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 help 99 This option enables a task-based RCU implementation that uses 100 explicit rcu_read_lock_trace() read-side markers, and allows 101 these readers to appear in the idle loop as well as on the CPU 102 hotplug code paths. It can force IPIs on online CPUs, including 103 idle ones, so use with caution. 104 105config RCU_STALL_COMMON 106 def_bool TREE_RCU 107 help 108 This option enables RCU CPU stall code that is common between 109 the TINY and TREE variants of RCU. The purpose is to allow 110 the tiny variants to disable RCU CPU stall warnings, while 111 making these warnings mandatory for the tree variants. 112 113config RCU_NEED_SEGCBLIST 114 def_bool ( TREE_RCU || TREE_SRCU ) 115 116config RCU_FANOUT 117 int "Tree-based hierarchical RCU fanout value" 118 range 2 64 if 64BIT 119 range 2 32 if !64BIT 120 depends on TREE_RCU && RCU_EXPERT 121 default 64 if 64BIT 122 default 32 if !64BIT 123 help 124 This option controls the fanout of hierarchical implementations 125 of RCU, allowing RCU to work efficiently on machines with 126 large numbers of CPUs. This value must be at least the fourth 127 root of NR_CPUS, which allows NR_CPUS to be insanely large. 128 The default value of RCU_FANOUT should be used for production 129 systems, but if you are stress-testing the RCU implementation 130 itself, small RCU_FANOUT values allow you to test large-system 131 code paths on small(er) systems. 132 133 Select a specific number if testing RCU itself. 134 Take the default if unsure. 135 136config RCU_FANOUT_LEAF 137 int "Tree-based hierarchical RCU leaf-level fanout value" 138 range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD 139 range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD 140 range 2 3 if RCU_STRICT_GRACE_PERIOD 141 depends on TREE_RCU && RCU_EXPERT 142 default 16 if !RCU_STRICT_GRACE_PERIOD 143 default 2 if RCU_STRICT_GRACE_PERIOD 144 help 145 This option controls the leaf-level fanout of hierarchical 146 implementations of RCU, and allows trading off cache misses 147 against lock contention. Systems that synchronize their 148 scheduling-clock interrupts for energy-efficiency reasons will 149 want the default because the smaller leaf-level fanout keeps 150 lock contention levels acceptably low. Very large systems 151 (hundreds or thousands of CPUs) will instead want to set this 152 value to the maximum value possible in order to reduce the 153 number of cache misses incurred during RCU's grace-period 154 initialization. These systems tend to run CPU-bound, and thus 155 are not helped by synchronized interrupts, and thus tend to 156 skew them, which reduces lock contention enough that large 157 leaf-level fanouts work well. That said, setting leaf-level 158 fanout to a large number will likely cause problematic 159 lock contention on the leaf-level rcu_node structures unless 160 you boot with the skew_tick kernel parameter. 161 162 Select a specific number if testing RCU itself. 163 164 Select the maximum permissible value for large systems, but 165 please understand that you may also need to set the skew_tick 166 kernel boot parameter to avoid contention on the rcu_node 167 structure's locks. 168 169 Take the default if unsure. 170 171config RCU_FAST_NO_HZ 172 bool "Accelerate last non-dyntick-idle CPU's grace periods" 173 depends on NO_HZ_COMMON && SMP && RCU_EXPERT 174 default n 175 help 176 This option permits CPUs to enter dynticks-idle state even if 177 they have RCU callbacks queued, and prevents RCU from waking 178 these CPUs up more than roughly once every four jiffies (by 179 default, you can adjust this using the rcutree.rcu_idle_gp_delay 180 parameter), thus improving energy efficiency. On the other 181 hand, this option increases the duration of RCU grace periods, 182 for example, slowing down synchronize_rcu(). 183 184 Say Y if energy efficiency is critically important, and you 185 don't care about increased grace-period durations. 186 187 Say N if you are unsure. 188 189config RCU_BOOST 190 bool "Enable RCU priority boosting" 191 depends on RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT 192 default n 193 help 194 This option boosts the priority of preempted RCU readers that 195 block the current preemptible RCU grace period for too long. 196 This option also prevents heavy loads from blocking RCU 197 callback invocation. 198 199 Say Y here if you are working with real-time apps or heavy loads 200 Say N here if you are unsure. 201 202config RCU_BOOST_DELAY 203 int "Milliseconds to delay boosting after RCU grace-period start" 204 range 0 3000 205 depends on RCU_BOOST 206 default 500 207 help 208 This option specifies the time to wait after the beginning of 209 a given grace period before priority-boosting preempted RCU 210 readers blocking that grace period. Note that any RCU reader 211 blocking an expedited RCU grace period is boosted immediately. 212 213 Accept the default if unsure. 214 215config RCU_NOCB_CPU 216 bool "Offload RCU callback processing from boot-selected CPUs" 217 depends on TREE_RCU 218 depends on RCU_EXPERT || NO_HZ_FULL 219 default n 220 help 221 Use this option to reduce OS jitter for aggressive HPC or 222 real-time workloads. It can also be used to offload RCU 223 callback invocation to energy-efficient CPUs in battery-powered 224 asymmetric multiprocessors. 225 226 This option offloads callback invocation from the set of CPUs 227 specified at boot time by the rcu_nocbs parameter. For each 228 such CPU, a kthread ("rcuox/N") will be created to invoke 229 callbacks, where the "N" is the CPU being offloaded, and where 230 the "p" for RCU-preempt (PREEMPTION kernels) and "s" for RCU-sched 231 (!PREEMPTION kernels). Nothing prevents this kthread from running 232 on the specified CPUs, but (1) the kthreads may be preempted 233 between each callback, and (2) affinity or cgroups can be used 234 to force the kthreads to run on whatever set of CPUs is desired. 235 236 Say Y here if you want to help to debug reduced OS jitter. 237 Say N here if you are unsure. 238 239config TASKS_TRACE_RCU_READ_MB 240 bool "Tasks Trace RCU readers use memory barriers in user and idle" 241 depends on RCU_EXPERT 242 default PREEMPT_RT || NR_CPUS < 8 243 help 244 Use this option to further reduce the number of IPIs sent 245 to CPUs executing in userspace or idle during tasks trace 246 RCU grace periods. Given that a reasonable setting of 247 the rcupdate.rcu_task_ipi_delay kernel boot parameter 248 eliminates such IPIs for many workloads, proper setting 249 of this Kconfig option is important mostly for aggressive 250 real-time installations and for battery-powered devices, 251 hence the default chosen above. 252 253 Say Y here if you hate IPIs. 254 Say N here if you hate read-side memory barriers. 255 Take the default if you are unsure. 256 257endmenu # "RCU Subsystem" 258