kernel/rcu/Kconfig

   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         help
  12           This option selects the RCU implementation that is
  13           designed for very large SMP system with hundreds or
  14           thousands of CPUs.  It also scales down nicely to
  15           smaller systems.
  16
  17 config PREEMPT_RCU
  18         bool
  19         default y if PREEMPTION
  20         select TREE_RCU
  21         help
  22           This option selects the RCU implementation that is
  23           designed for very large SMP systems with hundreds or
  24           thousands of CPUs, but for which real-time response
  25           is also required.  It also scales down nicely to
  26           smaller systems.
  27
  28           Select this option if you are unsure.
  29
  30 config TINY_RCU
  31         bool
  32         default y if !PREEMPTION && !SMP
  33         help
  34           This option selects the RCU implementation that is
  35           designed for UP systems from which real-time response
  36           is not required.  This option greatly reduces the
  37           memory footprint of RCU.
  38
  39 config RCU_EXPERT
  40         bool "Make expert-level adjustments to RCU configuration"
  41         default n
  42         help
  43           This option needs to be enabled if you wish to make
  44           expert-level adjustments to RCU configuration.  By default,
  45           no such adjustments can be made, which has the often-beneficial
  46           side-effect of preventing "make oldconfig" from asking you all
  47           sorts of detailed questions about how you would like numerous
  48           obscure RCU options to be set up.
  49
  50           Say Y if you need to make expert-level adjustments to RCU.
  51
  52           Say N if you are unsure.
  53
  54 config SRCU
  55         bool
  56         help
  57           This option selects the sleepable version of RCU. This version
  58           permits arbitrary sleeping or blocking within RCU read-side critical
  59           sections.
  60
  61 config TINY_SRCU
  62         bool
  63         default y if SRCU && TINY_RCU
  64         help
  65           This option selects the single-CPU non-preemptible version of SRCU.
  66
  67 config TREE_SRCU
  68         bool
  69         default y if SRCU && !TINY_RCU
  70         help
  71           This option selects the full-fledged version of SRCU.
  72
  73 config TASKS_RCU
  74         def_bool PREEMPTION
  75         select SRCU
  76         help
  77           This option enables a task-based RCU implementation that uses
  78           only voluntary context switch (not preemption!), idle, and
  79           user-mode execution as quiescent states.
  80
  81 config RCU_STALL_COMMON
  82         def_bool TREE_RCU
  83         help
  84           This option enables RCU CPU stall code that is common between
  85           the TINY and TREE variants of RCU.  The purpose is to allow
  86           the tiny variants to disable RCU CPU stall warnings, while
  87           making these warnings mandatory for the tree variants.
  88
  89 config RCU_NEED_SEGCBLIST
  90         def_bool ( TREE_RCU || TREE_SRCU )
  91
  92 config RCU_FANOUT
  93         int "Tree-based hierarchical RCU fanout value"
  94         range 2 64 if 64BIT
  95         range 2 32 if !64BIT
  96         depends on TREE_RCU && RCU_EXPERT
  97         default 64 if 64BIT
  98         default 32 if !64BIT
  99         help
 100           This option controls the fanout of hierarchical implementations
 101           of RCU, allowing RCU to work efficiently on machines with
 102           large numbers of CPUs.  This value must be at least the fourth
 103           root of NR_CPUS, which allows NR_CPUS to be insanely large.
 104           The default value of RCU_FANOUT should be used for production
 105           systems, but if you are stress-testing the RCU implementation
 106           itself, small RCU_FANOUT values allow you to test large-system
 107           code paths on small(er) systems.
 108
 109           Select a specific number if testing RCU itself.
 110           Take the default if unsure.
 111
 112 config RCU_FANOUT_LEAF
 113         int "Tree-based hierarchical RCU leaf-level fanout value"
 114         range 2 64 if 64BIT
 115         range 2 32 if !64BIT
 116         depends on TREE_RCU && RCU_EXPERT
 117         default 16
 118         help
 119           This option controls the leaf-level fanout of hierarchical
 120           implementations of RCU, and allows trading off cache misses
 121           against lock contention.  Systems that synchronize their
 122           scheduling-clock interrupts for energy-efficiency reasons will
 123           want the default because the smaller leaf-level fanout keeps
 124           lock contention levels acceptably low.  Very large systems
 125           (hundreds or thousands of CPUs) will instead want to set this
 126           value to the maximum value possible in order to reduce the
 127           number of cache misses incurred during RCU's grace-period
 128           initialization.  These systems tend to run CPU-bound, and thus
 129           are not helped by synchronized interrupts, and thus tend to
 130           skew them, which reduces lock contention enough that large
 131           leaf-level fanouts work well.  That said, setting leaf-level
 132           fanout to a large number will likely cause problematic
 133           lock contention on the leaf-level rcu_node structures unless
 134           you boot with the skew_tick kernel parameter.
 135
 136           Select a specific number if testing RCU itself.
 137
 138           Select the maximum permissible value for large systems, but
 139           please understand that you may also need to set the skew_tick
 140           kernel boot parameter to avoid contention on the rcu_node
 141           structure's locks.
 142
 143           Take the default if unsure.
 144
 145 config RCU_FAST_NO_HZ
 146         bool "Accelerate last non-dyntick-idle CPU's grace periods"
 147         depends on NO_HZ_COMMON && SMP && RCU_EXPERT
 148         default n
 149         help
 150           This option permits CPUs to enter dynticks-idle state even if
 151           they have RCU callbacks queued, and prevents RCU from waking
 152           these CPUs up more than roughly once every four jiffies (by
 153           default, you can adjust this using the rcutree.rcu_idle_gp_delay
 154           parameter), thus improving energy efficiency.  On the other
 155           hand, this option increases the duration of RCU grace periods,
 156           for example, slowing down synchronize_rcu().
 157
 158           Say Y if energy efficiency is critically important, and you
 159                 don't care about increased grace-period durations.
 160
 161           Say N if you are unsure.
 162
 163 config RCU_BOOST
 164         bool "Enable RCU priority boosting"
 165         depends on RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT
 166         default n
 167         help
 168           This option boosts the priority of preempted RCU readers that
 169           block the current preemptible RCU grace period for too long.
 170           This option also prevents heavy loads from blocking RCU
 171           callback invocation.
 172
 173           Say Y here if you are working with real-time apps or heavy loads
 174           Say N here if you are unsure.
 175
 176 config RCU_BOOST_DELAY
 177         int "Milliseconds to delay boosting after RCU grace-period start"
 178         range 0 3000
 179         depends on RCU_BOOST
 180         default 500
 181         help
 182           This option specifies the time to wait after the beginning of
 183           a given grace period before priority-boosting preempted RCU
 184           readers blocking that grace period.  Note that any RCU reader
 185           blocking an expedited RCU grace period is boosted immediately.
 186
 187           Accept the default if unsure.
 188
 189 config RCU_NOCB_CPU
 190         bool "Offload RCU callback processing from boot-selected CPUs"
 191         depends on TREE_RCU
 192         depends on RCU_EXPERT || NO_HZ_FULL
 193         default n
 194         help
 195           Use this option to reduce OS jitter for aggressive HPC or
 196           real-time workloads.  It can also be used to offload RCU
 197           callback invocation to energy-efficient CPUs in battery-powered
 198           asymmetric multiprocessors.
 199
 200           This option offloads callback invocation from the set of CPUs
 201           specified at boot time by the rcu_nocbs parameter.  For each
 202           such CPU, a kthread ("rcuox/N") will be created to invoke
 203           callbacks, where the "N" is the CPU being offloaded, and where
 204           the "p" for RCU-preempt (PREEMPTION kernels) and "s" for RCU-sched
 205           (!PREEMPTION kernels).  Nothing prevents this kthread from running
 206           on the specified CPUs, but (1) the kthreads may be preempted
 207           between each callback, and (2) affinity or cgroups can be used
 208           to force the kthreads to run on whatever set of CPUs is desired.
 209
 210           Say Y here if you want to help to debug reduced OS jitter.
 211           Say N here if you are unsure.
 212
 213 endmenu # "RCU Subsystem"