[linux.git] / kernel / rcu / Kconfig

#
# RCU-related configuration options
#

menu "RCU Subsystem"

config TREE_RCU
	bool
	default y if !PREEMPT && SMP
	help
	  This option selects the RCU implementation that is
	  designed for very large SMP system with hundreds or
	  thousands of CPUs.  It also scales down nicely to
	  smaller systems.

config PREEMPT_RCU
	bool
	default y if PREEMPT
	help
	  This option selects the RCU implementation that is
	  designed for very large SMP systems with hundreds or
	  thousands of CPUs, but for which real-time response
	  is also required.  It also scales down nicely to
	  smaller systems.

	  Select this option if you are unsure.

config TINY_RCU
	bool
	default y if !PREEMPT && !SMP
	help
	  This option selects the RCU implementation that is
	  designed for UP systems from which real-time response
	  is not required.  This option greatly reduces the
	  memory footprint of RCU.

config RCU_EXPERT
	bool "Make expert-level adjustments to RCU configuration"
	default n
	help
	  This option needs to be enabled if you wish to make
	  expert-level adjustments to RCU configuration.  By default,
	  no such adjustments can be made, which has the often-beneficial
	  side-effect of preventing "make oldconfig" from asking you all
	  sorts of detailed questions about how you would like numerous
	  obscure RCU options to be set up.

	  Say Y if you need to make expert-level adjustments to RCU.

	  Say N if you are unsure.

config SRCU
	bool
	help
	  This option selects the sleepable version of RCU. This version
	  permits arbitrary sleeping or blocking within RCU read-side critical
	  sections.

config TINY_SRCU
	bool
	default y if SRCU && TINY_RCU
	help
	  This option selects the single-CPU non-preemptible version of SRCU.

config TREE_SRCU
	bool
	default y if SRCU && !TINY_RCU
	help
	  This option selects the full-fledged version of SRCU.

config TASKS_RCU
	def_bool PREEMPT
	select SRCU
	help
	  This option enables a task-based RCU implementation that uses
	  only voluntary context switch (not preemption!), idle, and
	  user-mode execution as quiescent states.

config RCU_STALL_COMMON
	def_bool ( TREE_RCU || PREEMPT_RCU )
	help
	  This option enables RCU CPU stall code that is common between
	  the TINY and TREE variants of RCU.  The purpose is to allow
	  the tiny variants to disable RCU CPU stall warnings, while
	  making these warnings mandatory for the tree variants.

config RCU_NEED_SEGCBLIST
	def_bool ( TREE_RCU || PREEMPT_RCU || TREE_SRCU )

config CONTEXT_TRACKING
       bool

config CONTEXT_TRACKING_FORCE
	bool "Force context tracking"
	depends on CONTEXT_TRACKING
	default y if !NO_HZ_FULL
	help
	  The major pre-requirement for full dynticks to work is to
	  support the context tracking subsystem. But there are also
	  other dependencies to provide in order to make the full
	  dynticks working.

	  This option stands for testing when an arch implements the
	  context tracking backend but doesn't yet fullfill all the
	  requirements to make the full dynticks feature working.
	  Without the full dynticks, there is no way to test the support
	  for context tracking and the subsystems that rely on it: RCU
	  userspace extended quiescent state and tickless cputime
	  accounting. This option copes with the absence of the full
	  dynticks subsystem by forcing the context tracking on all
	  CPUs in the system.

	  Say Y only if you're working on the development of an
	  architecture backend for the context tracking.

	  Say N otherwise, this option brings an overhead that you
	  don't want in production.


config RCU_FANOUT
	int "Tree-based hierarchical RCU fanout value"
	range 2 64 if 64BIT
	range 2 32 if !64BIT
	depends on (TREE_RCU || PREEMPT_RCU) && RCU_EXPERT
	default 64 if 64BIT
	default 32 if !64BIT
	help
	  This option controls the fanout of hierarchical implementations
	  of RCU, allowing RCU to work efficiently on machines with
	  large numbers of CPUs.  This value must be at least the fourth
	  root of NR_CPUS, which allows NR_CPUS to be insanely large.
	  The default value of RCU_FANOUT should be used for production
	  systems, but if you are stress-testing the RCU implementation
	  itself, small RCU_FANOUT values allow you to test large-system
	  code paths on small(er) systems.

	  Select a specific number if testing RCU itself.
	  Take the default if unsure.

config RCU_FANOUT_LEAF
	int "Tree-based hierarchical RCU leaf-level fanout value"
	range 2 64 if 64BIT
	range 2 32 if !64BIT
	depends on (TREE_RCU || PREEMPT_RCU) && RCU_EXPERT
	default 16
	help
	  This option controls the leaf-level fanout of hierarchical
	  implementations of RCU, and allows trading off cache misses
	  against lock contention.  Systems that synchronize their
	  scheduling-clock interrupts for energy-efficiency reasons will
	  want the default because the smaller leaf-level fanout keeps
	  lock contention levels acceptably low.  Very large systems
	  (hundreds or thousands of CPUs) will instead want to set this
	  value to the maximum value possible in order to reduce the
	  number of cache misses incurred during RCU's grace-period
	  initialization.  These systems tend to run CPU-bound, and thus
	  are not helped by synchronized interrupts, and thus tend to
	  skew them, which reduces lock contention enough that large
	  leaf-level fanouts work well.  That said, setting leaf-level
	  fanout to a large number will likely cause problematic
	  lock contention on the leaf-level rcu_node structures unless
	  you boot with the skew_tick kernel parameter.

	  Select a specific number if testing RCU itself.

	  Select the maximum permissible value for large systems, but
	  please understand that you may also need to set the skew_tick
	  kernel boot parameter to avoid contention on the rcu_node
	  structure's locks.

	  Take the default if unsure.

config RCU_FAST_NO_HZ
	bool "Accelerate last non-dyntick-idle CPU's grace periods"
	depends on NO_HZ_COMMON && SMP && RCU_EXPERT
	default n
	help
	  This option permits CPUs to enter dynticks-idle state even if
	  they have RCU callbacks queued, and prevents RCU from waking
	  these CPUs up more than roughly once every four jiffies (by
	  default, you can adjust this using the rcutree.rcu_idle_gp_delay
	  parameter), thus improving energy efficiency.  On the other
	  hand, this option increases the duration of RCU grace periods,
	  for example, slowing down synchronize_rcu().

	  Say Y if energy efficiency is critically important, and you
	  	don't care about increased grace-period durations.

	  Say N if you are unsure.

config RCU_BOOST
	bool "Enable RCU priority boosting"
	depends on RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT
	default n
	help
	  This option boosts the priority of preempted RCU readers that
	  block the current preemptible RCU grace period for too long.
	  This option also prevents heavy loads from blocking RCU
	  callback invocation for all flavors of RCU.

	  Say Y here if you are working with real-time apps or heavy loads
	  Say N here if you are unsure.

config RCU_BOOST_DELAY
	int "Milliseconds to delay boosting after RCU grace-period start"
	range 0 3000
	depends on RCU_BOOST
	default 500
	help
	  This option specifies the time to wait after the beginning of
	  a given grace period before priority-boosting preempted RCU
	  readers blocking that grace period.  Note that any RCU reader
	  blocking an expedited RCU grace period is boosted immediately.

	  Accept the default if unsure.

config RCU_NOCB_CPU
	bool "Offload RCU callback processing from boot-selected CPUs"
	depends on TREE_RCU || PREEMPT_RCU
	depends on RCU_EXPERT || NO_HZ_FULL
	default n
	help
	  Use this option to reduce OS jitter for aggressive HPC or
	  real-time workloads.	It can also be used to offload RCU
	  callback invocation to energy-efficient CPUs in battery-powered
	  asymmetric multiprocessors.

	  This option offloads callback invocation from the set of
	  CPUs specified at boot time by the rcu_nocbs parameter.
	  For each such CPU, a kthread ("rcuox/N") will be created to
	  invoke callbacks, where the "N" is the CPU being offloaded,
	  and where the "x" is "b" for RCU-bh, "p" for RCU-preempt, and
	  "s" for RCU-sched.  Nothing prevents this kthread from running
	  on the specified CPUs, but (1) the kthreads may be preempted
	  between each callback, and (2) affinity or cgroups can be used
	  to force the kthreads to run on whatever set of CPUs is desired.

	  Say Y here if you want to help to debug reduced OS jitter.
	  Say N here if you are unsure.

endmenu # "RCU Subsystem"
Commit	Line	Data
0af92d46 PM	1	#
	2	# RCU-related configuration options
	3	#
	4
	5	menu "RCU Subsystem"
	6
	7	config TREE_RCU
	8	bool
	9	default y if !PREEMPT && SMP
	10	help
	11	This option selects the RCU implementation that is
	12	designed for very large SMP system with hundreds or
	13	thousands of CPUs. It also scales down nicely to
	14	smaller systems.
	15
	16	config PREEMPT_RCU
	17	bool
	18	default y if PREEMPT
	19	help
	20	This option selects the RCU implementation that is
	21	designed for very large SMP systems with hundreds or
	22	thousands of CPUs, but for which real-time response
	23	is also required. It also scales down nicely to
	24	smaller systems.
	25
	26	Select this option if you are unsure.
	27
	28	config TINY_RCU
	29	bool
	30	default y if !PREEMPT && !SMP
	31	help
	32	This option selects the RCU implementation that is
	33	designed for UP systems from which real-time response
	34	is not required. This option greatly reduces the
	35	memory footprint of RCU.
	36
	37	config RCU_EXPERT
	38	bool "Make expert-level adjustments to RCU configuration"
	39	default n
	40	help
	41	This option needs to be enabled if you wish to make
	42	expert-level adjustments to RCU configuration. By default,
	43	no such adjustments can be made, which has the often-beneficial
	44	side-effect of preventing "make oldconfig" from asking you all
	45	sorts of detailed questions about how you would like numerous
	46	obscure RCU options to be set up.
	47
	48	Say Y if you need to make expert-level adjustments to RCU.
	49
	50	Say N if you are unsure.
	51
	52	config SRCU
	53	bool
	54	help
	55	This option selects the sleepable version of RCU. This version
	56	permits arbitrary sleeping or blocking within RCU read-side critical
	57	sections.
	58
	59	config TINY_SRCU
	60	bool
	61	default y if SRCU && TINY_RCU
	62	help
	63	This option selects the single-CPU non-preemptible version of SRCU.
	64
65	config TREE_SRCU
66	bool
67	default y if SRCU && !TINY_RCU
68	help
69	This option selects the full-fledged version of SRCU.
70
71	config TASKS_RCU
7e42776d	72	def_bool PREEMPT
0af92d46 PM	73	select SRCU
	74	help
	75	This option enables a task-based RCU implementation that uses
	76	only voluntary context switch (not preemption!), idle, and
	77	user-mode execution as quiescent states.
	78
	79	config RCU_STALL_COMMON
6d48152e	80	def_bool ( TREE_RCU \|\| PREEMPT_RCU )
0af92d46 PM	81	help
	82	This option enables RCU CPU stall code that is common between
	83	the TINY and TREE variants of RCU. The purpose is to allow
	84	the tiny variants to disable RCU CPU stall warnings, while
	85	making these warnings mandatory for the tree variants.
	86
	87	config RCU_NEED_SEGCBLIST
	88	def_bool ( TREE_RCU \|\| PREEMPT_RCU \|\| TREE_SRCU )
	89
	90	config CONTEXT_TRACKING
	91	bool
	92
	93	config CONTEXT_TRACKING_FORCE
	94	bool "Force context tracking"
	95	depends on CONTEXT_TRACKING
	96	default y if !NO_HZ_FULL
	97	help
	98	The major pre-requirement for full dynticks to work is to
	99	support the context tracking subsystem. But there are also
	100	other dependencies to provide in order to make the full
	101	dynticks working.
	102
	103	This option stands for testing when an arch implements the
	104	context tracking backend but doesn't yet fullfill all the
	105	requirements to make the full dynticks feature working.
	106	Without the full dynticks, there is no way to test the support
	107	for context tracking and the subsystems that rely on it: RCU
	108	userspace extended quiescent state and tickless cputime
	109	accounting. This option copes with the absence of the full
	110	dynticks subsystem by forcing the context tracking on all
	111	CPUs in the system.
	112
	113	Say Y only if you're working on the development of an
	114	architecture backend for the context tracking.
	115
	116	Say N otherwise, this option brings an overhead that you
	117	don't want in production.
	118
	119
	120	config RCU_FANOUT
	121	int "Tree-based hierarchical RCU fanout value"
	122	range 2 64 if 64BIT
	123	range 2 32 if !64BIT
	124	depends on (TREE_RCU \|\| PREEMPT_RCU) && RCU_EXPERT
	125	default 64 if 64BIT
	126	default 32 if !64BIT
	127	help
	128	This option controls the fanout of hierarchical implementations
	129	of RCU, allowing RCU to work efficiently on machines with
	130	large numbers of CPUs. This value must be at least the fourth
	131	root of NR_CPUS, which allows NR_CPUS to be insanely large.
	132	The default value of RCU_FANOUT should be used for production
	133	systems, but if you are stress-testing the RCU implementation
	134	itself, small RCU_FANOUT values allow you to test large-system
	135	code paths on small(er) systems.
	136
	137	Select a specific number if testing RCU itself.
	138	Take the default if unsure.
	139
	140	config RCU_FANOUT_LEAF
	141	int "Tree-based hierarchical RCU leaf-level fanout value"
	142	range 2 64 if 64BIT
	143	range 2 32 if !64BIT
	144	depends on (TREE_RCU \|\| PREEMPT_RCU) && RCU_EXPERT
145	default 16
146	help
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.
163
164	Select a specific number if testing RCU itself.
165
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
169	structure's locks.
170
171	Take the default if unsure.
172
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
176	default n
177	help
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().
185
186	Say Y if energy efficiency is critically important, and you
187	don't care about increased grace-period durations.
188
189	Say N if you are unsure.
190
191	config RCU_BOOST
192	bool "Enable RCU priority boosting"
193	depends on RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT
194	default n
195	help
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
199	callback invocation for all flavors of RCU.
200
201	Say Y here if you are working with real-time apps or heavy loads
202	Say N here if you are unsure.
203
204	config RCU_BOOST_DELAY
205	int "Milliseconds to delay boosting after RCU grace-period start"
206	range 0 3000
207	depends on RCU_BOOST
208	default 500
209	help
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.
214
215	Accept the default if unsure.
216
217	config RCU_NOCB_CPU
218	bool "Offload RCU callback processing from boot-selected CPUs"
219	depends on TREE_RCU \|\| PREEMPT_RCU
220	depends on RCU_EXPERT \|\| NO_HZ_FULL
221	default n
222	help
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.
227
228	This option offloads callback invocation from the set of
229	CPUs specified at boot time by the rcu_nocbs parameter.
230	For each such CPU, a kthread ("rcuox/N") will be created to
231	invoke callbacks, where the "N" is the CPU being offloaded,
232	and where the "x" is "b" for RCU-bh, "p" for RCU-preempt, and
233	"s" for RCU-sched. Nothing prevents this kthread from running
234	on the specified CPUs, but (1) the kthreads may be preempted
235	between each callback, and (2) affinity or cgroups can be used
236	to force the kthreads to run on whatever set of CPUs is desired.
237
238	Say Y here if you want to help to debug reduced OS jitter.
239	Say N here if you are unsure.
240
241	endmenu # "RCU Subsystem"