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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 TINY_SRCU
57 bool
58 default y if TINY_RCU
59 help
60 This option selects the single-CPU non-preemptible version of SRCU.
61
62config TREE_SRCU
63 bool
64 default y if !TINY_RCU
65 help
66 This option selects the full-fledged version of SRCU.
67
68config NEED_SRCU_NMI_SAFE
69 def_bool HAVE_NMI && !ARCH_HAS_NMI_SAFE_THIS_CPU_OPS && !TINY_SRCU
70
71config TASKS_RCU_GENERIC
72 def_bool TASKS_RCU || TASKS_RUDE_RCU || TASKS_TRACE_RCU
73 help
74 This option enables generic infrastructure code supporting
75 task-based RCU implementations. Not for manual selection.
76
77config FORCE_TASKS_RCU
78 bool "Force selection of TASKS_RCU"
79 depends on RCU_EXPERT
80 select TASKS_RCU
81 default n
82 help
83 This option force-enables a task-based RCU implementation
84 that uses only voluntary context switch (not preemption!),
85 idle, and user-mode execution as quiescent states. Not for
86 manual selection in most cases.
87
88config TASKS_RCU
89 bool
90 default n
91 select IRQ_WORK
92
93config FORCE_TASKS_RUDE_RCU
94 bool "Force selection of Tasks Rude RCU"
95 depends on RCU_EXPERT
96 select TASKS_RUDE_RCU
97 default n
98 help
99 This option force-enables a task-based RCU implementation
100 that uses only context switch (including preemption) and
101 user-mode execution as quiescent states. It forces IPIs and
102 context switches on all online CPUs, including idle ones,
103 so use with caution. Not for manual selection in most cases.
104
105config TASKS_RUDE_RCU
106 bool
107 default n
108 select IRQ_WORK
109
110config FORCE_TASKS_TRACE_RCU
111 bool "Force selection of Tasks Trace RCU"
112 depends on RCU_EXPERT
113 select TASKS_TRACE_RCU
114 default n
115 help
116 This option enables a task-based RCU implementation that uses
117 explicit rcu_read_lock_trace() read-side markers, and allows
118 these readers to appear in the idle loop as well as on the
119 CPU hotplug code paths. It can force IPIs on online CPUs,
120 including idle ones, so use with caution. Not for manual
121 selection in most cases.
122
123config TASKS_TRACE_RCU
124 bool
125 default n
126 select IRQ_WORK
127
128config RCU_STALL_COMMON
129 def_bool TREE_RCU
130 help
131 This option enables RCU CPU stall code that is common between
132 the TINY and TREE variants of RCU. The purpose is to allow
133 the tiny variants to disable RCU CPU stall warnings, while
134 making these warnings mandatory for the tree variants.
135
136config RCU_NEED_SEGCBLIST
137 def_bool ( TREE_RCU || TREE_SRCU || TASKS_RCU_GENERIC )
138
139config RCU_FANOUT
140 int "Tree-based hierarchical RCU fanout value"
141 range 2 64 if 64BIT
142 range 2 32 if !64BIT
143 depends on TREE_RCU && RCU_EXPERT
144 default 64 if 64BIT
145 default 32 if !64BIT
146 help
147 This option controls the fanout of hierarchical implementations
148 of RCU, allowing RCU to work efficiently on machines with
149 large numbers of CPUs. This value must be at least the fourth
150 root of NR_CPUS, which allows NR_CPUS to be insanely large.
151 The default value of RCU_FANOUT should be used for production
152 systems, but if you are stress-testing the RCU implementation
153 itself, small RCU_FANOUT values allow you to test large-system
154 code paths on small(er) systems.
155
156 Select a specific number if testing RCU itself.
157 Take the default if unsure.
158
159config RCU_FANOUT_LEAF
160 int "Tree-based hierarchical RCU leaf-level fanout value"
161 range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD
162 range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD
163 range 2 3 if RCU_STRICT_GRACE_PERIOD
164 depends on TREE_RCU && RCU_EXPERT
165 default 16 if !RCU_STRICT_GRACE_PERIOD
166 default 2 if RCU_STRICT_GRACE_PERIOD
167 help
168 This option controls the leaf-level fanout of hierarchical
169 implementations of RCU, and allows trading off cache misses
170 against lock contention. Systems that synchronize their
171 scheduling-clock interrupts for energy-efficiency reasons will
172 want the default because the smaller leaf-level fanout keeps
173 lock contention levels acceptably low. Very large systems
174 (hundreds or thousands of CPUs) will instead want to set this
175 value to the maximum value possible in order to reduce the
176 number of cache misses incurred during RCU's grace-period
177 initialization. These systems tend to run CPU-bound, and thus
178 are not helped by synchronized interrupts, and thus tend to
179 skew them, which reduces lock contention enough that large
180 leaf-level fanouts work well. That said, setting leaf-level
181 fanout to a large number will likely cause problematic
182 lock contention on the leaf-level rcu_node structures unless
183 you boot with the skew_tick kernel parameter.
184
185 Select a specific number if testing RCU itself.
186
187 Select the maximum permissible value for large systems, but
188 please understand that you may also need to set the skew_tick
189 kernel boot parameter to avoid contention on the rcu_node
190 structure's locks.
191
192 Take the default if unsure.
193
194config RCU_BOOST
195 bool "Enable RCU priority boosting"
196 depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT
197 default y if PREEMPT_RT
198 help
199 This option boosts the priority of preempted RCU readers that
200 block the current preemptible RCU grace period for too long.
201 This option also prevents heavy loads from blocking RCU
202 callback invocation.
203
204 Say Y here if you are working with real-time apps or heavy loads
205 Say N here if you are unsure.
206
207config RCU_BOOST_DELAY
208 int "Milliseconds to delay boosting after RCU grace-period start"
209 range 0 3000
210 depends on RCU_BOOST
211 default 500
212 help
213 This option specifies the time to wait after the beginning of
214 a given grace period before priority-boosting preempted RCU
215 readers blocking that grace period. Note that any RCU reader
216 blocking an expedited RCU grace period is boosted immediately.
217
218 Accept the default if unsure.
219
220config RCU_EXP_KTHREAD
221 bool "Perform RCU expedited work in a real-time kthread"
222 depends on RCU_BOOST && RCU_EXPERT
223 default !PREEMPT_RT && NR_CPUS <= 32
224 help
225 Use this option to further reduce the latencies of expedited
226 grace periods at the expense of being more disruptive.
227
228 This option is disabled by default on PREEMPT_RT=y kernels which
229 disable expedited grace periods after boot by unconditionally
230 setting rcupdate.rcu_normal_after_boot=1.
231
232 Accept the default if unsure.
233
234config RCU_NOCB_CPU
235 bool "Offload RCU callback processing from boot-selected CPUs"
236 depends on TREE_RCU
237 depends on RCU_EXPERT || NO_HZ_FULL
238 default n
239 help
240 Use this option to reduce OS jitter for aggressive HPC or
241 real-time workloads. It can also be used to offload RCU
242 callback invocation to energy-efficient CPUs in battery-powered
243 asymmetric multiprocessors. The price of this reduced jitter
244 is that the overhead of call_rcu() increases and that some
245 workloads will incur significant increases in context-switch
246 rates.
247
248 This option offloads callback invocation from the set of CPUs
249 specified at boot time by the rcu_nocbs parameter. For each
250 such CPU, a kthread ("rcuox/N") will be created to invoke
251 callbacks, where the "N" is the CPU being offloaded, and where
252 the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
253 RCU-sched (!PREEMPTION kernels). Nothing prevents this kthread
254 from running on the specified CPUs, but (1) the kthreads may be
255 preempted between each callback, and (2) affinity or cgroups can
256 be used to force the kthreads to run on whatever set of CPUs is
257 desired.
258
259 Say Y here if you need reduced OS jitter, despite added overhead.
260 Say N here if you are unsure.
261
262config RCU_NOCB_CPU_DEFAULT_ALL
263 bool "Offload RCU callback processing from all CPUs by default"
264 depends on RCU_NOCB_CPU
265 default n
266 help
267 Use this option to offload callback processing from all CPUs
268 by default, in the absence of the rcu_nocbs or nohz_full boot
269 parameter. This also avoids the need to use any boot parameters
270 to achieve the effect of offloading all CPUs on boot.
271
272 Say Y here if you want offload all CPUs by default on boot.
273 Say N here if you are unsure.
274
275config RCU_NOCB_CPU_CB_BOOST
276 bool "Offload RCU callback from real-time kthread"
277 depends on RCU_NOCB_CPU && RCU_BOOST
278 default y if PREEMPT_RT
279 help
280 Use this option to invoke offloaded callbacks as SCHED_FIFO
281 to avoid starvation by heavy SCHED_OTHER background load.
282 Of course, running as SCHED_FIFO during callback floods will
283 cause the rcuo[ps] kthreads to monopolize the CPU for hundreds
284 of milliseconds or more. Therefore, when enabling this option,
285 it is your responsibility to ensure that latency-sensitive
286 tasks either run with higher priority or run on some other CPU.
287
288 Say Y here if you want to set RT priority for offloading kthreads.
289 Say N here if you are building a !PREEMPT_RT kernel and are unsure.
290
291config TASKS_TRACE_RCU_READ_MB
292 bool "Tasks Trace RCU readers use memory barriers in user and idle"
293 depends on RCU_EXPERT && TASKS_TRACE_RCU
294 default PREEMPT_RT || NR_CPUS < 8
295 help
296 Use this option to further reduce the number of IPIs sent
297 to CPUs executing in userspace or idle during tasks trace
298 RCU grace periods. Given that a reasonable setting of
299 the rcupdate.rcu_task_ipi_delay kernel boot parameter
300 eliminates such IPIs for many workloads, proper setting
301 of this Kconfig option is important mostly for aggressive
302 real-time installations and for battery-powered devices,
303 hence the default chosen above.
304
305 Say Y here if you hate IPIs.
306 Say N here if you hate read-side memory barriers.
307 Take the default if you are unsure.
308
309config RCU_LAZY
310 bool "RCU callback lazy invocation functionality"
311 depends on RCU_NOCB_CPU
312 default n
313 help
314 To save power, batch RCU callbacks and flush after delay, memory
315 pressure, or callback list growing too big.
316
317 Requires rcu_nocbs=all to be set.
318
319 Use rcutree.enable_rcu_lazy=0 to turn it off at boot time.
320
321config RCU_LAZY_DEFAULT_OFF
322 bool "Turn RCU lazy invocation off by default"
323 depends on RCU_LAZY
324 default n
325 help
326 Allows building the kernel with CONFIG_RCU_LAZY=y yet keep it default
327 off. Boot time param rcutree.enable_rcu_lazy=1 can be used to switch
328 it back on.
329
330config RCU_DOUBLE_CHECK_CB_TIME
331 bool "RCU callback-batch backup time check"
332 depends on RCU_EXPERT
333 default n
334 help
335 Use this option to provide more precise enforcement of the
336 rcutree.rcu_resched_ns module parameter in situations where
337 a single RCU callback might run for hundreds of microseconds,
338 thus defeating the 32-callback batching used to amortize the
339 cost of the fine-grained but expensive local_clock() function.
340
341 This option rounds rcutree.rcu_resched_ns up to the next
342 jiffy, and overrides the 32-callback batching if this limit
343 is exceeded.
344
345 Say Y here if you need tighter callback-limit enforcement.
346 Say N here if you are unsure.
347
348endmenu # "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 !PREEMPTION && 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 help
21 This option selects the RCU implementation that is
22 designed for very large SMP systems with hundreds or
23 thousands of CPUs, but for which real-time response
24 is also required. It also scales down nicely to
25 smaller systems.
26
27 Select this option if you are unsure.
28
29config TINY_RCU
30 bool
31 default y if !PREEMPTION && !SMP
32 help
33 This option selects the RCU implementation that is
34 designed for UP systems from which real-time response
35 is not required. This option greatly reduces the
36 memory footprint of RCU.
37
38config RCU_EXPERT
39 bool "Make expert-level adjustments to RCU configuration"
40 default n
41 help
42 This option needs to be enabled if you wish to make
43 expert-level adjustments to RCU configuration. By default,
44 no such adjustments can be made, which has the often-beneficial
45 side-effect of preventing "make oldconfig" from asking you all
46 sorts of detailed questions about how you would like numerous
47 obscure RCU options to be set up.
48
49 Say Y if you need to make expert-level adjustments to RCU.
50
51 Say N if you are unsure.
52
53config SRCU
54 bool
55 help
56 This option selects the sleepable version of RCU. This version
57 permits arbitrary sleeping or blocking within RCU read-side critical
58 sections.
59
60config TINY_SRCU
61 bool
62 default y if SRCU && TINY_RCU
63 help
64 This option selects the single-CPU non-preemptible version of SRCU.
65
66config TREE_SRCU
67 bool
68 default y if SRCU && !TINY_RCU
69 help
70 This option selects the full-fledged version of SRCU.
71
72config TASKS_RCU
73 def_bool PREEMPTION
74 select SRCU
75 help
76 This option enables a task-based RCU implementation that uses
77 only voluntary context switch (not preemption!), idle, and
78 user-mode execution as quiescent states.
79
80config RCU_STALL_COMMON
81 def_bool ( TREE_RCU || PREEMPT_RCU )
82 help
83 This option enables RCU CPU stall code that is common between
84 the TINY and TREE variants of RCU. The purpose is to allow
85 the tiny variants to disable RCU CPU stall warnings, while
86 making these warnings mandatory for the tree variants.
87
88config RCU_NEED_SEGCBLIST
89 def_bool ( TREE_RCU || PREEMPT_RCU || TREE_SRCU )
90
91config RCU_FANOUT
92 int "Tree-based hierarchical RCU fanout value"
93 range 2 64 if 64BIT
94 range 2 32 if !64BIT
95 depends on (TREE_RCU || PREEMPT_RCU) && RCU_EXPERT
96 default 64 if 64BIT
97 default 32 if !64BIT
98 help
99 This option controls the fanout of hierarchical implementations
100 of RCU, allowing RCU to work efficiently on machines with
101 large numbers of CPUs. This value must be at least the fourth
102 root of NR_CPUS, which allows NR_CPUS to be insanely large.
103 The default value of RCU_FANOUT should be used for production
104 systems, but if you are stress-testing the RCU implementation
105 itself, small RCU_FANOUT values allow you to test large-system
106 code paths on small(er) systems.
107
108 Select a specific number if testing RCU itself.
109 Take the default if unsure.
110
111config RCU_FANOUT_LEAF
112 int "Tree-based hierarchical RCU leaf-level fanout value"
113 range 2 64 if 64BIT
114 range 2 32 if !64BIT
115 depends on (TREE_RCU || PREEMPT_RCU) && RCU_EXPERT
116 default 16
117 help
118 This option controls the leaf-level fanout of hierarchical
119 implementations of RCU, and allows trading off cache misses
120 against lock contention. Systems that synchronize their
121 scheduling-clock interrupts for energy-efficiency reasons will
122 want the default because the smaller leaf-level fanout keeps
123 lock contention levels acceptably low. Very large systems
124 (hundreds or thousands of CPUs) will instead want to set this
125 value to the maximum value possible in order to reduce the
126 number of cache misses incurred during RCU's grace-period
127 initialization. These systems tend to run CPU-bound, and thus
128 are not helped by synchronized interrupts, and thus tend to
129 skew them, which reduces lock contention enough that large
130 leaf-level fanouts work well. That said, setting leaf-level
131 fanout to a large number will likely cause problematic
132 lock contention on the leaf-level rcu_node structures unless
133 you boot with the skew_tick kernel parameter.
134
135 Select a specific number if testing RCU itself.
136
137 Select the maximum permissible value for large systems, but
138 please understand that you may also need to set the skew_tick
139 kernel boot parameter to avoid contention on the rcu_node
140 structure's locks.
141
142 Take the default if unsure.
143
144config RCU_FAST_NO_HZ
145 bool "Accelerate last non-dyntick-idle CPU's grace periods"
146 depends on NO_HZ_COMMON && SMP && RCU_EXPERT
147 default n
148 help
149 This option permits CPUs to enter dynticks-idle state even if
150 they have RCU callbacks queued, and prevents RCU from waking
151 these CPUs up more than roughly once every four jiffies (by
152 default, you can adjust this using the rcutree.rcu_idle_gp_delay
153 parameter), thus improving energy efficiency. On the other
154 hand, this option increases the duration of RCU grace periods,
155 for example, slowing down synchronize_rcu().
156
157 Say Y if energy efficiency is critically important, and you
158 don't care about increased grace-period durations.
159
160 Say N if you are unsure.
161
162config RCU_BOOST
163 bool "Enable RCU priority boosting"
164 depends on RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT
165 default n
166 help
167 This option boosts the priority of preempted RCU readers that
168 block the current preemptible RCU grace period for too long.
169 This option also prevents heavy loads from blocking RCU
170 callback invocation.
171
172 Say Y here if you are working with real-time apps or heavy loads
173 Say N here if you are unsure.
174
175config RCU_BOOST_DELAY
176 int "Milliseconds to delay boosting after RCU grace-period start"
177 range 0 3000
178 depends on RCU_BOOST
179 default 500
180 help
181 This option specifies the time to wait after the beginning of
182 a given grace period before priority-boosting preempted RCU
183 readers blocking that grace period. Note that any RCU reader
184 blocking an expedited RCU grace period is boosted immediately.
185
186 Accept the default if unsure.
187
188config RCU_NOCB_CPU
189 bool "Offload RCU callback processing from boot-selected CPUs"
190 depends on TREE_RCU || PREEMPT_RCU
191 depends on RCU_EXPERT || NO_HZ_FULL
192 default n
193 help
194 Use this option to reduce OS jitter for aggressive HPC or
195 real-time workloads. It can also be used to offload RCU
196 callback invocation to energy-efficient CPUs in battery-powered
197 asymmetric multiprocessors.
198
199 This option offloads callback invocation from the set of CPUs
200 specified at boot time by the rcu_nocbs parameter. For each
201 such CPU, a kthread ("rcuox/N") will be created to invoke
202 callbacks, where the "N" is the CPU being offloaded, and where
203 the "p" for RCU-preempt (PREEMPT kernels) and "s" for RCU-sched
204 (!PREEMPT kernels). Nothing prevents this kthread from running
205 on the specified CPUs, but (1) the kthreads may be preempted
206 between each callback, and (2) affinity or cgroups can be used
207 to force the kthreads to run on whatever set of CPUs is desired.
208
209 Say Y here if you want to help to debug reduced OS jitter.
210 Say N here if you are unsure.
211
212endmenu # "RCU Subsystem"