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_GENERIC
  74         def_bool TASKS_RCU || TASKS_RUDE_RCU || TASKS_TRACE_RCU
  75         select SRCU
  76         help
  77           This option enables generic infrastructure code supporting
  78           task-based RCU implementations.  Not for manual selection.
  79
  80 config TASKS_RCU
  81         def_bool PREEMPTION
  82         help
  83           This option enables a task-based RCU implementation that uses
  84           only voluntary context switch (not preemption!), idle, and
  85           user-mode execution as quiescent states.  Not for manual selection.
  86
  87 config TASKS_RUDE_RCU
  88         def_bool 0
  89         help
  90           This option enables a task-based RCU implementation that uses
  91           only context switch (including preemption) and user-mode
  92           execution as quiescent states.  It forces IPIs and context
  93           switches on all online CPUs, including idle ones, so use
  94           with caution.
  95
  96 config TASKS_TRACE_RCU
  97         def_bool 0
  98         select IRQ_WORK
  99         help
 100           This option enables a task-based RCU implementation that uses
 101           explicit rcu_read_lock_trace() read-side markers, and allows
 102           these readers to appear in the idle loop as well as on the CPU
 103           hotplug code paths.  It can force IPIs on online CPUs, including
 104           idle ones, so use with caution.
 105
 106 config RCU_STALL_COMMON
 107         def_bool TREE_RCU
 108         help
 109           This option enables RCU CPU stall code that is common between
 110           the TINY and TREE variants of RCU.  The purpose is to allow
 111           the tiny variants to disable RCU CPU stall warnings, while
 112           making these warnings mandatory for the tree variants.
 113
 114 config RCU_NEED_SEGCBLIST
 115         def_bool ( TREE_RCU || TREE_SRCU )
 116
 117 config RCU_FANOUT
 118         int "Tree-based hierarchical RCU fanout value"
 119         range 2 64 if 64BIT
 120         range 2 32 if !64BIT
 121         depends on TREE_RCU && RCU_EXPERT
 122         default 64 if 64BIT
 123         default 32 if !64BIT
 124         help
 125           This option controls the fanout of hierarchical implementations
 126           of RCU, allowing RCU to work efficiently on machines with
 127           large numbers of CPUs.  This value must be at least the fourth
 128           root of NR_CPUS, which allows NR_CPUS to be insanely large.
 129           The default value of RCU_FANOUT should be used for production
 130           systems, but if you are stress-testing the RCU implementation
 131           itself, small RCU_FANOUT values allow you to test large-system
 132           code paths on small(er) systems.
 133
 134           Select a specific number if testing RCU itself.
 135           Take the default if unsure.
 136
 137 config RCU_FANOUT_LEAF
 138         int "Tree-based hierarchical RCU leaf-level fanout value"
 139         range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD
 140         range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD
 141         range 2 3 if RCU_STRICT_GRACE_PERIOD
 142         depends on TREE_RCU && RCU_EXPERT
 143         default 16 if !RCU_STRICT_GRACE_PERIOD
 144         default 2 if RCU_STRICT_GRACE_PERIOD
 145         help
 146           This option controls the leaf-level fanout of hierarchical
 147           implementations of RCU, and allows trading off cache misses
 148           against lock contention.  Systems that synchronize their
 149           scheduling-clock interrupts for energy-efficiency reasons will
 150           want the default because the smaller leaf-level fanout keeps
 151           lock contention levels acceptably low.  Very large systems
 152           (hundreds or thousands of CPUs) will instead want to set this
 153           value to the maximum value possible in order to reduce the
 154           number of cache misses incurred during RCU's grace-period
 155           initialization.  These systems tend to run CPU-bound, and thus
 156           are not helped by synchronized interrupts, and thus tend to
 157           skew them, which reduces lock contention enough that large
 158           leaf-level fanouts work well.  That said, setting leaf-level
 159           fanout to a large number will likely cause problematic
 160           lock contention on the leaf-level rcu_node structures unless
 161           you boot with the skew_tick kernel parameter.
 162
 163           Select a specific number if testing RCU itself.
 164
 165           Select the maximum permissible value for large systems, but
 166           please understand that you may also need to set the skew_tick
 167           kernel boot parameter to avoid contention on the rcu_node
 168           structure's locks.
 169
 170           Take the default if unsure.
 171
 172 config RCU_FAST_NO_HZ
 173         bool "Accelerate last non-dyntick-idle CPU's grace periods"
 174         depends on NO_HZ_COMMON && SMP && RCU_EXPERT
 175         default n
 176         help
 177           This option permits CPUs to enter dynticks-idle state even if
 178           they have RCU callbacks queued, and prevents RCU from waking
 179           these CPUs up more than roughly once every four jiffies (by
 180           default, you can adjust this using the rcutree.rcu_idle_gp_delay
 181           parameter), thus improving energy efficiency.  On the other
 182           hand, this option increases the duration of RCU grace periods,
 183           for example, slowing down synchronize_rcu().
 184
 185           Say Y if energy efficiency is critically important, and you
 186                 don't care about increased grace-period durations.
 187
 188           Say N if you are unsure.
 189
 190 config RCU_BOOST
 191         bool "Enable RCU priority boosting"
 192         depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT
 193         default y if PREEMPT_RT
 194         help
 195           This option boosts the priority of preempted RCU readers that
 196           block the current preemptible RCU grace period for too long.
 197           This option also prevents heavy loads from blocking RCU
 198           callback invocation.
 199
 200           Say Y here if you are working with real-time apps or heavy loads
 201           Say N here if you are unsure.
 202
 203 config RCU_BOOST_DELAY
 204         int "Milliseconds to delay boosting after RCU grace-period start"
 205         range 0 3000
 206         depends on RCU_BOOST
 207         default 500
 208         help
 209           This option specifies the time to wait after the beginning of
 210           a given grace period before priority-boosting preempted RCU
 211           readers blocking that grace period.  Note that any RCU reader
 212           blocking an expedited RCU grace period is boosted immediately.
 213
 214           Accept the default if unsure.
 215
 216 config RCU_NOCB_CPU
 217         bool "Offload RCU callback processing from boot-selected CPUs"
 218         depends on TREE_RCU
 219         depends on RCU_EXPERT || NO_HZ_FULL
 220         default n
 221         help
 222           Use this option to reduce OS jitter for aggressive HPC or
 223           real-time workloads.  It can also be used to offload RCU
 224           callback invocation to energy-efficient CPUs in battery-powered
 225           asymmetric multiprocessors.  The price of this reduced jitter
 226           is that the overhead of call_rcu() increases and that some
 227           workloads will incur significant increases in context-switch
 228           rates.
 229
 230           This option offloads callback invocation from the set of CPUs
 231           specified at boot time by the rcu_nocbs parameter.  For each
 232           such CPU, a kthread ("rcuox/N") will be created to invoke
 233           callbacks, where the "N" is the CPU being offloaded, and where
 234           the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
 235           RCU-sched (!PREEMPTION kernels).  Nothing prevents this kthread
 236           from running on the specified CPUs, but (1) the kthreads may be
 237           preempted between each callback, and (2) affinity or cgroups can
 238           be used to force the kthreads to run on whatever set of CPUs is
 239           desired.
 240
 241           Say Y here if you need reduced OS jitter, despite added overhead.
 242           Say N here if you are unsure.
 243
 244 config TASKS_TRACE_RCU_READ_MB
 245         bool "Tasks Trace RCU readers use memory barriers in user and idle"
 246         depends on RCU_EXPERT
 247         default PREEMPT_RT || NR_CPUS < 8
 248         help
 249           Use this option to further reduce the number of IPIs sent
 250           to CPUs executing in userspace or idle during tasks trace
 251           RCU grace periods.  Given that a reasonable setting of
 252           the rcupdate.rcu_task_ipi_delay kernel boot parameter
 253           eliminates such IPIs for many workloads, proper setting
 254           of this Kconfig option is important mostly for aggressive
 255           real-time installations and for battery-powered devices,
 256           hence the default chosen above.
 257
 258           Say Y here if you hate IPIs.
 259           Say N here if you hate read-side memory barriers.
 260           Take the default if you are unsure.
 261
 262 endmenu # "RCU Subsystem"