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 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 select TREE_RCU 21 help 22 This option selects the RCU implementation that is 23 designed for very large SMP systems with hundreds or 24 thousands of CPUs, but for which real-time response 25 is also required. It also scales down nicely to 26 smaller systems. 27 28 Select this option if you are unsure. 29 30config TINY_RCU 31 bool 32 default y if !PREEMPTION && !SMP 33 help 34 This option selects the RCU implementation that is 35 designed for UP systems from which real-time response 36 is not required. This option greatly reduces the 37 memory footprint of RCU. 38 39config RCU_EXPERT 40 bool "Make expert-level adjustments to RCU configuration" 41 default n 42 help 43 This option needs to be enabled if you wish to make 44 expert-level adjustments to RCU configuration. By default, 45 no such adjustments can be made, which has the often-beneficial 46 side-effect of preventing "make oldconfig" from asking you all 47 sorts of detailed questions about how you would like numerous 48 obscure RCU options to be set up. 49 50 Say Y if you need to make expert-level adjustments to RCU. 51 52 Say N if you are unsure. 53 54config SRCU 55 bool 56 help 57 This option selects the sleepable version of RCU. This version 58 permits arbitrary sleeping or blocking within RCU read-side critical 59 sections. 60 61config TINY_SRCU 62 bool 63 default y if SRCU && TINY_RCU 64 help 65 This option selects the single-CPU non-preemptible version of SRCU. 66 67config TREE_SRCU 68 bool 69 default y if SRCU && !TINY_RCU 70 help 71 This option selects the full-fledged version of SRCU. 72 73config TASKS_RCU_GENERIC 74 def_bool TASKS_RCU || TASKS_RUDE_RCU || TASKS_TRACE_RCU 75 select SRCU 76 help 77 This option enables generic infrastructure code supporting 78 task-based RCU implementations. Not for manual selection. 79 80config FORCE_TASKS_RCU 81 bool "Force selection of TASKS_RCU" 82 depends on RCU_EXPERT 83 select TASKS_RCU 84 default n 85 help 86 This option force-enables a task-based RCU implementation 87 that uses only voluntary context switch (not preemption!), 88 idle, and user-mode execution as quiescent states. Not for 89 manual selection in most cases. 90 91config TASKS_RCU 92 bool 93 default n 94 select IRQ_WORK 95 96config TASKS_RUDE_RCU 97 def_bool 0 98 select IRQ_WORK 99 help 100 This option enables a task-based RCU implementation that uses 101 only context switch (including preemption) and user-mode 102 execution as quiescent states. It forces IPIs and context 103 switches on all online CPUs, including idle ones, so use 104 with caution. 105 106config FORCE_TASKS_TRACE_RCU 107 bool "Force selection of Tasks Trace RCU" 108 depends on RCU_EXPERT 109 select TASKS_TRACE_RCU 110 default n 111 help 112 This option enables a task-based RCU implementation that uses 113 explicit rcu_read_lock_trace() read-side markers, and allows 114 these readers to appear in the idle loop as well as on the 115 CPU hotplug code paths. It can force IPIs on online CPUs, 116 including idle ones, so use with caution. Not for manual 117 selection in most cases. 118 119config TASKS_TRACE_RCU 120 bool 121 default n 122 select IRQ_WORK 123 124config RCU_STALL_COMMON 125 def_bool TREE_RCU 126 help 127 This option enables RCU CPU stall code that is common between 128 the TINY and TREE variants of RCU. The purpose is to allow 129 the tiny variants to disable RCU CPU stall warnings, while 130 making these warnings mandatory for the tree variants. 131 132config RCU_NEED_SEGCBLIST 133 def_bool ( TREE_RCU || TREE_SRCU || TASKS_RCU_GENERIC ) 134 135config RCU_FANOUT 136 int "Tree-based hierarchical RCU fanout value" 137 range 2 64 if 64BIT 138 range 2 32 if !64BIT 139 depends on TREE_RCU && RCU_EXPERT 140 default 64 if 64BIT 141 default 32 if !64BIT 142 help 143 This option controls the fanout of hierarchical implementations 144 of RCU, allowing RCU to work efficiently on machines with 145 large numbers of CPUs. This value must be at least the fourth 146 root of NR_CPUS, which allows NR_CPUS to be insanely large. 147 The default value of RCU_FANOUT should be used for production 148 systems, but if you are stress-testing the RCU implementation 149 itself, small RCU_FANOUT values allow you to test large-system 150 code paths on small(er) systems. 151 152 Select a specific number if testing RCU itself. 153 Take the default if unsure. 154 155config RCU_FANOUT_LEAF 156 int "Tree-based hierarchical RCU leaf-level fanout value" 157 range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD 158 range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD 159 range 2 3 if RCU_STRICT_GRACE_PERIOD 160 depends on TREE_RCU && RCU_EXPERT 161 default 16 if !RCU_STRICT_GRACE_PERIOD 162 default 2 if RCU_STRICT_GRACE_PERIOD 163 help 164 This option controls the leaf-level fanout of hierarchical 165 implementations of RCU, and allows trading off cache misses 166 against lock contention. Systems that synchronize their 167 scheduling-clock interrupts for energy-efficiency reasons will 168 want the default because the smaller leaf-level fanout keeps 169 lock contention levels acceptably low. Very large systems 170 (hundreds or thousands of CPUs) will instead want to set this 171 value to the maximum value possible in order to reduce the 172 number of cache misses incurred during RCU's grace-period 173 initialization. These systems tend to run CPU-bound, and thus 174 are not helped by synchronized interrupts, and thus tend to 175 skew them, which reduces lock contention enough that large 176 leaf-level fanouts work well. That said, setting leaf-level 177 fanout to a large number will likely cause problematic 178 lock contention on the leaf-level rcu_node structures unless 179 you boot with the skew_tick kernel parameter. 180 181 Select a specific number if testing RCU itself. 182 183 Select the maximum permissible value for large systems, but 184 please understand that you may also need to set the skew_tick 185 kernel boot parameter to avoid contention on the rcu_node 186 structure's locks. 187 188 Take the default if unsure. 189 190config RCU_BOOST 191 bool "Enable RCU priority boosting" 192 depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT 193 default y if PREEMPT_RT 194 help 195 This option boosts the priority of preempted RCU readers that 196 block the current preemptible RCU grace period for too long. 197 This option also prevents heavy loads from blocking RCU 198 callback invocation. 199 200 Say Y here if you are working with real-time apps or heavy loads 201 Say N here if you are unsure. 202 203config RCU_BOOST_DELAY 204 int "Milliseconds to delay boosting after RCU grace-period start" 205 range 0 3000 206 depends on RCU_BOOST 207 default 500 208 help 209 This option specifies the time to wait after the beginning of 210 a given grace period before priority-boosting preempted RCU 211 readers blocking that grace period. Note that any RCU reader 212 blocking an expedited RCU grace period is boosted immediately. 213 214 Accept the default if unsure. 215 216config RCU_NOCB_CPU 217 bool "Offload RCU callback processing from boot-selected CPUs" 218 depends on TREE_RCU 219 depends on RCU_EXPERT || NO_HZ_FULL 220 default n 221 help 222 Use this option to reduce OS jitter for aggressive HPC or 223 real-time workloads. It can also be used to offload RCU 224 callback invocation to energy-efficient CPUs in battery-powered 225 asymmetric multiprocessors. The price of this reduced jitter 226 is that the overhead of call_rcu() increases and that some 227 workloads will incur significant increases in context-switch 228 rates. 229 230 This option offloads callback invocation from the set of CPUs 231 specified at boot time by the rcu_nocbs parameter. For each 232 such CPU, a kthread ("rcuox/N") will be created to invoke 233 callbacks, where the "N" is the CPU being offloaded, and where 234 the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for 235 RCU-sched (!PREEMPTION kernels). Nothing prevents this kthread 236 from running on the specified CPUs, but (1) the kthreads may be 237 preempted between each callback, and (2) affinity or cgroups can 238 be used to force the kthreads to run on whatever set of CPUs is 239 desired. 240 241 Say Y here if you need reduced OS jitter, despite added overhead. 242 Say N here if you are unsure. 243 244config TASKS_TRACE_RCU_READ_MB 245 bool "Tasks Trace RCU readers use memory barriers in user and idle" 246 depends on RCU_EXPERT && TASKS_TRACE_RCU 247 default PREEMPT_RT || NR_CPUS < 8 248 help 249 Use this option to further reduce the number of IPIs sent 250 to CPUs executing in userspace or idle during tasks trace 251 RCU grace periods. Given that a reasonable setting of 252 the rcupdate.rcu_task_ipi_delay kernel boot parameter 253 eliminates such IPIs for many workloads, proper setting 254 of this Kconfig option is important mostly for aggressive 255 real-time installations and for battery-powered devices, 256 hence the default chosen above. 257 258 Say Y here if you hate IPIs. 259 Say N here if you hate read-side memory barriers. 260 Take the default if you are unsure. 261 262endmenu # "RCU Subsystem" 263