1# 2# RCU-related configuration options 3# 4 5menu "RCU Subsystem" 6 7config TREE_RCU 8 bool 9 default y if !PREEMPT && SMP 10 help 11 This option selects the RCU implementation that is 12 designed for very large SMP system with hundreds or 13 thousands of CPUs. It also scales down nicely to 14 smaller systems. 15 16config PREEMPT_RCU 17 bool 18 default y if PREEMPT 19 help 20 This option selects the RCU implementation that is 21 designed for very large SMP systems with hundreds or 22 thousands of CPUs, but for which real-time response 23 is also required. It also scales down nicely to 24 smaller systems. 25 26 Select this option if you are unsure. 27 28config TINY_RCU 29 bool 30 default y if !PREEMPT && !SMP 31 help 32 This option selects the RCU implementation that is 33 designed for UP systems from which real-time response 34 is not required. This option greatly reduces the 35 memory footprint of RCU. 36 37config RCU_EXPERT 38 bool "Make expert-level adjustments to RCU configuration" 39 default n 40 help 41 This option needs to be enabled if you wish to make 42 expert-level adjustments to RCU configuration. By default, 43 no such adjustments can be made, which has the often-beneficial 44 side-effect of preventing "make oldconfig" from asking you all 45 sorts of detailed questions about how you would like numerous 46 obscure RCU options to be set up. 47 48 Say Y if you need to make expert-level adjustments to RCU. 49 50 Say N if you are unsure. 51 52config SRCU 53 bool 54 help 55 This option selects the sleepable version of RCU. This version 56 permits arbitrary sleeping or blocking within RCU read-side critical 57 sections. 58 59config TINY_SRCU 60 bool 61 default y if SRCU && TINY_RCU 62 help 63 This option selects the single-CPU non-preemptible version of SRCU. 64 65config TREE_SRCU 66 bool 67 default y if SRCU && !TINY_RCU 68 help 69 This option selects the full-fledged version of SRCU. 70 71config TASKS_RCU 72 bool 73 default n 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 CONTEXT_TRACKING 92 bool 93 94config CONTEXT_TRACKING_FORCE 95 bool "Force context tracking" 96 depends on CONTEXT_TRACKING 97 default y if !NO_HZ_FULL 98 help 99 The major pre-requirement for full dynticks to work is to 100 support the context tracking subsystem. But there are also 101 other dependencies to provide in order to make the full 102 dynticks working. 103 104 This option stands for testing when an arch implements the 105 context tracking backend but doesn't yet fullfill all the 106 requirements to make the full dynticks feature working. 107 Without the full dynticks, there is no way to test the support 108 for context tracking and the subsystems that rely on it: RCU 109 userspace extended quiescent state and tickless cputime 110 accounting. This option copes with the absence of the full 111 dynticks subsystem by forcing the context tracking on all 112 CPUs in the system. 113 114 Say Y only if you're working on the development of an 115 architecture backend for the context tracking. 116 117 Say N otherwise, this option brings an overhead that you 118 don't want in production. 119 120 121config RCU_FANOUT 122 int "Tree-based hierarchical RCU fanout value" 123 range 2 64 if 64BIT 124 range 2 32 if !64BIT 125 depends on (TREE_RCU || PREEMPT_RCU) && RCU_EXPERT 126 default 64 if 64BIT 127 default 32 if !64BIT 128 help 129 This option controls the fanout of hierarchical implementations 130 of RCU, allowing RCU to work efficiently on machines with 131 large numbers of CPUs. This value must be at least the fourth 132 root of NR_CPUS, which allows NR_CPUS to be insanely large. 133 The default value of RCU_FANOUT should be used for production 134 systems, but if you are stress-testing the RCU implementation 135 itself, small RCU_FANOUT values allow you to test large-system 136 code paths on small(er) systems. 137 138 Select a specific number if testing RCU itself. 139 Take the default if unsure. 140 141config RCU_FANOUT_LEAF 142 int "Tree-based hierarchical RCU leaf-level fanout value" 143 range 2 64 if 64BIT 144 range 2 32 if !64BIT 145 depends on (TREE_RCU || PREEMPT_RCU) && RCU_EXPERT 146 default 16 147 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_FAST_NO_HZ 175 bool "Accelerate last non-dyntick-idle CPU's grace periods" 176 depends on NO_HZ_COMMON && SMP && RCU_EXPERT 177 default n 178 help 179 This option permits CPUs to enter dynticks-idle state even if 180 they have RCU callbacks queued, and prevents RCU from waking 181 these CPUs up more than roughly once every four jiffies (by 182 default, you can adjust this using the rcutree.rcu_idle_gp_delay 183 parameter), thus improving energy efficiency. On the other 184 hand, this option increases the duration of RCU grace periods, 185 for example, slowing down synchronize_rcu(). 186 187 Say Y if energy efficiency is critically important, and you 188 don't care about increased grace-period durations. 189 190 Say N if you are unsure. 191 192config RCU_BOOST 193 bool "Enable RCU priority boosting" 194 depends on RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT 195 default n 196 help 197 This option boosts the priority of preempted RCU readers that 198 block the current preemptible RCU grace period for too long. 199 This option also prevents heavy loads from blocking RCU 200 callback invocation for all flavors of RCU. 201 202 Say Y here if you are working with real-time apps or heavy loads 203 Say N here if you are unsure. 204 205config RCU_BOOST_DELAY 206 int "Milliseconds to delay boosting after RCU grace-period start" 207 range 0 3000 208 depends on RCU_BOOST 209 default 500 210 help 211 This option specifies the time to wait after the beginning of 212 a given grace period before priority-boosting preempted RCU 213 readers blocking that grace period. Note that any RCU reader 214 blocking an expedited RCU grace period is boosted immediately. 215 216 Accept the default if unsure. 217 218config RCU_NOCB_CPU 219 bool "Offload RCU callback processing from boot-selected CPUs" 220 depends on TREE_RCU || PREEMPT_RCU 221 depends on RCU_EXPERT || NO_HZ_FULL 222 default n 223 help 224 Use this option to reduce OS jitter for aggressive HPC or 225 real-time workloads. It can also be used to offload RCU 226 callback invocation to energy-efficient CPUs in battery-powered 227 asymmetric multiprocessors. 228 229 This option offloads callback invocation from the set of 230 CPUs specified at boot time by the rcu_nocbs parameter. 231 For each such CPU, a kthread ("rcuox/N") will be created to 232 invoke callbacks, where the "N" is the CPU being offloaded, 233 and where the "x" is "b" for RCU-bh, "p" for RCU-preempt, and 234 "s" for RCU-sched. Nothing prevents this kthread from running 235 on the specified CPUs, but (1) the kthreads may be preempted 236 between each callback, and (2) affinity or cgroups can be used 237 to force the kthreads to run on whatever set of CPUs is desired. 238 239 Say Y here if you want to help to debug reduced OS jitter. 240 Say N here if you are unsure. 241 242endmenu # "RCU Subsystem" 243