1 # SPDX-License-Identifier: GPL-2.0-only
3 # RCU-related configuration options
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
19 default y if PREEMPTION
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
28 Select this option if you are unsure.
32 default y if !PREEMPTION && !SMP
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.
40 bool "Make expert-level adjustments to RCU configuration"
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.
50 Say Y if you need to make expert-level adjustments to RCU.
52 Say N if you are unsure.
57 This option selects the sleepable version of RCU. This version
58 permits arbitrary sleeping or blocking within RCU read-side critical
63 default y if SRCU && TINY_RCU
65 This option selects the single-CPU non-preemptible version of SRCU.
69 default y if SRCU && !TINY_RCU
71 This option selects the full-fledged version of SRCU.
73 config TASKS_RCU_GENERIC
74 def_bool TASKS_RCU || TASKS_RUDE_RCU || TASKS_TRACE_RCU
77 This option enables generic infrastructure code supporting
78 task-based RCU implementations. Not for manual selection.
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.
91 This option enables a task-based RCU implementation that uses
92 only context switch (including preemption) and user-mode
93 execution as quiescent states. It forces IPIs and context
94 switches on all online CPUs, including idle ones, so use
97 config TASKS_TRACE_RCU
101 This option enables a task-based RCU implementation that uses
102 explicit rcu_read_lock_trace() read-side markers, and allows
103 these readers to appear in the idle loop as well as on the CPU
104 hotplug code paths. It can force IPIs on online CPUs, including
105 idle ones, so use with caution.
107 config RCU_STALL_COMMON
110 This option enables RCU CPU stall code that is common between
111 the TINY and TREE variants of RCU. The purpose is to allow
112 the tiny variants to disable RCU CPU stall warnings, while
113 making these warnings mandatory for the tree variants.
115 config RCU_NEED_SEGCBLIST
116 def_bool ( TREE_RCU || TREE_SRCU )
119 int "Tree-based hierarchical RCU fanout value"
122 depends on TREE_RCU && RCU_EXPERT
126 This option controls the fanout of hierarchical implementations
127 of RCU, allowing RCU to work efficiently on machines with
128 large numbers of CPUs. This value must be at least the fourth
129 root of NR_CPUS, which allows NR_CPUS to be insanely large.
130 The default value of RCU_FANOUT should be used for production
131 systems, but if you are stress-testing the RCU implementation
132 itself, small RCU_FANOUT values allow you to test large-system
133 code paths on small(er) systems.
135 Select a specific number if testing RCU itself.
136 Take the default if unsure.
138 config RCU_FANOUT_LEAF
139 int "Tree-based hierarchical RCU leaf-level fanout value"
140 range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD
141 range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD
142 range 2 3 if RCU_STRICT_GRACE_PERIOD
143 depends on TREE_RCU && RCU_EXPERT
144 default 16 if !RCU_STRICT_GRACE_PERIOD
145 default 2 if RCU_STRICT_GRACE_PERIOD
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.
164 Select a specific number if testing RCU itself.
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
171 Take the default if unsure.
173 config RCU_FAST_NO_HZ
174 bool "Accelerate last non-dyntick-idle CPU's grace periods"
175 depends on NO_HZ_COMMON && SMP && RCU_EXPERT
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().
186 Say Y if energy efficiency is critically important, and you
187 don't care about increased grace-period durations.
189 Say N if you are unsure.
192 bool "Enable RCU priority boosting"
193 depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT
194 default y if PREEMPT_RT
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
201 Say Y here if you are working with real-time apps or heavy loads
202 Say N here if you are unsure.
204 config RCU_BOOST_DELAY
205 int "Milliseconds to delay boosting after RCU grace-period start"
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.
215 Accept the default if unsure.
218 bool "Offload RCU callback processing from boot-selected CPUs"
220 depends on RCU_EXPERT || NO_HZ_FULL
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. The price of this reduced jitter
227 is that the overhead of call_rcu() increases and that some
228 workloads will incur significant increases in context-switch
231 This option offloads callback invocation from the set of CPUs
232 specified at boot time by the rcu_nocbs parameter. For each
233 such CPU, a kthread ("rcuox/N") will be created to invoke
234 callbacks, where the "N" is the CPU being offloaded, and where
235 the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
236 RCU-sched (!PREEMPTION kernels). Nothing prevents this kthread
237 from running on the specified CPUs, but (1) the kthreads may be
238 preempted between each callback, and (2) affinity or cgroups can
239 be used to force the kthreads to run on whatever set of CPUs is
242 Say Y here if you need reduced OS jitter, despite added overhead.
243 Say N here if you are unsure.
245 config TASKS_TRACE_RCU_READ_MB
246 bool "Tasks Trace RCU readers use memory barriers in user and idle"
247 depends on RCU_EXPERT
248 default PREEMPT_RT || NR_CPUS < 8
250 Use this option to further reduce the number of IPIs sent
251 to CPUs executing in userspace or idle during tasks trace
252 RCU grace periods. Given that a reasonable setting of
253 the rcupdate.rcu_task_ipi_delay kernel boot parameter
254 eliminates such IPIs for many workloads, proper setting
255 of this Kconfig option is important mostly for aggressive
256 real-time installations and for battery-powered devices,
257 hence the default chosen above.
259 Say Y here if you hate IPIs.
260 Say N here if you hate read-side memory barriers.
261 Take the default if you are unsure.
263 endmenu # "RCU Subsystem"