RCU
97 bool "Force selection of Tasks Rude RCU"
98 depends on RCU_EXPERT
99 select TASKS_RUDE_RCU
100 default n
101 help
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.
107
108config TASKS_RUDE_RCU
109 bool
110 default n
111 select IRQ_WORK
112
113config FORCE_TASKS_TRACE_RCU
114 bool "Force selection of Tasks Trace RCU"
115 depends on RCU_EXPERT
116 select TASKS_TRACE_RCU
117 default n
118 help
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.
125
126config TASKS_TRACE_RCU
127 bool
128 default n
129 select IRQ_WORK
130
131config RCU_STALL_COMMON
132 def_bool TREE_RCU
133 help
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.
138
139config RCU_NEED_SEGCBLIST
140 def_bool ( TREE_RCU || TREE_SRCU || TASKS_RCU_GENERIC )
141
142config RCU_FANOUT
143 int "Tree-based hierarchical RCU fanout value"
144 range 2 64 if 64BIT
145 range 2 32 if !64BIT
146 depends on TREE_RCU && RCU_EXPERT
147 default 64 if 64BIT
148 default 32 if !64BIT
149 help
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.
158
159 Select a specific number if testing RCU itself.
160 Take the default if unsure.
161
162config 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
170 help
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.
187
188 Select a specific number if testing RCU itself.
189
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
193 structure's locks.
194
195 Take the default if unsure.
196
197config RCU_BOOST
198 bool "Enable RCU priority boosting"
199 depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT
200 default y if PREEMPT_RT
201 help
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
205 callback invocation.
206
207 Say Y here if you are working with real-time apps or heavy loads
208 Say N here if you are unsure.
209
210config RCU_BOOST_DELAY
211 int "Milliseconds to delay boosting after RCU grace-period start"
212 range 0 3000
213 depends on RCU_BOOST
214 default 500
215 help
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.
220
221 Accept the default if unsure.
222
223config 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
227 help
228 Use this option to further reduce the latencies of expedited
229 grace periods at the expense of being more disruptive.
230
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.
234
235 Accept the default if unsure.
236
237config RCU_NOCB_CPU
238 bool "Offload RCU callback processing from boot-selected CPUs"
239 depends on TREE_RCU
240 depends on RCU_EXPERT || NO_HZ_FULL
241 default n
242 help
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
249 rates.
250
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
260 desired.
261
262 Say Y here if you need reduced OS jitter, despite added overhead.
263 Say N here if you are unsure.
264
265config RCU_NOCB_CPU_DEFAULT_ALL
266 bool "Offload RCU callback processing from all CPUs by default"
267 depends on RCU_NOCB_CPU
268 default n
269 help
270 Use this option to offload callback processing from all CPUs
271 by default, in the absence of the rcu_nocbs or nohz_full boot
272 parameter. This also avoids the need to use any boot parameters
273 to achieve the effect of offloading all CPUs on boot.
274
275 Say Y here if you want offload all CPUs by default on boot.
276 Say N here if you are unsure.
277
278config RCU_NOCB_CPU_CB_BOOST
279 bool "Offload RCU callback from real-time kthread"
280 depends on RCU_NOCB_CPU && RCU_BOOST
281 default y if PREEMPT_RT
282 help
283 Use this option to invoke offloaded callbacks as SCHED_FIFO
284 to avoid starvation by heavy SCHED_OTHER background load.
285 Of course, running as SCHED_FIFO during callback floods will
286 cause the rcuo[ps] kthreads to monopolize the CPU for hundreds
287 of milliseconds or more. Therefore, when enabling this option,
288 it is your responsibility to ensure that latency-sensitive
289 tasks either run with higher priority or run on some other CPU.
290
291 Say Y here if you want to set RT priority for offloading kthreads.
292 Say N here if you are building a !PREEMPT_RT kernel and are unsure.
293
294config TASKS_TRACE_RCU_READ_MB
295 bool "Tasks Trace RCU readers use memory barriers in user and idle"
296 depends on RCU_EXPERT && TASKS_TRACE_RCU
297 default PREEMPT_RT || NR_CPUS < 8
298 help
299 Use this option to further reduce the number of IPIs sent
300 to CPUs executing in userspace or idle during tasks trace
301 RCU grace periods. Given that a reasonable setting of
302 the rcupdate.rcu_task_ipi_delay kernel boot parameter
303 eliminates such IPIs for many workloads, proper setting
304 of this Kconfig option is important mostly for aggressive
305 real-time installations and for battery-powered devices,
306 hence the default chosen above.
307
308 Say Y here if you hate IPIs.
309 Say N here if you hate read-side memory barriers.
310 Take the default if you are unsure.
311
312config RCU_LAZY
313 bool "RCU callback lazy invocation functionality"
314 depends on RCU_NOCB_CPU
315 default n
316 help
317 To save power, batch RCU callbacks and flush after delay, memory
318 pressure, or callback list growing too big.
319
320endmenu # "RCU Subsystem"
1# SPDX-License-Identifier: GPL-2.0-only
2#
3# RCU-related configuration options
4#
5
6menu "RCU Subsystem"
7
8config TREE_RCU
9 bool
10 default y if SMP
11 help
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
15 smaller systems.
16
17config PREEMPT_RCU
18 bool
19 default y if PREEMPTION
20 select TREE_RCU
21 help
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
26 smaller systems.
27
28 Select this option if you are unsure.
29
30config TINY_RCU
31 bool
32 default y if !PREEMPTION && !SMP
33 help
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.
38
39config RCU_EXPERT
40 bool "Make expert-level adjustments to RCU configuration"
41 default n
42 help
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.
49
50 Say Y if you need to make expert-level adjustments to RCU.
51
52 Say N if you are unsure.
53
54config SRCU
55 bool
56 help
57 This option selects the sleepable version of RCU. This version
58 permits arbitrary sleeping or blocking within RCU read-side critical
59 sections.
60
61config TINY_SRCU
62 bool
63 default y if SRCU && TINY_RCU
64 help
65 This option selects the single-CPU non-preemptible version of SRCU.
66
67config TREE_SRCU
68 bool
69 default y if SRCU && !TINY_RCU
70 help
71 This option selects the full-fledged version of SRCU.
72
73config TASKS_RCU_GENERIC
74 def_bool TASKS_RCU || TASKS_RUDE_RCU || TASKS_TRACE_RCU
75 select SRCU
76 help
77 This option enables generic infrastructure code supporting
78 task-based RCU implementations. Not for manual selection.
79
80config TASKS_RCU
81 def_bool PREEMPTION
82 help
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.
86
87config TASKS_RUDE_RCU
88 def_bool 0
89 help
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
94 with caution.
95
96config TASKS_TRACE_RCU
97 def_bool 0
98 help
99 This option enables a task-based RCU implementation that uses
100 explicit rcu_read_lock_trace() read-side markers, and allows
101 these readers to appear in the idle loop as well as on the CPU
102 hotplug code paths. It can force IPIs on online CPUs, including
103 idle ones, so use with caution.
104
105config RCU_STALL_COMMON
106 def_bool TREE_RCU
107 help
108 This option enables RCU CPU stall code that is common between
109 the TINY and TREE variants of RCU. The purpose is to allow
110 the tiny variants to disable RCU CPU stall warnings, while
111 making these warnings mandatory for the tree variants.
112
113config RCU_NEED_SEGCBLIST
114 def_bool ( TREE_RCU || TREE_SRCU )
115
116config RCU_FANOUT
117 int "Tree-based hierarchical RCU fanout value"
118 range 2 64 if 64BIT
119 range 2 32 if !64BIT
120 depends on TREE_RCU && RCU_EXPERT
121 default 64 if 64BIT
122 default 32 if !64BIT
123 help
124 This option controls the fanout of hierarchical implementations
125 of RCU, allowing RCU to work efficiently on machines with
126 large numbers of CPUs. This value must be at least the fourth
127 root of NR_CPUS, which allows NR_CPUS to be insanely large.
128 The default value of RCU_FANOUT should be used for production
129 systems, but if you are stress-testing the RCU implementation
130 itself, small RCU_FANOUT values allow you to test large-system
131 code paths on small(er) systems.
132
133 Select a specific number if testing RCU itself.
134 Take the default if unsure.
135
136config RCU_FANOUT_LEAF
137 int "Tree-based hierarchical RCU leaf-level fanout value"
138 range 2 64 if 64BIT
139 range 2 32 if !64BIT
140 depends on TREE_RCU && RCU_EXPERT
141 default 16
142 help
143 This option controls the leaf-level fanout of hierarchical
144 implementations of RCU, and allows trading off cache misses
145 against lock contention. Systems that synchronize their
146 scheduling-clock interrupts for energy-efficiency reasons will
147 want the default because the smaller leaf-level fanout keeps
148 lock contention levels acceptably low. Very large systems
149 (hundreds or thousands of CPUs) will instead want to set this
150 value to the maximum value possible in order to reduce the
151 number of cache misses incurred during RCU's grace-period
152 initialization. These systems tend to run CPU-bound, and thus
153 are not helped by synchronized interrupts, and thus tend to
154 skew them, which reduces lock contention enough that large
155 leaf-level fanouts work well. That said, setting leaf-level
156 fanout to a large number will likely cause problematic
157 lock contention on the leaf-level rcu_node structures unless
158 you boot with the skew_tick kernel parameter.
159
160 Select a specific number if testing RCU itself.
161
162 Select the maximum permissible value for large systems, but
163 please understand that you may also need to set the skew_tick
164 kernel boot parameter to avoid contention on the rcu_node
165 structure's locks.
166
167 Take the default if unsure.
168
169config RCU_FAST_NO_HZ
170 bool "Accelerate last non-dyntick-idle CPU's grace periods"
171 depends on NO_HZ_COMMON && SMP && RCU_EXPERT
172 default n
173 help
174 This option permits CPUs to enter dynticks-idle state even if
175 they have RCU callbacks queued, and prevents RCU from waking
176 these CPUs up more than roughly once every four jiffies (by
177 default, you can adjust this using the rcutree.rcu_idle_gp_delay
178 parameter), thus improving energy efficiency. On the other
179 hand, this option increases the duration of RCU grace periods,
180 for example, slowing down synchronize_rcu().
181
182 Say Y if energy efficiency is critically important, and you
183 don't care about increased grace-period durations.
184
185 Say N if you are unsure.
186
187config RCU_BOOST
188 bool "Enable RCU priority boosting"
189 depends on RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT
190 default n
191 help
192 This option boosts the priority of preempted RCU readers that
193 block the current preemptible RCU grace period for too long.
194 This option also prevents heavy loads from blocking RCU
195 callback invocation.
196
197 Say Y here if you are working with real-time apps or heavy loads
198 Say N here if you are unsure.
199
200config RCU_BOOST_DELAY
201 int "Milliseconds to delay boosting after RCU grace-period start"
202 range 0 3000
203 depends on RCU_BOOST
204 default 500
205 help
206 This option specifies the time to wait after the beginning of
207 a given grace period before priority-boosting preempted RCU
208 readers blocking that grace period. Note that any RCU reader
209 blocking an expedited RCU grace period is boosted immediately.
210
211 Accept the default if unsure.
212
213config RCU_NOCB_CPU
214 bool "Offload RCU callback processing from boot-selected CPUs"
215 depends on TREE_RCU
216 depends on RCU_EXPERT || NO_HZ_FULL
217 default n
218 help
219 Use this option to reduce OS jitter for aggressive HPC or
220 real-time workloads. It can also be used to offload RCU
221 callback invocation to energy-efficient CPUs in battery-powered
222 asymmetric multiprocessors.
223
224 This option offloads callback invocation from the set of CPUs
225 specified at boot time by the rcu_nocbs parameter. For each
226 such CPU, a kthread ("rcuox/N") will be created to invoke
227 callbacks, where the "N" is the CPU being offloaded, and where
228 the "p" for RCU-preempt (PREEMPTION kernels) and "s" for RCU-sched
229 (!PREEMPTION kernels). Nothing prevents this kthread from running
230 on the specified CPUs, but (1) the kthreads may be preempted
231 between each callback, and (2) affinity or cgroups can be used
232 to force the kthreads to run on whatever set of CPUs is desired.
233
234 Say Y here if you want to help to debug reduced OS jitter.
235 Say N here if you are unsure.
236
237config TASKS_TRACE_RCU_READ_MB
238 bool "Tasks Trace RCU readers use memory barriers in user and idle"
239 depends on RCU_EXPERT
240 default PREEMPT_RT || NR_CPUS < 8
241 help
242 Use this option to further reduce the number of IPIs sent
243 to CPUs executing in userspace or idle during tasks trace
244 RCU grace periods. Given that a reasonable setting of
245 the rcupdate.rcu_task_ipi_delay kernel boot parameter
246 eliminates such IPIs for many workloads, proper setting
247 of this Kconfig option is important mostly for aggressive
248 real-time installations and for battery-powered devices,
249 hence the default chosen above.
250
251 Say Y here if you hate IPIs.
252 Say N here if you hate read-side memory barriers.
253 Take the default if you are unsure.
254
255endmenu # "RCU Subsystem"