kernel/rcu/Kconfig

   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         def_bool PREEMPT
  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
  80         def_bool ( TREE_RCU || PREEMPT_RCU )
  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"