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6menu "RCU Subsystem"
7
8config TREE_RCU
9 bool
10 default y if 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 PREEMPTION
20 select TREE_RCU
21 help
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
26 smaller systems.
27
28 Select this option if you are unsure.
29
30config TINY_RCU
31 bool
32 default y if !PREEMPTION && !SMP
33 help
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.
38
39config RCU_EXPERT
40 bool "Make expert-level adjustments to RCU configuration"
41 default n
42 help
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.
49
50 Say Y if you need to make expert-level adjustments to RCU.
51
52 Say N if you are unsure.
53
54config SRCU
55 bool
56 help
57 This option selects the sleepable version of RCU. This version
58 permits arbitrary sleeping or blocking within RCU read-side critical
59 sections.
60
61config TINY_SRCU
62 bool
63 default y if SRCU && TINY_RCU
64 help
65 This option selects the single-CPU non-preemptible version of SRCU.
66
67config TREE_SRCU
68 bool
69 default y if SRCU && !TINY_RCU
70 help
71 This option selects the full-fledged version of SRCU.
72
73config TASKS_RCU_GENERIC
74 def_bool TASKS_RCU || TASKS_RUDE_RCU || TASKS_TRACE_RCU
75 select SRCU
76 help
77 This option enables generic infrastructure code supporting
78 task-based RCU implementations. Not for manual selection.
79
80config TASKS_RCU
81 def_bool PREEMPTION
82 help
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.
86
87config TASKS_RUDE_RCU
88 def_bool 0
89 help
90 This option enables a task-based RCU implementation that uses
91 only context switch (including preemption) and user-mode
92 execution as quiescent states. It forces IPIs and context
93 switches on all online CPUs, including idle ones, so use
94 with caution.
95
96config TASKS_TRACE_RCU
97 def_bool 0
98 select IRQ_WORK
99 help
100 This option enables a task-based RCU implementation that uses
101 explicit rcu_read_lock_trace() read-side markers, and allows
102 these readers to appear in the idle loop as well as on the CPU
103 hotplug code paths. It can force IPIs on online CPUs, including
104 idle ones, so use with caution.
105
106config RCU_STALL_COMMON
107 def_bool TREE_RCU
108 help
109 This option enables RCU CPU stall code that is common between
110 the TINY and TREE variants of RCU. The purpose is to allow
111 the tiny variants to disable RCU CPU stall warnings, while
112 making these warnings mandatory for the tree variants.
113
114config RCU_NEED_SEGCBLIST
115 def_bool ( TREE_RCU || TREE_SRCU )
116
117config RCU_FANOUT
118 int "Tree-based hierarchical RCU fanout value"
119 range 2 64 if 64BIT
120 range 2 32 if !64BIT
121 depends on TREE_RCU && RCU_EXPERT
122 default 64 if 64BIT
123 default 32 if !64BIT
124 help
125 This option controls the fanout of hierarchical implementations
126 of RCU, allowing RCU to work efficiently on machines with
127 large numbers of CPUs. This value must be at least the fourth
128 root of NR_CPUS, which allows NR_CPUS to be insanely large.
129 The default value of RCU_FANOUT should be used for production
130 systems, but if you are stress-testing the RCU implementation
131 itself, small RCU_FANOUT values allow you to test large-system
132 code paths on small(er) systems.
133
134 Select a specific number if testing RCU itself.
135 Take the default if unsure.
136
137config RCU_FANOUT_LEAF
138 int "Tree-based hierarchical RCU leaf-level fanout value"
139 range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD
140 range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD
141 range 2 3 if RCU_STRICT_GRACE_PERIOD
142 depends on TREE_RCU && RCU_EXPERT
143 default 16 if !RCU_STRICT_GRACE_PERIOD
144 default 2 if RCU_STRICT_GRACE_PERIOD
145 help
146 This option controls the leaf-level fanout of hierarchical
147 implementations of RCU, and allows trading off cache misses
148 against lock contention. Systems that synchronize their
149 scheduling-clock interrupts for energy-efficiency reasons will
150 want the default because the smaller leaf-level fanout keeps
151 lock contention levels acceptably low. Very large systems
152 (hundreds or thousands of CPUs) will instead want to set this
153 value to the maximum value possible in order to reduce the
154 number of cache misses incurred during RCU's grace-period
155 initialization. These systems tend to run CPU-bound, and thus
156 are not helped by synchronized interrupts, and thus tend to
157 skew them, which reduces lock contention enough that large
158 leaf-level fanouts work well. That said, setting leaf-level
159 fanout to a large number will likely cause problematic
160 lock contention on the leaf-level rcu_node structures unless
161 you boot with the skew_tick kernel parameter.
162
163 Select a specific number if testing RCU itself.
164
165 Select the maximum permissible value for large systems, but
166 please understand that you may also need to set the skew_tick
167 kernel boot parameter to avoid contention on the rcu_node
168 structure's locks.
169
170 Take the default if unsure.
171
172config RCU_FAST_NO_HZ
173 bool "Accelerate last non-dyntick-idle CPU's grace periods"
174 depends on NO_HZ_COMMON && SMP && RCU_EXPERT
175 default n
176 help
177 This option permits CPUs to enter dynticks-idle state even if
178 they have RCU callbacks queued, and prevents RCU from waking
179 these CPUs up more than roughly once every four jiffies (by
180 default, you can adjust this using the rcutree.rcu_idle_gp_delay
181 parameter), thus improving energy efficiency. On the other
182 hand, this option increases the duration of RCU grace periods,
183 for example, slowing down synchronize_rcu().
184
185 Say Y if energy efficiency is critically important, and you
186 don't care about increased grace-period durations.
187
188 Say N if you are unsure.
189
190config RCU_BOOST
191 bool "Enable RCU priority boosting"
192 depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT
193 default y if PREEMPT_RT
194 help
195 This option boosts the priority of preempted RCU readers that
196 block the current preemptible RCU grace period for too long.
197 This option also prevents heavy loads from blocking RCU
198 callback invocation.
199
200 Say Y here if you are working with real-time apps or heavy loads
201 Say N here if you are unsure.
202
203config RCU_BOOST_DELAY
204 int "Milliseconds to delay boosting after RCU grace-period start"
205 range 0 3000
206 depends on RCU_BOOST
207 default 500
208 help
209 This option specifies the time to wait after the beginning of
210 a given grace period before priority-boosting preempted RCU
211 readers blocking that grace period. Note that any RCU reader
212 blocking an expedited RCU grace period is boosted immediately.
213
214 Accept the default if unsure.
215
216config RCU_NOCB_CPU
217 bool "Offload RCU callback processing from boot-selected CPUs"
218 depends on TREE_RCU
219 depends on RCU_EXPERT || NO_HZ_FULL
220 default n
221 help
222 Use this option to reduce OS jitter for aggressive HPC or
223 real-time workloads. It can also be used to offload RCU
224 callback invocation to energy-efficient CPUs in battery-powered
225 asymmetric multiprocessors. The price of this reduced jitter
226 is that the overhead of call_rcu() increases and that some
227 workloads will incur significant increases in context-switch
228 rates.
229
230 This option offloads callback invocation from the set of CPUs
231 specified at boot time by the rcu_nocbs parameter. For each
232 such CPU, a kthread ("rcuox/N") will be created to invoke
233 callbacks, where the "N" is the CPU being offloaded, and where
234 the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
235 RCU-sched (!PREEMPTION kernels). Nothing prevents this kthread
236 from running on the specified CPUs, but (1) the kthreads may be
237 preempted between each callback, and (2) affinity or cgroups can
238 be used to force the kthreads to run on whatever set of CPUs is
239 desired.
240
241 Say Y here if you need reduced OS jitter, despite added overhead.
242 Say N here if you are unsure.
243
244config TASKS_TRACE_RCU_READ_MB
245 bool "Tasks Trace RCU readers use memory barriers in user and idle"
246 depends on RCU_EXPERT
247 default PREEMPT_RT || NR_CPUS < 8
248 help
249 Use this option to further reduce the number of IPIs sent
250 to CPUs executing in userspace or idle during tasks trace
251 RCU grace periods. Given that a reasonable setting of
252 the rcupdate.rcu_task_ipi_delay kernel boot parameter
253 eliminates such IPIs for many workloads, proper setting
254 of this Kconfig option is important mostly for aggressive
255 real-time installations and for battery-powered devices,
256 hence the default chosen above.
257
258 Say Y here if you hate IPIs.
259 Say N here if you hate read-side memory barriers.
260 Take the default if you are unsure.
261
262endmenu
263
