1 # SPDX-License-Identifier: GPL-2.0-only 1 # SPDX-License-Identifier: GPL-2.0-only
2 # 2 #
3 # RCU-related configuration options 3 # RCU-related configuration options
4 # 4 #
5 5
6 menu "RCU Subsystem" 6 menu "RCU Subsystem"
7 7
8 config TREE_RCU 8 config TREE_RCU
9 bool 9 bool
10 default y if SMP 10 default y if SMP
11 # Dynticks-idle tracking 11 # Dynticks-idle tracking
12 select CONTEXT_TRACKING_IDLE 12 select CONTEXT_TRACKING_IDLE
13 help 13 help
14 This option selects the RCU implemen 14 This option selects the RCU implementation that is
15 designed for very large SMP system w 15 designed for very large SMP system with hundreds or
16 thousands of CPUs. It also scales d 16 thousands of CPUs. It also scales down nicely to
17 smaller systems. 17 smaller systems.
18 18
19 config PREEMPT_RCU 19 config PREEMPT_RCU
20 bool 20 bool
21 default y if PREEMPTION 21 default y if PREEMPTION
22 select TREE_RCU 22 select TREE_RCU
23 help 23 help
24 This option selects the RCU implemen 24 This option selects the RCU implementation that is
25 designed for very large SMP systems 25 designed for very large SMP systems with hundreds or
26 thousands of CPUs, but for which rea 26 thousands of CPUs, but for which real-time response
27 is also required. It also scales do 27 is also required. It also scales down nicely to
28 smaller systems. 28 smaller systems.
29 29
30 Select this option if you are unsure 30 Select this option if you are unsure.
31 31
32 config TINY_RCU 32 config TINY_RCU
33 bool 33 bool
34 default y if !PREEMPT_RCU && !SMP !! 34 default y if !PREEMPTION && !SMP
35 help 35 help
36 This option selects the RCU implemen 36 This option selects the RCU implementation that is
37 designed for UP systems from which r 37 designed for UP systems from which real-time response
38 is not required. This option greatl 38 is not required. This option greatly reduces the
39 memory footprint of RCU. 39 memory footprint of RCU.
40 40
41 config RCU_EXPERT 41 config RCU_EXPERT
42 bool "Make expert-level adjustments to 42 bool "Make expert-level adjustments to RCU configuration"
43 default n 43 default n
44 help 44 help
45 This option needs to be enabled if y 45 This option needs to be enabled if you wish to make
46 expert-level adjustments to RCU conf 46 expert-level adjustments to RCU configuration. By default,
47 no such adjustments can be made, whi 47 no such adjustments can be made, which has the often-beneficial
48 side-effect of preventing "make oldc 48 side-effect of preventing "make oldconfig" from asking you all
49 sorts of detailed questions about ho 49 sorts of detailed questions about how you would like numerous
50 obscure RCU options to be set up. 50 obscure RCU options to be set up.
51 51
52 Say Y if you need to make expert-lev 52 Say Y if you need to make expert-level adjustments to RCU.
53 53
54 Say N if you are unsure. 54 Say N if you are unsure.
55 55
56 config TINY_SRCU 56 config TINY_SRCU
57 bool 57 bool
58 default y if TINY_RCU 58 default y if TINY_RCU
59 help 59 help
60 This option selects the single-CPU n 60 This option selects the single-CPU non-preemptible version of SRCU.
61 61
62 config TREE_SRCU 62 config TREE_SRCU
63 bool 63 bool
64 default y if !TINY_RCU 64 default y if !TINY_RCU
65 help 65 help
66 This option selects the full-fledged 66 This option selects the full-fledged version of SRCU.
67 67
68 config NEED_SRCU_NMI_SAFE 68 config NEED_SRCU_NMI_SAFE
69 def_bool HAVE_NMI && !ARCH_HAS_NMI_SAF 69 def_bool HAVE_NMI && !ARCH_HAS_NMI_SAFE_THIS_CPU_OPS && !TINY_SRCU
70 70
71 config TASKS_RCU_GENERIC 71 config TASKS_RCU_GENERIC
72 def_bool TASKS_RCU || TASKS_RUDE_RCU | 72 def_bool TASKS_RCU || TASKS_RUDE_RCU || TASKS_TRACE_RCU
73 help 73 help
74 This option enables generic infrastr 74 This option enables generic infrastructure code supporting
75 task-based RCU implementations. Not 75 task-based RCU implementations. Not for manual selection.
76 76
77 config FORCE_TASKS_RCU 77 config FORCE_TASKS_RCU
78 bool "Force selection of TASKS_RCU" 78 bool "Force selection of TASKS_RCU"
79 depends on RCU_EXPERT 79 depends on RCU_EXPERT
80 select TASKS_RCU 80 select TASKS_RCU
81 default n 81 default n
82 help 82 help
83 This option force-enables a task-bas 83 This option force-enables a task-based RCU implementation
84 that uses only voluntary context swi 84 that uses only voluntary context switch (not preemption!),
85 idle, and user-mode execution as qui 85 idle, and user-mode execution as quiescent states. Not for
86 manual selection in most cases. 86 manual selection in most cases.
87 87
88 config NEED_TASKS_RCU <<
89 bool <<
90 default n <<
91 <<
92 config TASKS_RCU 88 config TASKS_RCU
93 bool 89 bool
94 default NEED_TASKS_RCU && (PREEMPTION !! 90 default n
95 select IRQ_WORK 91 select IRQ_WORK
96 92
97 config FORCE_TASKS_RUDE_RCU 93 config FORCE_TASKS_RUDE_RCU
98 bool "Force selection of Tasks Rude RC 94 bool "Force selection of Tasks Rude RCU"
99 depends on RCU_EXPERT 95 depends on RCU_EXPERT
100 select TASKS_RUDE_RCU 96 select TASKS_RUDE_RCU
101 default n 97 default n
102 help 98 help
103 This option force-enables a task-bas 99 This option force-enables a task-based RCU implementation
104 that uses only context switch (inclu 100 that uses only context switch (including preemption) and
105 user-mode execution as quiescent sta 101 user-mode execution as quiescent states. It forces IPIs and
106 context switches on all online CPUs, 102 context switches on all online CPUs, including idle ones,
107 so use with caution. Not for manual 103 so use with caution. Not for manual selection in most cases.
108 104
109 config TASKS_RUDE_RCU 105 config TASKS_RUDE_RCU
110 bool 106 bool
111 default n 107 default n
112 select IRQ_WORK 108 select IRQ_WORK
113 109
114 config FORCE_TASKS_TRACE_RCU 110 config FORCE_TASKS_TRACE_RCU
115 bool "Force selection of Tasks Trace R 111 bool "Force selection of Tasks Trace RCU"
116 depends on RCU_EXPERT 112 depends on RCU_EXPERT
117 select TASKS_TRACE_RCU 113 select TASKS_TRACE_RCU
118 default n 114 default n
119 help 115 help
120 This option enables a task-based RCU 116 This option enables a task-based RCU implementation that uses
121 explicit rcu_read_lock_trace() read- 117 explicit rcu_read_lock_trace() read-side markers, and allows
122 these readers to appear in the idle 118 these readers to appear in the idle loop as well as on the
123 CPU hotplug code paths. It can forc 119 CPU hotplug code paths. It can force IPIs on online CPUs,
124 including idle ones, so use with cau 120 including idle ones, so use with caution. Not for manual
125 selection in most cases. 121 selection in most cases.
126 122
127 config TASKS_TRACE_RCU 123 config TASKS_TRACE_RCU
128 bool 124 bool
129 default n 125 default n
130 select IRQ_WORK 126 select IRQ_WORK
131 127
132 config RCU_STALL_COMMON 128 config RCU_STALL_COMMON
133 def_bool TREE_RCU 129 def_bool TREE_RCU
134 help 130 help
135 This option enables RCU CPU stall co 131 This option enables RCU CPU stall code that is common between
136 the TINY and TREE variants of RCU. 132 the TINY and TREE variants of RCU. The purpose is to allow
137 the tiny variants to disable RCU CPU 133 the tiny variants to disable RCU CPU stall warnings, while
138 making these warnings mandatory for 134 making these warnings mandatory for the tree variants.
139 135
140 config RCU_NEED_SEGCBLIST 136 config RCU_NEED_SEGCBLIST
141 def_bool ( TREE_RCU || TREE_SRCU || TA 137 def_bool ( TREE_RCU || TREE_SRCU || TASKS_RCU_GENERIC )
142 138
143 config RCU_FANOUT 139 config RCU_FANOUT
144 int "Tree-based hierarchical RCU fanou 140 int "Tree-based hierarchical RCU fanout value"
145 range 2 64 if 64BIT 141 range 2 64 if 64BIT
146 range 2 32 if !64BIT 142 range 2 32 if !64BIT
147 depends on TREE_RCU && RCU_EXPERT 143 depends on TREE_RCU && RCU_EXPERT
148 default 64 if 64BIT 144 default 64 if 64BIT
149 default 32 if !64BIT 145 default 32 if !64BIT
150 help 146 help
151 This option controls the fanout of h 147 This option controls the fanout of hierarchical implementations
152 of RCU, allowing RCU to work efficie 148 of RCU, allowing RCU to work efficiently on machines with
153 large numbers of CPUs. This value m 149 large numbers of CPUs. This value must be at least the fourth
154 root of NR_CPUS, which allows NR_CPU 150 root of NR_CPUS, which allows NR_CPUS to be insanely large.
155 The default value of RCU_FANOUT shou 151 The default value of RCU_FANOUT should be used for production
156 systems, but if you are stress-testi 152 systems, but if you are stress-testing the RCU implementation
157 itself, small RCU_FANOUT values allo 153 itself, small RCU_FANOUT values allow you to test large-system
158 code paths on small(er) systems. 154 code paths on small(er) systems.
159 155
160 Select a specific number if testing 156 Select a specific number if testing RCU itself.
161 Take the default if unsure. 157 Take the default if unsure.
162 158
163 config RCU_FANOUT_LEAF 159 config RCU_FANOUT_LEAF
164 int "Tree-based hierarchical RCU leaf- 160 int "Tree-based hierarchical RCU leaf-level fanout value"
165 range 2 64 if 64BIT && !RCU_STRICT_GRA 161 range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD
166 range 2 32 if !64BIT && !RCU_STRICT_GR 162 range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD
167 range 2 3 if RCU_STRICT_GRACE_PERIOD 163 range 2 3 if RCU_STRICT_GRACE_PERIOD
168 depends on TREE_RCU && RCU_EXPERT 164 depends on TREE_RCU && RCU_EXPERT
169 default 16 if !RCU_STRICT_GRACE_PERIOD 165 default 16 if !RCU_STRICT_GRACE_PERIOD
170 default 2 if RCU_STRICT_GRACE_PERIOD 166 default 2 if RCU_STRICT_GRACE_PERIOD
171 help 167 help
172 This option controls the leaf-level 168 This option controls the leaf-level fanout of hierarchical
173 implementations of RCU, and allows t 169 implementations of RCU, and allows trading off cache misses
174 against lock contention. Systems th 170 against lock contention. Systems that synchronize their
175 scheduling-clock interrupts for ener 171 scheduling-clock interrupts for energy-efficiency reasons will
176 want the default because the smaller 172 want the default because the smaller leaf-level fanout keeps
177 lock contention levels acceptably lo 173 lock contention levels acceptably low. Very large systems
178 (hundreds or thousands of CPUs) will 174 (hundreds or thousands of CPUs) will instead want to set this
179 value to the maximum value possible 175 value to the maximum value possible in order to reduce the
180 number of cache misses incurred duri 176 number of cache misses incurred during RCU's grace-period
181 initialization. These systems tend 177 initialization. These systems tend to run CPU-bound, and thus
182 are not helped by synchronized inter 178 are not helped by synchronized interrupts, and thus tend to
183 skew them, which reduces lock conten 179 skew them, which reduces lock contention enough that large
184 leaf-level fanouts work well. That 180 leaf-level fanouts work well. That said, setting leaf-level
185 fanout to a large number will likely 181 fanout to a large number will likely cause problematic
186 lock contention on the leaf-level rc 182 lock contention on the leaf-level rcu_node structures unless
187 you boot with the skew_tick kernel p 183 you boot with the skew_tick kernel parameter.
188 184
189 Select a specific number if testing 185 Select a specific number if testing RCU itself.
190 186
191 Select the maximum permissible value 187 Select the maximum permissible value for large systems, but
192 please understand that you may also 188 please understand that you may also need to set the skew_tick
193 kernel boot parameter to avoid conte 189 kernel boot parameter to avoid contention on the rcu_node
194 structure's locks. 190 structure's locks.
195 191
196 Take the default if unsure. 192 Take the default if unsure.
197 193
198 config RCU_BOOST 194 config RCU_BOOST
199 bool "Enable RCU priority boosting" 195 bool "Enable RCU priority boosting"
200 depends on (RT_MUTEXES && PREEMPT_RCU 196 depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT
201 default y if PREEMPT_RT 197 default y if PREEMPT_RT
202 help 198 help
203 This option boosts the priority of p 199 This option boosts the priority of preempted RCU readers that
204 block the current preemptible RCU gr 200 block the current preemptible RCU grace period for too long.
205 This option also prevents heavy load 201 This option also prevents heavy loads from blocking RCU
206 callback invocation. 202 callback invocation.
207 203
208 Say Y here if you are working with r 204 Say Y here if you are working with real-time apps or heavy loads
209 Say N here if you are unsure. 205 Say N here if you are unsure.
210 206
211 config RCU_BOOST_DELAY 207 config RCU_BOOST_DELAY
212 int "Milliseconds to delay boosting af 208 int "Milliseconds to delay boosting after RCU grace-period start"
213 range 0 3000 209 range 0 3000
214 depends on RCU_BOOST 210 depends on RCU_BOOST
215 default 500 211 default 500
216 help 212 help
217 This option specifies the time to wa 213 This option specifies the time to wait after the beginning of
218 a given grace period before priority 214 a given grace period before priority-boosting preempted RCU
219 readers blocking that grace period. 215 readers blocking that grace period. Note that any RCU reader
220 blocking an expedited RCU grace peri 216 blocking an expedited RCU grace period is boosted immediately.
221 217
222 Accept the default if unsure. 218 Accept the default if unsure.
223 219
224 config RCU_EXP_KTHREAD 220 config RCU_EXP_KTHREAD
225 bool "Perform RCU expedited work in a 221 bool "Perform RCU expedited work in a real-time kthread"
226 depends on RCU_BOOST && RCU_EXPERT 222 depends on RCU_BOOST && RCU_EXPERT
227 default !PREEMPT_RT && NR_CPUS <= 32 223 default !PREEMPT_RT && NR_CPUS <= 32
228 help 224 help
229 Use this option to further reduce th 225 Use this option to further reduce the latencies of expedited
230 grace periods at the expense of bein 226 grace periods at the expense of being more disruptive.
231 227
232 This option is disabled by default o 228 This option is disabled by default on PREEMPT_RT=y kernels which
233 disable expedited grace periods afte 229 disable expedited grace periods after boot by unconditionally
234 setting rcupdate.rcu_normal_after_bo 230 setting rcupdate.rcu_normal_after_boot=1.
235 231
236 Accept the default if unsure. 232 Accept the default if unsure.
237 233
238 config RCU_NOCB_CPU 234 config RCU_NOCB_CPU
239 bool "Offload RCU callback processing 235 bool "Offload RCU callback processing from boot-selected CPUs"
240 depends on TREE_RCU 236 depends on TREE_RCU
241 depends on RCU_EXPERT || NO_HZ_FULL 237 depends on RCU_EXPERT || NO_HZ_FULL
242 default n 238 default n
243 help 239 help
244 Use this option to reduce OS jitter 240 Use this option to reduce OS jitter for aggressive HPC or
245 real-time workloads. It can also be 241 real-time workloads. It can also be used to offload RCU
246 callback invocation to energy-effici 242 callback invocation to energy-efficient CPUs in battery-powered
247 asymmetric multiprocessors. The pri 243 asymmetric multiprocessors. The price of this reduced jitter
248 is that the overhead of call_rcu() i 244 is that the overhead of call_rcu() increases and that some
249 workloads will incur significant inc 245 workloads will incur significant increases in context-switch
250 rates. 246 rates.
251 247
252 This option offloads callback invoca 248 This option offloads callback invocation from the set of CPUs
253 specified at boot time by the rcu_no 249 specified at boot time by the rcu_nocbs parameter. For each
254 such CPU, a kthread ("rcuox/N") will 250 such CPU, a kthread ("rcuox/N") will be created to invoke
255 callbacks, where the "N" is the CPU 251 callbacks, where the "N" is the CPU being offloaded, and where
256 the "x" is "p" for RCU-preempt (PREE 252 the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
257 RCU-sched (!PREEMPTION kernels). No 253 RCU-sched (!PREEMPTION kernels). Nothing prevents this kthread
258 from running on the specified CPUs, 254 from running on the specified CPUs, but (1) the kthreads may be
259 preempted between each callback, and 255 preempted between each callback, and (2) affinity or cgroups can
260 be used to force the kthreads to run 256 be used to force the kthreads to run on whatever set of CPUs is
261 desired. 257 desired.
262 258
263 Say Y here if you need reduced OS ji 259 Say Y here if you need reduced OS jitter, despite added overhead.
264 Say N here if you are unsure. 260 Say N here if you are unsure.
265 261
266 config RCU_NOCB_CPU_DEFAULT_ALL 262 config RCU_NOCB_CPU_DEFAULT_ALL
267 bool "Offload RCU callback processing 263 bool "Offload RCU callback processing from all CPUs by default"
268 depends on RCU_NOCB_CPU 264 depends on RCU_NOCB_CPU
269 default n 265 default n
270 help 266 help
271 Use this option to offload callback 267 Use this option to offload callback processing from all CPUs
272 by default, in the absence of the rc 268 by default, in the absence of the rcu_nocbs or nohz_full boot
273 parameter. This also avoids the need 269 parameter. This also avoids the need to use any boot parameters
274 to achieve the effect of offloading 270 to achieve the effect of offloading all CPUs on boot.
275 271
276 Say Y here if you want offload all C 272 Say Y here if you want offload all CPUs by default on boot.
277 Say N here if you are unsure. 273 Say N here if you are unsure.
278 274
279 config RCU_NOCB_CPU_CB_BOOST 275 config RCU_NOCB_CPU_CB_BOOST
280 bool "Offload RCU callback from real-t 276 bool "Offload RCU callback from real-time kthread"
281 depends on RCU_NOCB_CPU && RCU_BOOST 277 depends on RCU_NOCB_CPU && RCU_BOOST
282 default y if PREEMPT_RT 278 default y if PREEMPT_RT
283 help 279 help
284 Use this option to invoke offloaded 280 Use this option to invoke offloaded callbacks as SCHED_FIFO
285 to avoid starvation by heavy SCHED_O 281 to avoid starvation by heavy SCHED_OTHER background load.
286 Of course, running as SCHED_FIFO dur 282 Of course, running as SCHED_FIFO during callback floods will
287 cause the rcuo[ps] kthreads to monop 283 cause the rcuo[ps] kthreads to monopolize the CPU for hundreds
288 of milliseconds or more. Therefore, 284 of milliseconds or more. Therefore, when enabling this option,
289 it is your responsibility to ensure 285 it is your responsibility to ensure that latency-sensitive
290 tasks either run with higher priorit 286 tasks either run with higher priority or run on some other CPU.
291 287
292 Say Y here if you want to set RT pri 288 Say Y here if you want to set RT priority for offloading kthreads.
293 Say N here if you are building a !PR 289 Say N here if you are building a !PREEMPT_RT kernel and are unsure.
294 290
295 config TASKS_TRACE_RCU_READ_MB 291 config TASKS_TRACE_RCU_READ_MB
296 bool "Tasks Trace RCU readers use memo 292 bool "Tasks Trace RCU readers use memory barriers in user and idle"
297 depends on RCU_EXPERT && TASKS_TRACE_R 293 depends on RCU_EXPERT && TASKS_TRACE_RCU
298 default PREEMPT_RT || NR_CPUS < 8 294 default PREEMPT_RT || NR_CPUS < 8
299 help 295 help
300 Use this option to further reduce th 296 Use this option to further reduce the number of IPIs sent
301 to CPUs executing in userspace or id 297 to CPUs executing in userspace or idle during tasks trace
302 RCU grace periods. Given that a rea 298 RCU grace periods. Given that a reasonable setting of
303 the rcupdate.rcu_task_ipi_delay kern 299 the rcupdate.rcu_task_ipi_delay kernel boot parameter
304 eliminates such IPIs for many worklo 300 eliminates such IPIs for many workloads, proper setting
305 of this Kconfig option is important 301 of this Kconfig option is important mostly for aggressive
306 real-time installations and for batt 302 real-time installations and for battery-powered devices,
307 hence the default chosen above. 303 hence the default chosen above.
308 304
309 Say Y here if you hate IPIs. 305 Say Y here if you hate IPIs.
310 Say N here if you hate read-side mem 306 Say N here if you hate read-side memory barriers.
311 Take the default if you are unsure. 307 Take the default if you are unsure.
312 308
313 config RCU_LAZY 309 config RCU_LAZY
314 bool "RCU callback lazy invocation fun 310 bool "RCU callback lazy invocation functionality"
315 depends on RCU_NOCB_CPU 311 depends on RCU_NOCB_CPU
316 default n 312 default n
317 help 313 help
318 To save power, batch RCU callbacks a 314 To save power, batch RCU callbacks and flush after delay, memory
319 pressure, or callback list growing t 315 pressure, or callback list growing too big.
320 316
321 Requires rcu_nocbs=all to be set. 317 Requires rcu_nocbs=all to be set.
322 318
323 Use rcutree.enable_rcu_lazy=0 to tur 319 Use rcutree.enable_rcu_lazy=0 to turn it off at boot time.
324 320
325 config RCU_LAZY_DEFAULT_OFF 321 config RCU_LAZY_DEFAULT_OFF
326 bool "Turn RCU lazy invocation off by 322 bool "Turn RCU lazy invocation off by default"
327 depends on RCU_LAZY 323 depends on RCU_LAZY
328 default n 324 default n
329 help 325 help
330 Allows building the kernel with CONF 326 Allows building the kernel with CONFIG_RCU_LAZY=y yet keep it default
331 off. Boot time param rcutree.enable_ 327 off. Boot time param rcutree.enable_rcu_lazy=1 can be used to switch
332 it back on. 328 it back on.
333 329
334 config RCU_DOUBLE_CHECK_CB_TIME 330 config RCU_DOUBLE_CHECK_CB_TIME
335 bool "RCU callback-batch backup time c 331 bool "RCU callback-batch backup time check"
336 depends on RCU_EXPERT 332 depends on RCU_EXPERT
337 default n 333 default n
338 help 334 help
339 Use this option to provide more prec 335 Use this option to provide more precise enforcement of the
340 rcutree.rcu_resched_ns module parame 336 rcutree.rcu_resched_ns module parameter in situations where
341 a single RCU callback might run for 337 a single RCU callback might run for hundreds of microseconds,
342 thus defeating the 32-callback batch 338 thus defeating the 32-callback batching used to amortize the
343 cost of the fine-grained but expensi 339 cost of the fine-grained but expensive local_clock() function.
344 340
345 This option rounds rcutree.rcu_resch 341 This option rounds rcutree.rcu_resched_ns up to the next
346 jiffy, and overrides the 32-callback 342 jiffy, and overrides the 32-callback batching if this limit
347 is exceeded. 343 is exceeded.
348 344
349 Say Y here if you need tighter callb 345 Say Y here if you need tighter callback-limit enforcement.
350 Say N here if you are unsure. 346 Say N here if you are unsure.
351 347
352 endmenu # "RCU Subsystem" 348 endmenu # "RCU Subsystem"
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