xref: /openbmc/linux/kernel/rcu/Kconfig (revision 1ac731c529cd4d6adbce134754b51ff7d822b145)
1 # SPDX-License-Identifier: GPL-2.0-only
2 #
3 # RCU-related configuration options
4 #
5 
6 menu "RCU Subsystem"
7 
8 config TREE_RCU
9 	bool
10 	default y if SMP
11 	# Dynticks-idle tracking
12 	select CONTEXT_TRACKING_IDLE
13 	help
14 	  This option selects the RCU implementation that is
15 	  designed for very large SMP system with hundreds or
16 	  thousands of CPUs.  It also scales down nicely to
17 	  smaller systems.
18 
19 config PREEMPT_RCU
20 	bool
21 	default y if PREEMPTION
22 	select TREE_RCU
23 	help
24 	  This option selects the RCU implementation that is
25 	  designed for very large SMP systems with hundreds or
26 	  thousands of CPUs, but for which real-time response
27 	  is also required.  It also scales down nicely to
28 	  smaller systems.
29 
30 	  Select this option if you are unsure.
31 
32 config TINY_RCU
33 	bool
34 	default y if !PREEMPTION && !SMP
35 	help
36 	  This option selects the RCU implementation that is
37 	  designed for UP systems from which real-time response
38 	  is not required.  This option greatly reduces the
39 	  memory footprint of RCU.
40 
41 config RCU_EXPERT
42 	bool "Make expert-level adjustments to RCU configuration"
43 	default n
44 	help
45 	  This option needs to be enabled if you wish to make
46 	  expert-level adjustments to RCU configuration.  By default,
47 	  no such adjustments can be made, which has the often-beneficial
48 	  side-effect of preventing "make oldconfig" from asking you all
49 	  sorts of detailed questions about how you would like numerous
50 	  obscure RCU options to be set up.
51 
52 	  Say Y if you need to make expert-level adjustments to RCU.
53 
54 	  Say N if you are unsure.
55 
56 config TINY_SRCU
57 	bool
58 	default y if TINY_RCU
59 	help
60 	  This option selects the single-CPU non-preemptible version of SRCU.
61 
62 config TREE_SRCU
63 	bool
64 	default y if !TINY_RCU
65 	help
66 	  This option selects the full-fledged version of SRCU.
67 
68 config NEED_SRCU_NMI_SAFE
69 	def_bool HAVE_NMI && !ARCH_HAS_NMI_SAFE_THIS_CPU_OPS && !TINY_SRCU
70 
71 config TASKS_RCU_GENERIC
72 	def_bool TASKS_RCU || TASKS_RUDE_RCU || TASKS_TRACE_RCU
73 	help
74 	  This option enables generic infrastructure code supporting
75 	  task-based RCU implementations.  Not for manual selection.
76 
77 config FORCE_TASKS_RCU
78 	bool "Force selection of TASKS_RCU"
79 	depends on RCU_EXPERT
80 	select TASKS_RCU
81 	default n
82 	help
83 	  This option force-enables a task-based RCU implementation
84 	  that uses only voluntary context switch (not preemption!),
85 	  idle, and user-mode execution as quiescent states.  Not for
86 	  manual selection in most cases.
87 
88 config TASKS_RCU
89 	bool
90 	default n
91 	select IRQ_WORK
92 
93 config FORCE_TASKS_RUDE_RCU
94 	bool "Force selection of Tasks Rude RCU"
95 	depends on RCU_EXPERT
96 	select TASKS_RUDE_RCU
97 	default n
98 	help
99 	  This option force-enables a task-based RCU implementation
100 	  that uses only context switch (including preemption) and
101 	  user-mode execution as quiescent states.  It forces IPIs and
102 	  context switches on all online CPUs, including idle ones,
103 	  so use with caution.	Not for manual selection in most cases.
104 
105 config TASKS_RUDE_RCU
106 	bool
107 	default n
108 	select IRQ_WORK
109 
110 config FORCE_TASKS_TRACE_RCU
111 	bool "Force selection of Tasks Trace RCU"
112 	depends on RCU_EXPERT
113 	select TASKS_TRACE_RCU
114 	default n
115 	help
116 	  This option enables a task-based RCU implementation that uses
117 	  explicit rcu_read_lock_trace() read-side markers, and allows
118 	  these readers to appear in the idle loop as well as on the
119 	  CPU hotplug code paths.  It can force IPIs on online CPUs,
120 	  including idle ones, so use with caution.  Not for manual
121 	  selection in most cases.
122 
123 config TASKS_TRACE_RCU
124 	bool
125 	default n
126 	select IRQ_WORK
127 
128 config RCU_STALL_COMMON
129 	def_bool TREE_RCU
130 	help
131 	  This option enables RCU CPU stall code that is common between
132 	  the TINY and TREE variants of RCU.  The purpose is to allow
133 	  the tiny variants to disable RCU CPU stall warnings, while
134 	  making these warnings mandatory for the tree variants.
135 
136 config RCU_NEED_SEGCBLIST
137 	def_bool ( TREE_RCU || TREE_SRCU || TASKS_RCU_GENERIC )
138 
139 config RCU_FANOUT
140 	int "Tree-based hierarchical RCU fanout value"
141 	range 2 64 if 64BIT
142 	range 2 32 if !64BIT
143 	depends on TREE_RCU && RCU_EXPERT
144 	default 64 if 64BIT
145 	default 32 if !64BIT
146 	help
147 	  This option controls the fanout of hierarchical implementations
148 	  of RCU, allowing RCU to work efficiently on machines with
149 	  large numbers of CPUs.  This value must be at least the fourth
150 	  root of NR_CPUS, which allows NR_CPUS to be insanely large.
151 	  The default value of RCU_FANOUT should be used for production
152 	  systems, but if you are stress-testing the RCU implementation
153 	  itself, small RCU_FANOUT values allow you to test large-system
154 	  code paths on small(er) systems.
155 
156 	  Select a specific number if testing RCU itself.
157 	  Take the default if unsure.
158 
159 config RCU_FANOUT_LEAF
160 	int "Tree-based hierarchical RCU leaf-level fanout value"
161 	range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD
162 	range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD
163 	range 2 3 if RCU_STRICT_GRACE_PERIOD
164 	depends on TREE_RCU && RCU_EXPERT
165 	default 16 if !RCU_STRICT_GRACE_PERIOD
166 	default 2 if RCU_STRICT_GRACE_PERIOD
167 	help
168 	  This option controls the leaf-level fanout of hierarchical
169 	  implementations of RCU, and allows trading off cache misses
170 	  against lock contention.  Systems that synchronize their
171 	  scheduling-clock interrupts for energy-efficiency reasons will
172 	  want the default because the smaller leaf-level fanout keeps
173 	  lock contention levels acceptably low.  Very large systems
174 	  (hundreds or thousands of CPUs) will instead want to set this
175 	  value to the maximum value possible in order to reduce the
176 	  number of cache misses incurred during RCU's grace-period
177 	  initialization.  These systems tend to run CPU-bound, and thus
178 	  are not helped by synchronized interrupts, and thus tend to
179 	  skew them, which reduces lock contention enough that large
180 	  leaf-level fanouts work well.  That said, setting leaf-level
181 	  fanout to a large number will likely cause problematic
182 	  lock contention on the leaf-level rcu_node structures unless
183 	  you boot with the skew_tick kernel parameter.
184 
185 	  Select a specific number if testing RCU itself.
186 
187 	  Select the maximum permissible value for large systems, but
188 	  please understand that you may also need to set the skew_tick
189 	  kernel boot parameter to avoid contention on the rcu_node
190 	  structure's locks.
191 
192 	  Take the default if unsure.
193 
194 config RCU_BOOST
195 	bool "Enable RCU priority boosting"
196 	depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT
197 	default y if PREEMPT_RT
198 	help
199 	  This option boosts the priority of preempted RCU readers that
200 	  block the current preemptible RCU grace period for too long.
201 	  This option also prevents heavy loads from blocking RCU
202 	  callback invocation.
203 
204 	  Say Y here if you are working with real-time apps or heavy loads
205 	  Say N here if you are unsure.
206 
207 config RCU_BOOST_DELAY
208 	int "Milliseconds to delay boosting after RCU grace-period start"
209 	range 0 3000
210 	depends on RCU_BOOST
211 	default 500
212 	help
213 	  This option specifies the time to wait after the beginning of
214 	  a given grace period before priority-boosting preempted RCU
215 	  readers blocking that grace period.  Note that any RCU reader
216 	  blocking an expedited RCU grace period is boosted immediately.
217 
218 	  Accept the default if unsure.
219 
220 config RCU_EXP_KTHREAD
221 	bool "Perform RCU expedited work in a real-time kthread"
222 	depends on RCU_BOOST && RCU_EXPERT
223 	default !PREEMPT_RT && NR_CPUS <= 32
224 	help
225 	  Use this option to further reduce the latencies of expedited
226 	  grace periods at the expense of being more disruptive.
227 
228 	  This option is disabled by default on PREEMPT_RT=y kernels which
229 	  disable expedited grace periods after boot by unconditionally
230 	  setting rcupdate.rcu_normal_after_boot=1.
231 
232 	  Accept the default if unsure.
233 
234 config RCU_NOCB_CPU
235 	bool "Offload RCU callback processing from boot-selected CPUs"
236 	depends on TREE_RCU
237 	depends on RCU_EXPERT || NO_HZ_FULL
238 	default n
239 	help
240 	  Use this option to reduce OS jitter for aggressive HPC or
241 	  real-time workloads.	It can also be used to offload RCU
242 	  callback invocation to energy-efficient CPUs in battery-powered
243 	  asymmetric multiprocessors.  The price of this reduced jitter
244 	  is that the overhead of call_rcu() increases and that some
245 	  workloads will incur significant increases in context-switch
246 	  rates.
247 
248 	  This option offloads callback invocation from the set of CPUs
249 	  specified at boot time by the rcu_nocbs parameter.  For each
250 	  such CPU, a kthread ("rcuox/N") will be created to invoke
251 	  callbacks, where the "N" is the CPU being offloaded, and where
252 	  the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
253 	  RCU-sched (!PREEMPTION kernels).  Nothing prevents this kthread
254 	  from running on the specified CPUs, but (1) the kthreads may be
255 	  preempted between each callback, and (2) affinity or cgroups can
256 	  be used to force the kthreads to run on whatever set of CPUs is
257 	  desired.
258 
259 	  Say Y here if you need reduced OS jitter, despite added overhead.
260 	  Say N here if you are unsure.
261 
262 config RCU_NOCB_CPU_DEFAULT_ALL
263 	bool "Offload RCU callback processing from all CPUs by default"
264 	depends on RCU_NOCB_CPU
265 	default n
266 	help
267 	  Use this option to offload callback processing from all CPUs
268 	  by default, in the absence of the rcu_nocbs or nohz_full boot
269 	  parameter. This also avoids the need to use any boot parameters
270 	  to achieve the effect of offloading all CPUs on boot.
271 
272 	  Say Y here if you want offload all CPUs by default on boot.
273 	  Say N here if you are unsure.
274 
275 config RCU_NOCB_CPU_CB_BOOST
276 	bool "Offload RCU callback from real-time kthread"
277 	depends on RCU_NOCB_CPU && RCU_BOOST
278 	default y if PREEMPT_RT
279 	help
280 	  Use this option to invoke offloaded callbacks as SCHED_FIFO
281 	  to avoid starvation by heavy SCHED_OTHER background load.
282 	  Of course, running as SCHED_FIFO during callback floods will
283 	  cause the rcuo[ps] kthreads to monopolize the CPU for hundreds
284 	  of milliseconds or more.  Therefore, when enabling this option,
285 	  it is your responsibility to ensure that latency-sensitive
286 	  tasks either run with higher priority or run on some other CPU.
287 
288 	  Say Y here if you want to set RT priority for offloading kthreads.
289 	  Say N here if you are building a !PREEMPT_RT kernel and are unsure.
290 
291 config TASKS_TRACE_RCU_READ_MB
292 	bool "Tasks Trace RCU readers use memory barriers in user and idle"
293 	depends on RCU_EXPERT && TASKS_TRACE_RCU
294 	default PREEMPT_RT || NR_CPUS < 8
295 	help
296 	  Use this option to further reduce the number of IPIs sent
297 	  to CPUs executing in userspace or idle during tasks trace
298 	  RCU grace periods.  Given that a reasonable setting of
299 	  the rcupdate.rcu_task_ipi_delay kernel boot parameter
300 	  eliminates such IPIs for many workloads, proper setting
301 	  of this Kconfig option is important mostly for aggressive
302 	  real-time installations and for battery-powered devices,
303 	  hence the default chosen above.
304 
305 	  Say Y here if you hate IPIs.
306 	  Say N here if you hate read-side memory barriers.
307 	  Take the default if you are unsure.
308 
309 config RCU_LAZY
310 	bool "RCU callback lazy invocation functionality"
311 	depends on RCU_NOCB_CPU
312 	default n
313 	help
314 	  To save power, batch RCU callbacks and flush after delay, memory
315 	  pressure, or callback list growing too big.
316 
317 config RCU_DOUBLE_CHECK_CB_TIME
318 	bool "RCU callback-batch backup time check"
319 	depends on RCU_EXPERT
320 	default n
321 	help
322 	  Use this option to provide more precise enforcement of the
323 	  rcutree.rcu_resched_ns module parameter in situations where
324 	  a single RCU callback might run for hundreds of microseconds,
325 	  thus defeating the 32-callback batching used to amortize the
326 	  cost of the fine-grained but expensive local_clock() function.
327 
328 	  This option rounds rcutree.rcu_resched_ns up to the next
329 	  jiffy, and overrides the 32-callback batching if this limit
330 	  is exceeded.
331 
332 	  Say Y here if you need tighter callback-limit enforcement.
333 	  Say N here if you are unsure.
334 
335 endmenu # "RCU Subsystem"
336