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 TASKS_RCU 81 def_bool 0 82 select IRQ_WORK 83 help 84 This option enables a task-based RCU implementation that uses 85 only voluntary context switch (not preemption!), idle, and 86 user-mode execution as quiescent states. Not for manual selection. 87 88config TASKS_RUDE_RCU 89 def_bool 0 90 select IRQ_WORK 91 help 92 This option enables a task-based RCU implementation that uses 93 only context switch (including preemption) and user-mode 94 execution as quiescent states. It forces IPIs and context 95 switches on all online CPUs, including idle ones, so use 96 with caution. 97 98config TASKS_TRACE_RCU 99 def_bool 0 100 select IRQ_WORK 101 help 102 This option enables a task-based RCU implementation that uses 103 explicit rcu_read_lock_trace() read-side markers, and allows 104 these readers to appear in the idle loop as well as on the CPU 105 hotplug code paths. It can force IPIs on online CPUs, including 106 idle ones, so use with caution. 107 108config RCU_STALL_COMMON 109 def_bool TREE_RCU 110 help 111 This option enables RCU CPU stall code that is common between 112 the TINY and TREE variants of RCU. The purpose is to allow 113 the tiny variants to disable RCU CPU stall warnings, while 114 making these warnings mandatory for the tree variants. 115 116config RCU_NEED_SEGCBLIST 117 def_bool ( TREE_RCU || TREE_SRCU || TASKS_RCU_GENERIC ) 118 119config RCU_FANOUT 120 int "Tree-based hierarchical RCU fanout value" 121 range 2 64 if 64BIT 122 range 2 32 if !64BIT 123 depends on TREE_RCU && RCU_EXPERT 124 default 64 if 64BIT 125 default 32 if !64BIT 126 help 127 This option controls the fanout of hierarchical implementations 128 of RCU, allowing RCU to work efficiently on machines with 129 large numbers of CPUs. This value must be at least the fourth 130 root of NR_CPUS, which allows NR_CPUS to be insanely large. 131 The default value of RCU_FANOUT should be used for production 132 systems, but if you are stress-testing the RCU implementation 133 itself, small RCU_FANOUT values allow you to test large-system 134 code paths on small(er) systems. 135 136 Select a specific number if testing RCU itself. 137 Take the default if unsure. 138 139config RCU_FANOUT_LEAF 140 int "Tree-based hierarchical RCU leaf-level fanout value" 141 range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD 142 range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD 143 range 2 3 if RCU_STRICT_GRACE_PERIOD 144 depends on TREE_RCU && RCU_EXPERT 145 default 16 if !RCU_STRICT_GRACE_PERIOD 146 default 2 if RCU_STRICT_GRACE_PERIOD 147 help 148 This option controls the leaf-level fanout of hierarchical 149 implementations of RCU, and allows trading off cache misses 150 against lock contention. Systems that synchronize their 151 scheduling-clock interrupts for energy-efficiency reasons will 152 want the default because the smaller leaf-level fanout keeps 153 lock contention levels acceptably low. Very large systems 154 (hundreds or thousands of CPUs) will instead want to set this 155 value to the maximum value possible in order to reduce the 156 number of cache misses incurred during RCU's grace-period 157 initialization. These systems tend to run CPU-bound, and thus 158 are not helped by synchronized interrupts, and thus tend to 159 skew them, which reduces lock contention enough that large 160 leaf-level fanouts work well. That said, setting leaf-level 161 fanout to a large number will likely cause problematic 162 lock contention on the leaf-level rcu_node structures unless 163 you boot with the skew_tick kernel parameter. 164 165 Select a specific number if testing RCU itself. 166 167 Select the maximum permissible value for large systems, but 168 please understand that you may also need to set the skew_tick 169 kernel boot parameter to avoid contention on the rcu_node 170 structure's locks. 171 172 Take the default if unsure. 173 174config RCU_BOOST 175 bool "Enable RCU priority boosting" 176 depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT 177 default y if PREEMPT_RT 178 help 179 This option boosts the priority of preempted RCU readers that 180 block the current preemptible RCU grace period for too long. 181 This option also prevents heavy loads from blocking RCU 182 callback invocation. 183 184 Say Y here if you are working with real-time apps or heavy loads 185 Say N here if you are unsure. 186 187config RCU_BOOST_DELAY 188 int "Milliseconds to delay boosting after RCU grace-period start" 189 range 0 3000 190 depends on RCU_BOOST 191 default 500 192 help 193 This option specifies the time to wait after the beginning of 194 a given grace period before priority-boosting preempted RCU 195 readers blocking that grace period. Note that any RCU reader 196 blocking an expedited RCU grace period is boosted immediately. 197 198 Accept the default if unsure. 199 200config RCU_NOCB_CPU 201 bool "Offload RCU callback processing from boot-selected CPUs" 202 depends on TREE_RCU 203 depends on RCU_EXPERT || NO_HZ_FULL 204 default n 205 help 206 Use this option to reduce OS jitter for aggressive HPC or 207 real-time workloads. It can also be used to offload RCU 208 callback invocation to energy-efficient CPUs in battery-powered 209 asymmetric multiprocessors. The price of this reduced jitter 210 is that the overhead of call_rcu() increases and that some 211 workloads will incur significant increases in context-switch 212 rates. 213 214 This option offloads callback invocation from the set of CPUs 215 specified at boot time by the rcu_nocbs parameter. For each 216 such CPU, a kthread ("rcuox/N") will be created to invoke 217 callbacks, where the "N" is the CPU being offloaded, and where 218 the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for 219 RCU-sched (!PREEMPTION kernels). Nothing prevents this kthread 220 from running on the specified CPUs, but (1) the kthreads may be 221 preempted between each callback, and (2) affinity or cgroups can 222 be used to force the kthreads to run on whatever set of CPUs is 223 desired. 224 225 Say Y here if you need reduced OS jitter, despite added overhead. 226 Say N here if you are unsure. 227 228config TASKS_TRACE_RCU_READ_MB 229 bool "Tasks Trace RCU readers use memory barriers in user and idle" 230 depends on RCU_EXPERT 231 default PREEMPT_RT || NR_CPUS < 8 232 help 233 Use this option to further reduce the number of IPIs sent 234 to CPUs executing in userspace or idle during tasks trace 235 RCU grace periods. Given that a reasonable setting of 236 the rcupdate.rcu_task_ipi_delay kernel boot parameter 237 eliminates such IPIs for many workloads, proper setting 238 of this Kconfig option is important mostly for aggressive 239 real-time installations and for battery-powered devices, 240 hence the default chosen above. 241 242 Say Y here if you hate IPIs. 243 Say N here if you hate read-side memory barriers. 244 Take the default if you are unsure. 245 246endmenu # "RCU Subsystem" 247