kernel/Kconfig.preempt

   1 # SPDX-License-Identifier: GPL-2.0-only
   2
   3 choice
   4         prompt "Preemption Model"
   5         default PREEMPT_NONE
   6
   7 config PREEMPT_NONE
   8         bool "No Forced Preemption (Server)"
   9         help
  10           This is the traditional Linux preemption model, geared towards
  11           throughput. It will still provide good latencies most of the
  12           time, but there are no guarantees and occasional longer delays
  13           are possible.
  14
  15           Select this option if you are building a kernel for a server or
  16           scientific/computation system, or if you want to maximize the
  17           raw processing power of the kernel, irrespective of scheduling
  18           latencies.
  19
  20 config PREEMPT_VOLUNTARY
  21         bool "Voluntary Kernel Preemption (Desktop)"
  22         depends on !ARCH_NO_PREEMPT
  23         help
  24           This option reduces the latency of the kernel by adding more
  25           "explicit preemption points" to the kernel code. These new
  26           preemption points have been selected to reduce the maximum
  27           latency of rescheduling, providing faster application reactions,
  28           at the cost of slightly lower throughput.
  29
  30           This allows reaction to interactive events by allowing a
  31           low priority process to voluntarily preempt itself even if it
  32           is in kernel mode executing a system call. This allows
  33           applications to run more 'smoothly' even when the system is
  34           under load.
  35
  36           Select this if you are building a kernel for a desktop system.
  37
  38 config PREEMPT
  39         bool "Preemptible Kernel (Low-Latency Desktop)"
  40         depends on !ARCH_NO_PREEMPT
  41         select PREEMPTION
  42         select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
  43         select PREEMPT_DYNAMIC if HAVE_PREEMPT_DYNAMIC
  44         help
  45           This option reduces the latency of the kernel by making
  46           all kernel code (that is not executing in a critical section)
  47           preemptible.  This allows reaction to interactive events by
  48           permitting a low priority process to be preempted involuntarily
  49           even if it is in kernel mode executing a system call and would
  50           otherwise not be about to reach a natural preemption point.
  51           This allows applications to run more 'smoothly' even when the
  52           system is under load, at the cost of slightly lower throughput
  53           and a slight runtime overhead to kernel code.
  54
  55           Select this if you are building a kernel for a desktop or
  56           embedded system with latency requirements in the milliseconds
  57           range.
  58
  59 config PREEMPT_RT
  60         bool "Fully Preemptible Kernel (Real-Time)"
  61         depends on EXPERT && ARCH_SUPPORTS_RT
  62         select PREEMPTION
  63         help
  64           This option turns the kernel into a real-time kernel by replacing
  65           various locking primitives (spinlocks, rwlocks, etc.) with
  66           preemptible priority-inheritance aware variants, enforcing
  67           interrupt threading and introducing mechanisms to break up long
  68           non-preemptible sections. This makes the kernel, except for very
  69           low level and critical code paths (entry code, scheduler, low
  70           level interrupt handling) fully preemptible and brings most
  71           execution contexts under scheduler control.
  72
  73           Select this if you are building a kernel for systems which
  74           require real-time guarantees.
  75
  76 endchoice
  77
  78 config PREEMPT_COUNT
  79        bool
  80
  81 config PREEMPTION
  82        bool
  83        select PREEMPT_COUNT
  84
  85 config PREEMPT_DYNAMIC
  86         bool
  87         help
  88           This option allows to define the preemption model on the kernel
  89           command line parameter and thus override the default preemption
  90           model defined during compile time.
  91
  92           The feature is primarily interesting for Linux distributions which
  93           provide a pre-built kernel binary to reduce the number of kernel
  94           flavors they offer while still offering different usecases.
  95
  96           The runtime overhead is negligible with HAVE_STATIC_CALL_INLINE enabled
  97           but if runtime patching is not available for the specific architecture
  98           then the potential overhead should be considered.
  99
 100           Interesting if you want the same pre-built kernel should be used for
 101           both Server and Desktop workloads.
 102
 103 config SCHED_CORE
 104         bool "Core Scheduling for SMT"
 105         depends on SCHED_SMT
 106         help
 107           This option permits Core Scheduling, a means of coordinated task
 108           selection across SMT siblings. When enabled -- see
 109           prctl(PR_SCHED_CORE) -- task selection ensures that all SMT siblings
 110           will execute a task from the same 'core group', forcing idle when no
 111           matching task is found.
 112
 113           Use of this feature includes:
 114            - mitigation of some (not all) SMT side channels;
 115            - limiting SMT interference to improve determinism and/or performance.
 116
 117           SCHED_CORE is default disabled. When it is enabled and unused,
 118           which is the likely usage by Linux distributions, there should
 119           be no measurable impact on performance.
 120
 121