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 !PREEMPT_RCU && !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 NEED_TASKS_RCU 89 bool 90 default n 91 92config TASKS_RCU 93 bool 94 default NEED_TASKS_RCU && (PREEMPTION || PREEMPT_AUTO) 95 select IRQ_WORK 96 97config FORCE_TASKS_RUDE_RCU 98 bool "Force selection of Tasks Rude RCU" 99 depends on RCU_EXPERT 100 select TASKS_RUDE_RCU 101 default n 102 help 103 This option force-enables a task-based RCU implementation 104 that uses only context switch (including preemption) and 105 user-mode execution as quiescent states. It forces IPIs and 106 context switches on all online CPUs, including idle ones, 107 so use with caution. Not for manual selection in most cases. 108 109config TASKS_RUDE_RCU 110 bool 111 default n 112 select IRQ_WORK 113 114config FORCE_TASKS_TRACE_RCU 115 bool "Force selection of Tasks Trace RCU" 116 depends on RCU_EXPERT 117 select TASKS_TRACE_RCU 118 default n 119 help 120 This option enables a task-based RCU implementation that uses 121 explicit rcu_read_lock_trace() read-side markers, and allows 122 these readers to appear in the idle loop as well as on the 123 CPU hotplug code paths. It can force IPIs on online CPUs, 124 including idle ones, so use with caution. Not for manual 125 selection in most cases. 126 127config TASKS_TRACE_RCU 128 bool 129 default n 130 select IRQ_WORK 131 132config RCU_STALL_COMMON 133 def_bool TREE_RCU 134 help 135 This option enables RCU CPU stall code that is common between 136 the TINY and TREE variants of RCU. The purpose is to allow 137 the tiny variants to disable RCU CPU stall warnings, while 138 making these warnings mandatory for the tree variants. 139 140config RCU_NEED_SEGCBLIST 141 def_bool ( TREE_RCU || TREE_SRCU || TASKS_RCU_GENERIC ) 142 143config RCU_FANOUT 144 int "Tree-based hierarchical RCU fanout value" 145 range 2 64 if 64BIT 146 range 2 32 if !64BIT 147 depends on TREE_RCU && RCU_EXPERT 148 default 64 if 64BIT 149 default 32 if !64BIT 150 help 151 This option controls the fanout of hierarchical implementations 152 of RCU, allowing RCU to work efficiently on machines with 153 large numbers of CPUs. This value must be at least the fourth 154 root of NR_CPUS, which allows NR_CPUS to be insanely large. 155 The default value of RCU_FANOUT should be used for production 156 systems, but if you are stress-testing the RCU implementation 157 itself, small RCU_FANOUT values allow you to test large-system 158 code paths on small(er) systems. 159 160 Select a specific number if testing RCU itself. 161 Take the default if unsure. 162 163config RCU_FANOUT_LEAF 164 int "Tree-based hierarchical RCU leaf-level fanout value" 165 range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD 166 range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD 167 range 2 3 if RCU_STRICT_GRACE_PERIOD 168 depends on TREE_RCU && RCU_EXPERT 169 default 16 if !RCU_STRICT_GRACE_PERIOD 170 default 2 if RCU_STRICT_GRACE_PERIOD 171 help 172 This option controls the leaf-level fanout of hierarchical 173 implementations of RCU, and allows trading off cache misses 174 against lock contention. Systems that synchronize their 175 scheduling-clock interrupts for energy-efficiency reasons will 176 want the default because the smaller leaf-level fanout keeps 177 lock contention levels acceptably low. Very large systems 178 (hundreds or thousands of CPUs) will instead want to set this 179 value to the maximum value possible in order to reduce the 180 number of cache misses incurred during RCU's grace-period 181 initialization. These systems tend to run CPU-bound, and thus 182 are not helped by synchronized interrupts, and thus tend to 183 skew them, which reduces lock contention enough that large 184 leaf-level fanouts work well. That said, setting leaf-level 185 fanout to a large number will likely cause problematic 186 lock contention on the leaf-level rcu_node structures unless 187 you boot with the skew_tick kernel parameter. 188 189 Select a specific number if testing RCU itself. 190 191 Select the maximum permissible value for large systems, but 192 please understand that you may also need to set the skew_tick 193 kernel boot parameter to avoid contention on the rcu_node 194 structure's locks. 195 196 Take the default if unsure. 197 198config RCU_BOOST 199 bool "Enable RCU priority boosting" 200 depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT 201 default y if PREEMPT_RT 202 help 203 This option boosts the priority of preempted RCU readers that 204 block the current preemptible RCU grace period for too long. 205 This option also prevents heavy loads from blocking RCU 206 callback invocation. 207 208 Say Y here if you are working with real-time apps or heavy loads 209 Say N here if you are unsure. 210 211config RCU_BOOST_DELAY 212 int "Milliseconds to delay boosting after RCU grace-period start" 213 range 0 3000 214 depends on RCU_BOOST 215 default 500 216 help 217 This option specifies the time to wait after the beginning of 218 a given grace period before priority-boosting preempted RCU 219 readers blocking that grace period. Note that any RCU reader 220 blocking an expedited RCU grace period is boosted immediately. 221 222 Accept the default if unsure. 223 224config RCU_EXP_KTHREAD 225 bool "Perform RCU expedited work in a real-time kthread" 226 depends on RCU_BOOST && RCU_EXPERT 227 default !PREEMPT_RT && NR_CPUS <= 32 228 help 229 Use this option to further reduce the latencies of expedited 230 grace periods at the expense of being more disruptive. 231 232 This option is disabled by default on PREEMPT_RT=y kernels which 233 disable expedited grace periods after boot by unconditionally 234 setting rcupdate.rcu_normal_after_boot=1. 235 236 Accept the default if unsure. 237 238config RCU_NOCB_CPU 239 bool "Offload RCU callback processing from boot-selected CPUs" 240 depends on TREE_RCU 241 depends on RCU_EXPERT || NO_HZ_FULL 242 default n 243 help 244 Use this option to reduce OS jitter for aggressive HPC or 245 real-time workloads. It can also be used to offload RCU 246 callback invocation to energy-efficient CPUs in battery-powered 247 asymmetric multiprocessors. The price of this reduced jitter 248 is that the overhead of call_rcu() increases and that some 249 workloads will incur significant increases in context-switch 250 rates. 251 252 This option offloads callback invocation from the set of 253 CPUs specified at boot time by the rcu_nocbs parameter. 254 For each such CPU, a kthread ("rcuox/N") will be created to 255 invoke callbacks, where the "N" is the CPU being offloaded, 256 and where the "x" is "p" for RCU-preempt (PREEMPTION kernels) 257 and "s" for RCU-sched (!PREEMPTION kernels). This option 258 also creates another kthread for each sqrt(nr_cpu_ids) CPUs 259 ("rcuog/N", where N is the first CPU in that group to come 260 online), which handles grace periods for its group. Nothing 261 prevents these kthreads from running on the specified CPUs, 262 but (1) the kthreads may be preempted between each callback, 263 and (2) affinity or cgroups can be used to force the kthreads 264 to run on whatever set of CPUs is desired. 265 266 The sqrt(nr_cpu_ids) grouping may be overridden using the 267 rcutree.rcu_nocb_gp_stride kernel boot parameter. This can 268 be especially helpful for smaller numbers of CPUs, where 269 sqrt(nr_cpu_ids) can be a bit of a blunt instrument. 270 271 Say Y here if you need reduced OS jitter, despite added overhead. 272 Say N here if you are unsure. 273 274config RCU_NOCB_CPU_DEFAULT_ALL 275 bool "Offload RCU callback processing from all CPUs by default" 276 depends on RCU_NOCB_CPU 277 default n 278 help 279 Use this option to offload callback processing from all CPUs 280 by default, in the absence of the rcu_nocbs or nohz_full boot 281 parameter. This also avoids the need to use any boot parameters 282 to achieve the effect of offloading all CPUs on boot. 283 284 Say Y here if you want offload all CPUs by default on boot. 285 Say N here if you are unsure. 286 287config RCU_NOCB_CPU_CB_BOOST 288 bool "Offload RCU callback from real-time kthread" 289 depends on RCU_NOCB_CPU && RCU_BOOST 290 default y if PREEMPT_RT 291 help 292 Use this option to invoke offloaded callbacks as SCHED_FIFO 293 to avoid starvation by heavy SCHED_OTHER background load. 294 Of course, running as SCHED_FIFO during callback floods will 295 cause the rcuo[ps] kthreads to monopolize the CPU for hundreds 296 of milliseconds or more. Therefore, when enabling this option, 297 it is your responsibility to ensure that latency-sensitive 298 tasks either run with higher priority or run on some other CPU. 299 300 Say Y here if you want to set RT priority for offloading kthreads. 301 Say N here if you are building a !PREEMPT_RT kernel and are unsure. 302 303config TASKS_TRACE_RCU_READ_MB 304 bool "Tasks Trace RCU readers use memory barriers in user and idle" 305 depends on RCU_EXPERT && TASKS_TRACE_RCU 306 default PREEMPT_RT || NR_CPUS < 8 307 help 308 Use this option to further reduce the number of IPIs sent 309 to CPUs executing in userspace or idle during tasks trace 310 RCU grace periods. Given that a reasonable setting of 311 the rcupdate.rcu_task_ipi_delay kernel boot parameter 312 eliminates such IPIs for many workloads, proper setting 313 of this Kconfig option is important mostly for aggressive 314 real-time installations and for battery-powered devices, 315 hence the default chosen above. 316 317 Say Y here if you hate IPIs. 318 Say N here if you hate read-side memory barriers. 319 Take the default if you are unsure. 320 321config RCU_LAZY 322 bool "RCU callback lazy invocation functionality" 323 depends on RCU_NOCB_CPU 324 default n 325 help 326 To save power, batch RCU callbacks and flush after delay, memory 327 pressure, or callback list growing too big. 328 329 Requires rcu_nocbs=all to be set. 330 331 Use rcutree.enable_rcu_lazy=0 to turn it off at boot time. 332 333config RCU_LAZY_DEFAULT_OFF 334 bool "Turn RCU lazy invocation off by default" 335 depends on RCU_LAZY 336 default n 337 help 338 Allows building the kernel with CONFIG_RCU_LAZY=y yet keep it default 339 off. Boot time param rcutree.enable_rcu_lazy=1 can be used to switch 340 it back on. 341 342config RCU_DOUBLE_CHECK_CB_TIME 343 bool "RCU callback-batch backup time check" 344 depends on RCU_EXPERT 345 default n 346 help 347 Use this option to provide more precise enforcement of the 348 rcutree.rcu_resched_ns module parameter in situations where 349 a single RCU callback might run for hundreds of microseconds, 350 thus defeating the 32-callback batching used to amortize the 351 cost of the fine-grained but expensive local_clock() function. 352 353 This option rounds rcutree.rcu_resched_ns up to the next 354 jiffy, and overrides the 32-callback batching if this limit 355 is exceeded. 356 357 Say Y here if you need tighter callback-limit enforcement. 358 Say N here if you are unsure. 359 360endmenu # "RCU Subsystem" 361