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TOMOYO Linux Cross Reference
Linux/kernel/rcu/Kconfig

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  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 !PREEMPT_RCU && !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 NEED_TASKS_RCU
 89         bool
 90         default n
 91 
 92 config TASKS_RCU
 93         bool
 94         default NEED_TASKS_RCU && (PREEMPTION || PREEMPT_AUTO)
 95         select IRQ_WORK
 96 
 97 config FORCE_TASKS_RUDE_RCU
 98         bool "Force selection of Tasks Rude RCU"
 99         depends on RCU_EXPERT
100         select TASKS_RUDE_RCU
101         default n
102         help
103           This option force-enables a task-based RCU implementation
104           that uses only context switch (including preemption) and
105           user-mode execution as quiescent states.  It forces IPIs and
106           context switches on all online CPUs, including idle ones,
107           so use with caution.  Not for manual selection in most cases.
108 
109 config TASKS_RUDE_RCU
110         bool
111         default n
112         select IRQ_WORK
113 
114 config FORCE_TASKS_TRACE_RCU
115         bool "Force selection of Tasks Trace RCU"
116         depends on RCU_EXPERT
117         select TASKS_TRACE_RCU
118         default n
119         help
120           This option enables a task-based RCU implementation that uses
121           explicit rcu_read_lock_trace() read-side markers, and allows
122           these readers to appear in the idle loop as well as on the
123           CPU hotplug code paths.  It can force IPIs on online CPUs,
124           including idle ones, so use with caution.  Not for manual
125           selection in most cases.
126 
127 config TASKS_TRACE_RCU
128         bool
129         default n
130         select IRQ_WORK
131 
132 config RCU_STALL_COMMON
133         def_bool TREE_RCU
134         help
135           This option enables RCU CPU stall code that is common between
136           the TINY and TREE variants of RCU.  The purpose is to allow
137           the tiny variants to disable RCU CPU stall warnings, while
138           making these warnings mandatory for the tree variants.
139 
140 config RCU_NEED_SEGCBLIST
141         def_bool ( TREE_RCU || TREE_SRCU || TASKS_RCU_GENERIC )
142 
143 config RCU_FANOUT
144         int "Tree-based hierarchical RCU fanout value"
145         range 2 64 if 64BIT
146         range 2 32 if !64BIT
147         depends on TREE_RCU && RCU_EXPERT
148         default 64 if 64BIT
149         default 32 if !64BIT
150         help
151           This option controls the fanout of hierarchical implementations
152           of RCU, allowing RCU to work efficiently on machines with
153           large numbers of CPUs.  This value must be at least the fourth
154           root of NR_CPUS, which allows NR_CPUS to be insanely large.
155           The default value of RCU_FANOUT should be used for production
156           systems, but if you are stress-testing the RCU implementation
157           itself, small RCU_FANOUT values allow you to test large-system
158           code paths on small(er) systems.
159 
160           Select a specific number if testing RCU itself.
161           Take the default if unsure.
162 
163 config RCU_FANOUT_LEAF
164         int "Tree-based hierarchical RCU leaf-level fanout value"
165         range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD
166         range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD
167         range 2 3 if RCU_STRICT_GRACE_PERIOD
168         depends on TREE_RCU && RCU_EXPERT
169         default 16 if !RCU_STRICT_GRACE_PERIOD
170         default 2 if RCU_STRICT_GRACE_PERIOD
171         help
172           This option controls the leaf-level fanout of hierarchical
173           implementations of RCU, and allows trading off cache misses
174           against lock contention.  Systems that synchronize their
175           scheduling-clock interrupts for energy-efficiency reasons will
176           want the default because the smaller leaf-level fanout keeps
177           lock contention levels acceptably low.  Very large systems
178           (hundreds or thousands of CPUs) will instead want to set this
179           value to the maximum value possible in order to reduce the
180           number of cache misses incurred during RCU's grace-period
181           initialization.  These systems tend to run CPU-bound, and thus
182           are not helped by synchronized interrupts, and thus tend to
183           skew them, which reduces lock contention enough that large
184           leaf-level fanouts work well.  That said, setting leaf-level
185           fanout to a large number will likely cause problematic
186           lock contention on the leaf-level rcu_node structures unless
187           you boot with the skew_tick kernel parameter.
188 
189           Select a specific number if testing RCU itself.
190 
191           Select the maximum permissible value for large systems, but
192           please understand that you may also need to set the skew_tick
193           kernel boot parameter to avoid contention on the rcu_node
194           structure's locks.
195 
196           Take the default if unsure.
197 
198 config RCU_BOOST
199         bool "Enable RCU priority boosting"
200         depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT
201         default y if PREEMPT_RT
202         help
203           This option boosts the priority of preempted RCU readers that
204           block the current preemptible RCU grace period for too long.
205           This option also prevents heavy loads from blocking RCU
206           callback invocation.
207 
208           Say Y here if you are working with real-time apps or heavy loads
209           Say N here if you are unsure.
210 
211 config RCU_BOOST_DELAY
212         int "Milliseconds to delay boosting after RCU grace-period start"
213         range 0 3000
214         depends on RCU_BOOST
215         default 500
216         help
217           This option specifies the time to wait after the beginning of
218           a given grace period before priority-boosting preempted RCU
219           readers blocking that grace period.  Note that any RCU reader
220           blocking an expedited RCU grace period is boosted immediately.
221 
222           Accept the default if unsure.
223 
224 config RCU_EXP_KTHREAD
225         bool "Perform RCU expedited work in a real-time kthread"
226         depends on RCU_BOOST && RCU_EXPERT
227         default !PREEMPT_RT && NR_CPUS <= 32
228         help
229           Use this option to further reduce the latencies of expedited
230           grace periods at the expense of being more disruptive.
231 
232           This option is disabled by default on PREEMPT_RT=y kernels which
233           disable expedited grace periods after boot by unconditionally
234           setting rcupdate.rcu_normal_after_boot=1.
235 
236           Accept the default if unsure.
237 
238 config RCU_NOCB_CPU
239         bool "Offload RCU callback processing from boot-selected CPUs"
240         depends on TREE_RCU
241         depends on RCU_EXPERT || NO_HZ_FULL
242         default n
243         help
244           Use this option to reduce OS jitter for aggressive HPC or
245           real-time workloads.  It can also be used to offload RCU
246           callback invocation to energy-efficient CPUs in battery-powered
247           asymmetric multiprocessors.  The price of this reduced jitter
248           is that the overhead of call_rcu() increases and that some
249           workloads will incur significant increases in context-switch
250           rates.
251 
252           This option offloads callback invocation from the set of CPUs
253           specified at boot time by the rcu_nocbs parameter.  For each
254           such CPU, a kthread ("rcuox/N") will be created to invoke
255           callbacks, where the "N" is the CPU being offloaded, and where
256           the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
257           RCU-sched (!PREEMPTION kernels).  Nothing prevents this kthread
258           from running on the specified CPUs, but (1) the kthreads may be
259           preempted between each callback, and (2) affinity or cgroups can
260           be used to force the kthreads to run on whatever set of CPUs is
261           desired.
262 
263           Say Y here if you need reduced OS jitter, despite added overhead.
264           Say N here if you are unsure.
265 
266 config RCU_NOCB_CPU_DEFAULT_ALL
267         bool "Offload RCU callback processing from all CPUs by default"
268         depends on RCU_NOCB_CPU
269         default n
270         help
271           Use this option to offload callback processing from all CPUs
272           by default, in the absence of the rcu_nocbs or nohz_full boot
273           parameter. This also avoids the need to use any boot parameters
274           to achieve the effect of offloading all CPUs on boot.
275 
276           Say Y here if you want offload all CPUs by default on boot.
277           Say N here if you are unsure.
278 
279 config RCU_NOCB_CPU_CB_BOOST
280         bool "Offload RCU callback from real-time kthread"
281         depends on RCU_NOCB_CPU && RCU_BOOST
282         default y if PREEMPT_RT
283         help
284           Use this option to invoke offloaded callbacks as SCHED_FIFO
285           to avoid starvation by heavy SCHED_OTHER background load.
286           Of course, running as SCHED_FIFO during callback floods will
287           cause the rcuo[ps] kthreads to monopolize the CPU for hundreds
288           of milliseconds or more.  Therefore, when enabling this option,
289           it is your responsibility to ensure that latency-sensitive
290           tasks either run with higher priority or run on some other CPU.
291 
292           Say Y here if you want to set RT priority for offloading kthreads.
293           Say N here if you are building a !PREEMPT_RT kernel and are unsure.
294 
295 config TASKS_TRACE_RCU_READ_MB
296         bool "Tasks Trace RCU readers use memory barriers in user and idle"
297         depends on RCU_EXPERT && TASKS_TRACE_RCU
298         default PREEMPT_RT || NR_CPUS < 8
299         help
300           Use this option to further reduce the number of IPIs sent
301           to CPUs executing in userspace or idle during tasks trace
302           RCU grace periods.  Given that a reasonable setting of
303           the rcupdate.rcu_task_ipi_delay kernel boot parameter
304           eliminates such IPIs for many workloads, proper setting
305           of this Kconfig option is important mostly for aggressive
306           real-time installations and for battery-powered devices,
307           hence the default chosen above.
308 
309           Say Y here if you hate IPIs.
310           Say N here if you hate read-side memory barriers.
311           Take the default if you are unsure.
312 
313 config RCU_LAZY
314         bool "RCU callback lazy invocation functionality"
315         depends on RCU_NOCB_CPU
316         default n
317         help
318           To save power, batch RCU callbacks and flush after delay, memory
319           pressure, or callback list growing too big.
320 
321           Requires rcu_nocbs=all to be set.
322 
323           Use rcutree.enable_rcu_lazy=0 to turn it off at boot time.
324 
325 config RCU_LAZY_DEFAULT_OFF
326         bool "Turn RCU lazy invocation off by default"
327         depends on RCU_LAZY
328         default n
329         help
330           Allows building the kernel with CONFIG_RCU_LAZY=y yet keep it default
331           off. Boot time param rcutree.enable_rcu_lazy=1 can be used to switch
332           it back on.
333 
334 config RCU_DOUBLE_CHECK_CB_TIME
335         bool "RCU callback-batch backup time check"
336         depends on RCU_EXPERT
337         default n
338         help
339           Use this option to provide more precise enforcement of the
340           rcutree.rcu_resched_ns module parameter in situations where
341           a single RCU callback might run for hundreds of microseconds,
342           thus defeating the 32-callback batching used to amortize the
343           cost of the fine-grained but expensive local_clock() function.
344 
345           This option rounds rcutree.rcu_resched_ns up to the next
346           jiffy, and overrides the 32-callback batching if this limit
347           is exceeded.
348 
349           Say Y here if you need tighter callback-limit enforcement.
350           Say N here if you are unsure.
351 
352 endmenu # "RCU Subsystem"

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