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 317endmenu # "RCU Subsystem" 318