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 def_bool PREEMPT 73 select SRCU 74 help 75 This option enables a task-based RCU implementation that uses 76 only voluntary context switch (not preemption!), idle, and 77 user-mode execution as quiescent states. 78 79config RCU_STALL_COMMON 80 def_bool ( TREE_RCU || PREEMPT_RCU ) 81 help 82 This option enables RCU CPU stall code that is common between 83 the TINY and TREE variants of RCU. The purpose is to allow 84 the tiny variants to disable RCU CPU stall warnings, while 85 making these warnings mandatory for the tree variants. 86 87config RCU_NEED_SEGCBLIST 88 def_bool ( TREE_RCU || PREEMPT_RCU || TREE_SRCU ) 89 90config CONTEXT_TRACKING 91 bool 92 93config CONTEXT_TRACKING_FORCE 94 bool "Force context tracking" 95 depends on CONTEXT_TRACKING 96 default y if !NO_HZ_FULL 97 help 98 The major pre-requirement for full dynticks to work is to 99 support the context tracking subsystem. But there are also 100 other dependencies to provide in order to make the full 101 dynticks working. 102 103 This option stands for testing when an arch implements the 104 context tracking backend but doesn't yet fullfill all the 105 requirements to make the full dynticks feature working. 106 Without the full dynticks, there is no way to test the support 107 for context tracking and the subsystems that rely on it: RCU 108 userspace extended quiescent state and tickless cputime 109 accounting. This option copes with the absence of the full 110 dynticks subsystem by forcing the context tracking on all 111 CPUs in the system. 112 113 Say Y only if you're working on the development of an 114 architecture backend for the context tracking. 115 116 Say N otherwise, this option brings an overhead that you 117 don't want in production. 118 119 120config RCU_FANOUT 121 int "Tree-based hierarchical RCU fanout value" 122 range 2 64 if 64BIT 123 range 2 32 if !64BIT 124 depends on (TREE_RCU || PREEMPT_RCU) && RCU_EXPERT 125 default 64 if 64BIT 126 default 32 if !64BIT 127 help 128 This option controls the fanout of hierarchical implementations 129 of RCU, allowing RCU to work efficiently on machines with 130 large numbers of CPUs. This value must be at least the fourth 131 root of NR_CPUS, which allows NR_CPUS to be insanely large. 132 The default value of RCU_FANOUT should be used for production 133 systems, but if you are stress-testing the RCU implementation 134 itself, small RCU_FANOUT values allow you to test large-system 135 code paths on small(er) systems. 136 137 Select a specific number if testing RCU itself. 138 Take the default if unsure. 139 140config RCU_FANOUT_LEAF 141 int "Tree-based hierarchical RCU leaf-level fanout value" 142 range 2 64 if 64BIT 143 range 2 32 if !64BIT 144 depends on (TREE_RCU || PREEMPT_RCU) && RCU_EXPERT 145 default 16 146 help 147 This option controls the leaf-level fanout of hierarchical 148 implementations of RCU, and allows trading off cache misses 149 against lock contention. Systems that synchronize their 150 scheduling-clock interrupts for energy-efficiency reasons will 151 want the default because the smaller leaf-level fanout keeps 152 lock contention levels acceptably low. Very large systems 153 (hundreds or thousands of CPUs) will instead want to set this 154 value to the maximum value possible in order to reduce the 155 number of cache misses incurred during RCU's grace-period 156 initialization. These systems tend to run CPU-bound, and thus 157 are not helped by synchronized interrupts, and thus tend to 158 skew them, which reduces lock contention enough that large 159 leaf-level fanouts work well. That said, setting leaf-level 160 fanout to a large number will likely cause problematic 161 lock contention on the leaf-level rcu_node structures unless 162 you boot with the skew_tick kernel parameter. 163 164 Select a specific number if testing RCU itself. 165 166 Select the maximum permissible value for large systems, but 167 please understand that you may also need to set the skew_tick 168 kernel boot parameter to avoid contention on the rcu_node 169 structure's locks. 170 171 Take the default if unsure. 172 173config RCU_FAST_NO_HZ 174 bool "Accelerate last non-dyntick-idle CPU's grace periods" 175 depends on NO_HZ_COMMON && SMP && RCU_EXPERT 176 default n 177 help 178 This option permits CPUs to enter dynticks-idle state even if 179 they have RCU callbacks queued, and prevents RCU from waking 180 these CPUs up more than roughly once every four jiffies (by 181 default, you can adjust this using the rcutree.rcu_idle_gp_delay 182 parameter), thus improving energy efficiency. On the other 183 hand, this option increases the duration of RCU grace periods, 184 for example, slowing down synchronize_rcu(). 185 186 Say Y if energy efficiency is critically important, and you 187 don't care about increased grace-period durations. 188 189 Say N if you are unsure. 190 191config RCU_BOOST 192 bool "Enable RCU priority boosting" 193 depends on RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT 194 default n 195 help 196 This option boosts the priority of preempted RCU readers that 197 block the current preemptible RCU grace period for too long. 198 This option also prevents heavy loads from blocking RCU 199 callback invocation for all flavors of RCU. 200 201 Say Y here if you are working with real-time apps or heavy loads 202 Say N here if you are unsure. 203 204config RCU_BOOST_DELAY 205 int "Milliseconds to delay boosting after RCU grace-period start" 206 range 0 3000 207 depends on RCU_BOOST 208 default 500 209 help 210 This option specifies the time to wait after the beginning of 211 a given grace period before priority-boosting preempted RCU 212 readers blocking that grace period. Note that any RCU reader 213 blocking an expedited RCU grace period is boosted immediately. 214 215 Accept the default if unsure. 216 217config RCU_NOCB_CPU 218 bool "Offload RCU callback processing from boot-selected CPUs" 219 depends on TREE_RCU || PREEMPT_RCU 220 depends on RCU_EXPERT || NO_HZ_FULL 221 default n 222 help 223 Use this option to reduce OS jitter for aggressive HPC or 224 real-time workloads. It can also be used to offload RCU 225 callback invocation to energy-efficient CPUs in battery-powered 226 asymmetric multiprocessors. 227 228 This option offloads callback invocation from the set of 229 CPUs specified at boot time by the rcu_nocbs parameter. 230 For each such CPU, a kthread ("rcuox/N") will be created to 231 invoke callbacks, where the "N" is the CPU being offloaded, 232 and where the "x" is "b" for RCU-bh, "p" for RCU-preempt, and 233 "s" for RCU-sched. Nothing prevents this kthread from running 234 on the specified CPUs, but (1) the kthreads may be preempted 235 between each callback, and (2) affinity or cgroups can be used 236 to force the kthreads to run on whatever set of CPUs is desired. 237 238 Say Y here if you want to help to debug reduced OS jitter. 239 Say N here if you are unsure. 240 241endmenu # "RCU Subsystem" 242