[mirror_ubuntu-artful-kernel.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
	bool
	default n
	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
72	bool
73	default n
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
80	config RCU_STALL_COMMON
6d48152e	81	def_bool ( TREE_RCU \|\| PREEMPT_RCU )
0af92d46 PM	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
	88	config RCU_NEED_SEGCBLIST
	89	def_bool ( TREE_RCU \|\| PREEMPT_RCU \|\| TREE_SRCU )
	90
	91	config CONTEXT_TRACKING
	92	bool
	93
	94	config CONTEXT_TRACKING_FORCE
	95	bool "Force context tracking"
	96	depends on CONTEXT_TRACKING
	97	default y if !NO_HZ_FULL
	98	help
	99	The major pre-requirement for full dynticks to work is to
	100	support the context tracking subsystem. But there are also
	101	other dependencies to provide in order to make the full
	102	dynticks working.
	103
	104	This option stands for testing when an arch implements the
	105	context tracking backend but doesn't yet fullfill all the
	106	requirements to make the full dynticks feature working.
	107	Without the full dynticks, there is no way to test the support
	108	for context tracking and the subsystems that rely on it: RCU
	109	userspace extended quiescent state and tickless cputime
	110	accounting. This option copes with the absence of the full
	111	dynticks subsystem by forcing the context tracking on all
	112	CPUs in the system.
	113
	114	Say Y only if you're working on the development of an
	115	architecture backend for the context tracking.
	116
	117	Say N otherwise, this option brings an overhead that you
	118	don't want in production.
	119
	120
	121	config RCU_FANOUT
	122	int "Tree-based hierarchical RCU fanout value"
	123	range 2 64 if 64BIT
	124	range 2 32 if !64BIT
	125	depends on (TREE_RCU \|\| PREEMPT_RCU) && RCU_EXPERT
	126	default 64 if 64BIT
	127	default 32 if !64BIT
	128	help
	129	This option controls the fanout of hierarchical implementations
	130	of RCU, allowing RCU to work efficiently on machines with
	131	large numbers of CPUs. This value must be at least the fourth
	132	root of NR_CPUS, which allows NR_CPUS to be insanely large.
	133	The default value of RCU_FANOUT should be used for production
	134	systems, but if you are stress-testing the RCU implementation
	135	itself, small RCU_FANOUT values allow you to test large-system
	136	code paths on small(er) systems.
	137
	138	Select a specific number if testing RCU itself.
	139	Take the default if unsure.
	140
	141	config RCU_FANOUT_LEAF
	142	int "Tree-based hierarchical RCU leaf-level fanout value"
	143	range 2 64 if 64BIT
	144	range 2 32 if !64BIT
	145	depends on (TREE_RCU \|\| PREEMPT_RCU) && RCU_EXPERT
146	default 16
147	help
148	This option controls the leaf-level fanout of hierarchical
149	implementations of RCU, and allows trading off cache misses
150	against lock contention. Systems that synchronize their
151	scheduling-clock interrupts for energy-efficiency reasons will
152	want the default because the smaller leaf-level fanout keeps
153	lock contention levels acceptably low. Very large systems
154	(hundreds or thousands of CPUs) will instead want to set this
155	value to the maximum value possible in order to reduce the
156	number of cache misses incurred during RCU's grace-period
157	initialization. These systems tend to run CPU-bound, and thus
158	are not helped by synchronized interrupts, and thus tend to
159	skew them, which reduces lock contention enough that large
160	leaf-level fanouts work well. That said, setting leaf-level
161	fanout to a large number will likely cause problematic
162	lock contention on the leaf-level rcu_node structures unless
163	you boot with the skew_tick kernel parameter.
164
165	Select a specific number if testing RCU itself.
166
167	Select the maximum permissible value for large systems, but
168	please understand that you may also need to set the skew_tick
169	kernel boot parameter to avoid contention on the rcu_node
170	structure's locks.
171
172	Take the default if unsure.
173
174	config RCU_FAST_NO_HZ
175	bool "Accelerate last non-dyntick-idle CPU's grace periods"
176	depends on NO_HZ_COMMON && SMP && RCU_EXPERT
177	default n
178	help
179	This option permits CPUs to enter dynticks-idle state even if
180	they have RCU callbacks queued, and prevents RCU from waking
181	these CPUs up more than roughly once every four jiffies (by
182	default, you can adjust this using the rcutree.rcu_idle_gp_delay
183	parameter), thus improving energy efficiency. On the other
184	hand, this option increases the duration of RCU grace periods,
185	for example, slowing down synchronize_rcu().
186
187	Say Y if energy efficiency is critically important, and you
188	don't care about increased grace-period durations.
189
190	Say N if you are unsure.
191
192	config RCU_BOOST
193	bool "Enable RCU priority boosting"
194	depends on RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT
195	default n
196	help
197	This option boosts the priority of preempted RCU readers that
198	block the current preemptible RCU grace period for too long.
199	This option also prevents heavy loads from blocking RCU
200	callback invocation for all flavors of RCU.
201
202	Say Y here if you are working with real-time apps or heavy loads
203	Say N here if you are unsure.
204
205	config RCU_BOOST_DELAY
206	int "Milliseconds to delay boosting after RCU grace-period start"
207	range 0 3000
208	depends on RCU_BOOST
209	default 500
210	help
211	This option specifies the time to wait after the beginning of
212	a given grace period before priority-boosting preempted RCU
213	readers blocking that grace period. Note that any RCU reader
214	blocking an expedited RCU grace period is boosted immediately.
215
216	Accept the default if unsure.
217
218	config RCU_NOCB_CPU
219	bool "Offload RCU callback processing from boot-selected CPUs"
220	depends on TREE_RCU \|\| PREEMPT_RCU
221	depends on RCU_EXPERT \|\| NO_HZ_FULL
222	default n
223	help
224	Use this option to reduce OS jitter for aggressive HPC or
225	real-time workloads. It can also be used to offload RCU
226	callback invocation to energy-efficient CPUs in battery-powered
227	asymmetric multiprocessors.
228
229	This option offloads callback invocation from the set of
230	CPUs specified at boot time by the rcu_nocbs parameter.
231	For each such CPU, a kthread ("rcuox/N") will be created to
232	invoke callbacks, where the "N" is the CPU being offloaded,
233	and where the "x" is "b" for RCU-bh, "p" for RCU-preempt, and
234	"s" for RCU-sched. Nothing prevents this kthread from running
235	on the specified CPUs, but (1) the kthreads may be preempted
236	between each callback, and (2) affinity or cgroups can be used
237	to force the kthreads to run on whatever set of CPUs is desired.
238
239	Say Y here if you want to help to debug reduced OS jitter.
240	Say N here if you are unsure.
241
242	endmenu # "RCU Subsystem"