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 !PREEMPT && 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 PREEMPT 20 help 21 This option selects the RCU implementation that is 22 designed for very large SMP systems with hundreds or 23 thousands of CPUs, but for which real-time response 24 is also required. It also scales down nicely to 25 smaller systems. 26 27 Select this option if you are unsure. 28 29config TINY_RCU 30 bool 31 default y if !PREEMPT && !SMP 32 help 33 This option selects the RCU implementation that is 34 designed for UP systems from which real-time response 35 is not required. This option greatly reduces the 36 memory footprint of RCU. 37 38config RCU_EXPERT 39 bool "Make expert-level adjustments to RCU configuration" 40 default n 41 help 42 This option needs to be enabled if you wish to make 43 expert-level adjustments to RCU configuration. By default, 44 no such adjustments can be made, which has the often-beneficial 45 side-effect of preventing "make oldconfig" from asking you all 46 sorts of detailed questions about how you would like numerous 47 obscure RCU options to be set up. 48 49 Say Y if you need to make expert-level adjustments to RCU. 50 51 Say N if you are unsure. 52 53config SRCU 54 bool 55 help 56 This option selects the sleepable version of RCU. This version 57 permits arbitrary sleeping or blocking within RCU read-side critical 58 sections. 59 60config TINY_SRCU 61 bool 62 default y if SRCU && TINY_RCU 63 help 64 This option selects the single-CPU non-preemptible version of SRCU. 65 66config TREE_SRCU 67 bool 68 default y if SRCU && !TINY_RCU 69 help 70 This option selects the full-fledged version of SRCU. 71 72config TASKS_RCU 73 def_bool PREEMPT 74 select SRCU 75 help 76 This option enables a task-based RCU implementation that uses 77 only voluntary context switch (not preemption!), idle, and 78 user-mode execution as quiescent states. 79 80config RCU_STALL_COMMON 81 def_bool ( TREE_RCU || PREEMPT_RCU ) 82 help 83 This option enables RCU CPU stall code that is common between 84 the TINY and TREE variants of RCU. The purpose is to allow 85 the tiny variants to disable RCU CPU stall warnings, while 86 making these warnings mandatory for the tree variants. 87 88config RCU_NEED_SEGCBLIST 89 def_bool ( TREE_RCU || PREEMPT_RCU || TREE_SRCU ) 90 91config RCU_FANOUT 92 int "Tree-based hierarchical RCU fanout value" 93 range 2 64 if 64BIT 94 range 2 32 if !64BIT 95 depends on (TREE_RCU || PREEMPT_RCU) && RCU_EXPERT 96 default 64 if 64BIT 97 default 32 if !64BIT 98 help 99 This option controls the fanout of hierarchical implementations 100 of RCU, allowing RCU to work efficiently on machines with 101 large numbers of CPUs. This value must be at least the fourth 102 root of NR_CPUS, which allows NR_CPUS to be insanely large. 103 The default value of RCU_FANOUT should be used for production 104 systems, but if you are stress-testing the RCU implementation 105 itself, small RCU_FANOUT values allow you to test large-system 106 code paths on small(er) systems. 107 108 Select a specific number if testing RCU itself. 109 Take the default if unsure. 110 111config RCU_FANOUT_LEAF 112 int "Tree-based hierarchical RCU leaf-level fanout value" 113 range 2 64 if 64BIT 114 range 2 32 if !64BIT 115 depends on (TREE_RCU || PREEMPT_RCU) && RCU_EXPERT 116 default 16 117 help 118 This option controls the leaf-level fanout of hierarchical 119 implementations of RCU, and allows trading off cache misses 120 against lock contention. Systems that synchronize their 121 scheduling-clock interrupts for energy-efficiency reasons will 122 want the default because the smaller leaf-level fanout keeps 123 lock contention levels acceptably low. Very large systems 124 (hundreds or thousands of CPUs) will instead want to set this 125 value to the maximum value possible in order to reduce the 126 number of cache misses incurred during RCU's grace-period 127 initialization. These systems tend to run CPU-bound, and thus 128 are not helped by synchronized interrupts, and thus tend to 129 skew them, which reduces lock contention enough that large 130 leaf-level fanouts work well. That said, setting leaf-level 131 fanout to a large number will likely cause problematic 132 lock contention on the leaf-level rcu_node structures unless 133 you boot with the skew_tick kernel parameter. 134 135 Select a specific number if testing RCU itself. 136 137 Select the maximum permissible value for large systems, but 138 please understand that you may also need to set the skew_tick 139 kernel boot parameter to avoid contention on the rcu_node 140 structure's locks. 141 142 Take the default if unsure. 143 144config RCU_FAST_NO_HZ 145 bool "Accelerate last non-dyntick-idle CPU's grace periods" 146 depends on NO_HZ_COMMON && SMP && RCU_EXPERT 147 default n 148 help 149 This option permits CPUs to enter dynticks-idle state even if 150 they have RCU callbacks queued, and prevents RCU from waking 151 these CPUs up more than roughly once every four jiffies (by 152 default, you can adjust this using the rcutree.rcu_idle_gp_delay 153 parameter), thus improving energy efficiency. On the other 154 hand, this option increases the duration of RCU grace periods, 155 for example, slowing down synchronize_rcu(). 156 157 Say Y if energy efficiency is critically important, and you 158 don't care about increased grace-period durations. 159 160 Say N if you are unsure. 161 162config RCU_BOOST 163 bool "Enable RCU priority boosting" 164 depends on RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT 165 default n 166 help 167 This option boosts the priority of preempted RCU readers that 168 block the current preemptible RCU grace period for too long. 169 This option also prevents heavy loads from blocking RCU 170 callback invocation. 171 172 Say Y here if you are working with real-time apps or heavy loads 173 Say N here if you are unsure. 174 175config RCU_BOOST_DELAY 176 int "Milliseconds to delay boosting after RCU grace-period start" 177 range 0 3000 178 depends on RCU_BOOST 179 default 500 180 help 181 This option specifies the time to wait after the beginning of 182 a given grace period before priority-boosting preempted RCU 183 readers blocking that grace period. Note that any RCU reader 184 blocking an expedited RCU grace period is boosted immediately. 185 186 Accept the default if unsure. 187 188config RCU_NOCB_CPU 189 bool "Offload RCU callback processing from boot-selected CPUs" 190 depends on TREE_RCU || PREEMPT_RCU 191 depends on RCU_EXPERT || NO_HZ_FULL 192 default n 193 help 194 Use this option to reduce OS jitter for aggressive HPC or 195 real-time workloads. It can also be used to offload RCU 196 callback invocation to energy-efficient CPUs in battery-powered 197 asymmetric multiprocessors. 198 199 This option offloads callback invocation from the set of CPUs 200 specified at boot time by the rcu_nocbs parameter. For each 201 such CPU, a kthread ("rcuox/N") will be created to invoke 202 callbacks, where the "N" is the CPU being offloaded, and where 203 the "p" for RCU-preempt (PREEMPT kernels) and "s" for RCU-sched 204 (!PREEMPT kernels). Nothing prevents this kthread from running 205 on the specified CPUs, but (1) the kthreads may be preempted 206 between each callback, and (2) affinity or cgroups can be used 207 to force the kthreads to run on whatever set of CPUs is desired. 208 209 Say Y here if you want to help to debug reduced OS jitter. 210 Say N here if you are unsure. 211 212endmenu # "RCU Subsystem" 213