2 # RCU-related configuration options
9 default y if !PREEMPT && SMP
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
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
26 Select this option if you are unsure.
30 default y if !PREEMPT && !SMP
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.
38 bool "Make expert-level adjustments to RCU configuration"
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.
48 Say Y if you need to make expert-level adjustments to RCU.
50 Say N if you are unsure.
55 This option selects the sleepable version of RCU. This version
56 permits arbitrary sleeping or blocking within RCU read-side critical
61 default y if SRCU && TINY_RCU
63 This option selects the single-CPU non-preemptible version of SRCU.
67 default y if SRCU && !TINY_RCU
69 This option selects the full-fledged version of SRCU.
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.
79 config RCU_STALL_COMMON
80 def_bool ( TREE_RCU || PREEMPT_RCU )
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.
87 config RCU_NEED_SEGCBLIST
88 def_bool ( TREE_RCU || PREEMPT_RCU || TREE_SRCU )
91 int "Tree-based hierarchical RCU fanout value"
94 depends on (TREE_RCU || PREEMPT_RCU) && RCU_EXPERT
98 This option controls the fanout of hierarchical implementations
99 of RCU, allowing RCU to work efficiently on machines with
100 large numbers of CPUs. This value must be at least the fourth
101 root of NR_CPUS, which allows NR_CPUS to be insanely large.
102 The default value of RCU_FANOUT should be used for production
103 systems, but if you are stress-testing the RCU implementation
104 itself, small RCU_FANOUT values allow you to test large-system
105 code paths on small(er) systems.
107 Select a specific number if testing RCU itself.
108 Take the default if unsure.
110 config RCU_FANOUT_LEAF
111 int "Tree-based hierarchical RCU leaf-level fanout value"
114 depends on (TREE_RCU || PREEMPT_RCU) && RCU_EXPERT
117 This option controls the leaf-level fanout of hierarchical
118 implementations of RCU, and allows trading off cache misses
119 against lock contention. Systems that synchronize their
120 scheduling-clock interrupts for energy-efficiency reasons will
121 want the default because the smaller leaf-level fanout keeps
122 lock contention levels acceptably low. Very large systems
123 (hundreds or thousands of CPUs) will instead want to set this
124 value to the maximum value possible in order to reduce the
125 number of cache misses incurred during RCU's grace-period
126 initialization. These systems tend to run CPU-bound, and thus
127 are not helped by synchronized interrupts, and thus tend to
128 skew them, which reduces lock contention enough that large
129 leaf-level fanouts work well. That said, setting leaf-level
130 fanout to a large number will likely cause problematic
131 lock contention on the leaf-level rcu_node structures unless
132 you boot with the skew_tick kernel parameter.
134 Select a specific number if testing RCU itself.
136 Select the maximum permissible value for large systems, but
137 please understand that you may also need to set the skew_tick
138 kernel boot parameter to avoid contention on the rcu_node
141 Take the default if unsure.
143 config RCU_FAST_NO_HZ
144 bool "Accelerate last non-dyntick-idle CPU's grace periods"
145 depends on NO_HZ_COMMON && SMP && RCU_EXPERT
148 This option permits CPUs to enter dynticks-idle state even if
149 they have RCU callbacks queued, and prevents RCU from waking
150 these CPUs up more than roughly once every four jiffies (by
151 default, you can adjust this using the rcutree.rcu_idle_gp_delay
152 parameter), thus improving energy efficiency. On the other
153 hand, this option increases the duration of RCU grace periods,
154 for example, slowing down synchronize_rcu().
156 Say Y if energy efficiency is critically important, and you
157 don't care about increased grace-period durations.
159 Say N if you are unsure.
162 bool "Enable RCU priority boosting"
163 depends on RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT
166 This option boosts the priority of preempted RCU readers that
167 block the current preemptible RCU grace period for too long.
168 This option also prevents heavy loads from blocking RCU
171 Say Y here if you are working with real-time apps or heavy loads
172 Say N here if you are unsure.
174 config RCU_BOOST_DELAY
175 int "Milliseconds to delay boosting after RCU grace-period start"
180 This option specifies the time to wait after the beginning of
181 a given grace period before priority-boosting preempted RCU
182 readers blocking that grace period. Note that any RCU reader
183 blocking an expedited RCU grace period is boosted immediately.
185 Accept the default if unsure.
188 bool "Offload RCU callback processing from boot-selected CPUs"
189 depends on TREE_RCU || PREEMPT_RCU
190 depends on RCU_EXPERT || NO_HZ_FULL
193 Use this option to reduce OS jitter for aggressive HPC or
194 real-time workloads. It can also be used to offload RCU
195 callback invocation to energy-efficient CPUs in battery-powered
196 asymmetric multiprocessors.
198 This option offloads callback invocation from the set of CPUs
199 specified at boot time by the rcu_nocbs parameter. For each
200 such CPU, a kthread ("rcuox/N") will be created to invoke
201 callbacks, where the "N" is the CPU being offloaded, and where
202 the "p" for RCU-preempt (PREEMPT kernels) and "s" for RCU-sched
203 (!PREEMPT kernels). Nothing prevents this kthread from running
204 on the specified CPUs, but (1) the kthreads may be preempted
205 between each callback, and (2) affinity or cgroups can be used
206 to force the kthreads to run on whatever set of CPUs is desired.
208 Say Y here if you want to help to debug reduced OS jitter.
209 Say N here if you are unsure.
211 endmenu # "RCU Subsystem"