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