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 PREEMPTION 82 help 83 This option enables a task-based RCU implementation that uses 84 only voluntary context switch (not preemption!), idle, and 85 user-mode execution as quiescent states. Not for manual selection. 86 87config TASKS_RUDE_RCU 88 def_bool 0 89 help 90 This option enables a task-based RCU implementation that uses 91 only context switch (including preemption) and user-mode 92 execution as quiescent states. It forces IPIs and context 93 switches on all online CPUs, including idle ones, so use 94 with caution. 95 96config TASKS_TRACE_RCU 97 def_bool 0 98 select IRQ_WORK 99 help 100 This option enables a task-based RCU implementation that uses 101 explicit rcu_read_lock_trace() read-side markers, and allows 102 these readers to appear in the idle loop as well as on the CPU 103 hotplug code paths. It can force IPIs on online CPUs, including 104 idle ones, so use with caution. 105 106config RCU_STALL_COMMON 107 def_bool TREE_RCU 108 help 109 This option enables RCU CPU stall code that is common between 110 the TINY and TREE variants of RCU. The purpose is to allow 111 the tiny variants to disable RCU CPU stall warnings, while 112 making these warnings mandatory for the tree variants. 113 114config RCU_NEED_SEGCBLIST 115 def_bool ( TREE_RCU || TREE_SRCU ) 116 117config RCU_FANOUT 118 int "Tree-based hierarchical RCU fanout value" 119 range 2 64 if 64BIT 120 range 2 32 if !64BIT 121 depends on TREE_RCU && RCU_EXPERT 122 default 64 if 64BIT 123 default 32 if !64BIT 124 help 125 This option controls the fanout of hierarchical implementations 126 of RCU, allowing RCU to work efficiently on machines with 127 large numbers of CPUs. This value must be at least the fourth 128 root of NR_CPUS, which allows NR_CPUS to be insanely large. 129 The default value of RCU_FANOUT should be used for production 130 systems, but if you are stress-testing the RCU implementation 131 itself, small RCU_FANOUT values allow you to test large-system 132 code paths on small(er) systems. 133 134 Select a specific number if testing RCU itself. 135 Take the default if unsure. 136 137config RCU_FANOUT_LEAF 138 int "Tree-based hierarchical RCU leaf-level fanout value" 139 range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD 140 range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD 141 range 2 3 if RCU_STRICT_GRACE_PERIOD 142 depends on TREE_RCU && RCU_EXPERT 143 default 16 if !RCU_STRICT_GRACE_PERIOD 144 default 2 if RCU_STRICT_GRACE_PERIOD 145 help 146 This option controls the leaf-level fanout of hierarchical 147 implementations of RCU, and allows trading off cache misses 148 against lock contention. Systems that synchronize their 149 scheduling-clock interrupts for energy-efficiency reasons will 150 want the default because the smaller leaf-level fanout keeps 151 lock contention levels acceptably low. Very large systems 152 (hundreds or thousands of CPUs) will instead want to set this 153 value to the maximum value possible in order to reduce the 154 number of cache misses incurred during RCU's grace-period 155 initialization. These systems tend to run CPU-bound, and thus 156 are not helped by synchronized interrupts, and thus tend to 157 skew them, which reduces lock contention enough that large 158 leaf-level fanouts work well. That said, setting leaf-level 159 fanout to a large number will likely cause problematic 160 lock contention on the leaf-level rcu_node structures unless 161 you boot with the skew_tick kernel parameter. 162 163 Select a specific number if testing RCU itself. 164 165 Select the maximum permissible value for large systems, but 166 please understand that you may also need to set the skew_tick 167 kernel boot parameter to avoid contention on the rcu_node 168 structure's locks. 169 170 Take the default if unsure. 171 172config RCU_FAST_NO_HZ 173 bool "Accelerate last non-dyntick-idle CPU's grace periods" 174 depends on NO_HZ_COMMON && SMP && RCU_EXPERT 175 default n 176 help 177 This option permits CPUs to enter dynticks-idle state even if 178 they have RCU callbacks queued, and prevents RCU from waking 179 these CPUs up more than roughly once every four jiffies (by 180 default, you can adjust this using the rcutree.rcu_idle_gp_delay 181 parameter), thus improving energy efficiency. On the other 182 hand, this option increases the duration of RCU grace periods, 183 for example, slowing down synchronize_rcu(). 184 185 Say Y if energy efficiency is critically important, and you 186 don't care about increased grace-period durations. 187 188 Say N if you are 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 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