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