Loading...
1# SPDX-License-Identifier: GPL-2.0-only
2#
3# RCU-related configuration options
4#
5
6menu "RCU Subsystem"
7
8config TREE_RCU
9 bool
10 default y if !PREEMPTION && 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 help
21 This option selects the RCU implementation that is
22 designed for very large SMP systems with hundreds or
23 thousands of CPUs, but for which real-time response
24 is also required. It also scales down nicely to
25 smaller systems.
26
27 Select this option if you are unsure.
28
29config TINY_RCU
30 bool
31 default y if !PREEMPTION && !SMP
32 help
33 This option selects the RCU implementation that is
34 designed for UP systems from which real-time response
35 is not required. This option greatly reduces the
36 memory footprint of RCU.
37
38config RCU_EXPERT
39 bool "Make expert-level adjustments to RCU configuration"
40 default n
41 help
42 This option needs to be enabled if you wish to make
43 expert-level adjustments to RCU configuration. By default,
44 no such adjustments can be made, which has the often-beneficial
45 side-effect of preventing "make oldconfig" from asking you all
46 sorts of detailed questions about how you would like numerous
47 obscure RCU options to be set up.
48
49 Say Y if you need to make expert-level adjustments to RCU.
50
51 Say N if you are unsure.
52
53config SRCU
54 bool
55 help
56 This option selects the sleepable version of RCU. This version
57 permits arbitrary sleeping or blocking within RCU read-side critical
58 sections.
59
60config TINY_SRCU
61 bool
62 default y if SRCU && TINY_RCU
63 help
64 This option selects the single-CPU non-preemptible version of SRCU.
65
66config TREE_SRCU
67 bool
68 default y if SRCU && !TINY_RCU
69 help
70 This option selects the full-fledged version of SRCU.
71
72config TASKS_RCU
73 def_bool PREEMPTION
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 RCU_FANOUT
92 int "Tree-based hierarchical RCU fanout value"
93 range 2 64 if 64BIT
94 range 2 32 if !64BIT
95 depends on (TREE_RCU || PREEMPT_RCU) && RCU_EXPERT
96 default 64 if 64BIT
97 default 32 if !64BIT
98 help
99 This option controls the fanout of hierarchical implementations
100 of RCU, allowing RCU to work efficiently on machines with
101 large numbers of CPUs. This value must be at least the fourth
102 root of NR_CPUS, which allows NR_CPUS to be insanely large.
103 The default value of RCU_FANOUT should be used for production
104 systems, but if you are stress-testing the RCU implementation
105 itself, small RCU_FANOUT values allow you to test large-system
106 code paths on small(er) systems.
107
108 Select a specific number if testing RCU itself.
109 Take the default if unsure.
110
111config RCU_FANOUT_LEAF
112 int "Tree-based hierarchical RCU leaf-level fanout value"
113 range 2 64 if 64BIT
114 range 2 32 if !64BIT
115 depends on (TREE_RCU || PREEMPT_RCU) && RCU_EXPERT
116 default 16
117 help
118 This option controls the leaf-level fanout of hierarchical
119 implementations of RCU, and allows trading off cache misses
120 against lock contention. Systems that synchronize their
121 scheduling-clock interrupts for energy-efficiency reasons will
122 want the default because the smaller leaf-level fanout keeps
123 lock contention levels acceptably low. Very large systems
124 (hundreds or thousands of CPUs) will instead want to set this
125 value to the maximum value possible in order to reduce the
126 number of cache misses incurred during RCU's grace-period
127 initialization. These systems tend to run CPU-bound, and thus
128 are not helped by synchronized interrupts, and thus tend to
129 skew them, which reduces lock contention enough that large
130 leaf-level fanouts work well. That said, setting leaf-level
131 fanout to a large number will likely cause problematic
132 lock contention on the leaf-level rcu_node structures unless
133 you boot with the skew_tick kernel parameter.
134
135 Select a specific number if testing RCU itself.
136
137 Select the maximum permissible value for large systems, but
138 please understand that you may also need to set the skew_tick
139 kernel boot parameter to avoid contention on the rcu_node
140 structure's locks.
141
142 Take the default if unsure.
143
144config RCU_FAST_NO_HZ
145 bool "Accelerate last non-dyntick-idle CPU's grace periods"
146 depends on NO_HZ_COMMON && SMP && RCU_EXPERT
147 default n
148 help
149 This option permits CPUs to enter dynticks-idle state even if
150 they have RCU callbacks queued, and prevents RCU from waking
151 these CPUs up more than roughly once every four jiffies (by
152 default, you can adjust this using the rcutree.rcu_idle_gp_delay
153 parameter), thus improving energy efficiency. On the other
154 hand, this option increases the duration of RCU grace periods,
155 for example, slowing down synchronize_rcu().
156
157 Say Y if energy efficiency is critically important, and you
158 don't care about increased grace-period durations.
159
160 Say N if you are unsure.
161
162config RCU_BOOST
163 bool "Enable RCU priority boosting"
164 depends on RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT
165 default n
166 help
167 This option boosts the priority of preempted RCU readers that
168 block the current preemptible RCU grace period for too long.
169 This option also prevents heavy loads from blocking RCU
170 callback invocation.
171
172 Say Y here if you are working with real-time apps or heavy loads
173 Say N here if you are unsure.
174
175config RCU_BOOST_DELAY
176 int "Milliseconds to delay boosting after RCU grace-period start"
177 range 0 3000
178 depends on RCU_BOOST
179 default 500
180 help
181 This option specifies the time to wait after the beginning of
182 a given grace period before priority-boosting preempted RCU
183 readers blocking that grace period. Note that any RCU reader
184 blocking an expedited RCU grace period is boosted immediately.
185
186 Accept the default if unsure.
187
188config RCU_NOCB_CPU
189 bool "Offload RCU callback processing from boot-selected CPUs"
190 depends on TREE_RCU || PREEMPT_RCU
191 depends on RCU_EXPERT || NO_HZ_FULL
192 default n
193 help
194 Use this option to reduce OS jitter for aggressive HPC or
195 real-time workloads. It can also be used to offload RCU
196 callback invocation to energy-efficient CPUs in battery-powered
197 asymmetric multiprocessors.
198
199 This option offloads callback invocation from the set of CPUs
200 specified at boot time by the rcu_nocbs parameter. For each
201 such CPU, a kthread ("rcuox/N") will be created to invoke
202 callbacks, where the "N" is the CPU being offloaded, and where
203 the "p" for RCU-preempt (PREEMPT kernels) and "s" for RCU-sched
204 (!PREEMPT kernels). Nothing prevents this kthread from running
205 on the specified CPUs, but (1) the kthreads may be preempted
206 between each callback, and (2) affinity or cgroups can be used
207 to force the kthreads to run on whatever set of CPUs is desired.
208
209 Say Y here if you want to help to debug reduced OS jitter.
210 Say N here if you are unsure.
211
212endmenu # "RCU Subsystem"
1# SPDX-License-Identifier: GPL-2.0-only
2#
3# RCU-related configuration options
4#
5
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 # "RCU Subsystem"