linux/kernel/Kconfig.preempt

# SPDX-License-Identifier: GPL-2.0-only

choice
	prompt "Preemption Model"
	default PREEMPT_NONE

config PREEMPT_NONE
	bool "No Forced Preemption (Server)"
	help
	  This is the traditional Linux preemption model, geared towards
	  throughput. It will still provide good latencies most of the
	  time, but there are no guarantees and occasional longer delays
	  are possible.

	  Select this option if you are building a kernel for a server or
	  scientific/computation system, or if you want to maximize the
	  raw processing power of the kernel, irrespective of scheduling
	  latencies.

config PREEMPT_VOLUNTARY
	bool "Voluntary Kernel Preemption (Desktop)"
	depends on !ARCH_NO_PREEMPT
	help
	  This option reduces the latency of the kernel by adding more
	  "explicit preemption points" to the kernel code. These new
	  preemption points have been selected to reduce the maximum
	  latency of rescheduling, providing faster application reactions,
	  at the cost of slightly lower throughput.

	  This allows reaction to interactive events by allowing a
	  low priority process to voluntarily preempt itself even if it
	  is in kernel mode executing a system call. This allows
	  applications to run more 'smoothly' even when the system is
	  under load.

	  Select this if you are building a kernel for a desktop system.

config PREEMPT
	bool "Preemptible Kernel (Low-Latency Desktop)"
	depends on !ARCH_NO_PREEMPT
	select PREEMPTION
	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
	select PREEMPT_DYNAMIC if HAVE_PREEMPT_DYNAMIC
	help
	  This option reduces the latency of the kernel by making
	  all kernel code (that is not executing in a critical section)
	  preemptible.  This allows reaction to interactive events by
	  permitting a low priority process to be preempted involuntarily
	  even if it is in kernel mode executing a system call and would
	  otherwise not be about to reach a natural preemption point.
	  This allows applications to run more 'smoothly' even when the
	  system is under load, at the cost of slightly lower throughput
	  and a slight runtime overhead to kernel code.

	  Select this if you are building a kernel for a desktop or
	  embedded system with latency requirements in the milliseconds
	  range.

config PREEMPT_RT
	bool "Fully Preemptible Kernel (Real-Time)"
	depends on EXPERT && ARCH_SUPPORTS_RT
	select PREEMPTION
	help
	  This option turns the kernel into a real-time kernel by replacing
	  various locking primitives (spinlocks, rwlocks, etc.) with
	  preemptible priority-inheritance aware variants, enforcing
	  interrupt threading and introducing mechanisms to break up long
	  non-preemptible sections. This makes the kernel, except for very
	  low level and critical code paths (entry code, scheduler, low
	  level interrupt handling) fully preemptible and brings most
	  execution contexts under scheduler control.

	  Select this if you are building a kernel for systems which
	  require real-time guarantees.

endchoice

config PREEMPT_COUNT
       bool

config PREEMPTION
       bool
       select PREEMPT_COUNT

config PREEMPT_DYNAMIC
	bool
	help
	  This option allows to define the preemption model on the kernel
	  command line parameter and thus override the default preemption
	  model defined during compile time.

	  The feature is primarily interesting for Linux distributions which
	  provide a pre-built kernel binary to reduce the number of kernel
	  flavors they offer while still offering different usecases.

	  The runtime overhead is negligible with HAVE_STATIC_CALL_INLINE enabled
	  but if runtime patching is not available for the specific architecture
	  then the potential overhead should be considered.

	  Interesting if you want the same pre-built kernel should be used for
	  both Server and Desktop workloads.

config SCHED_CORE
	bool "Core Scheduling for SMT"
	depends on SCHED_SMT
	help
	  This option permits Core Scheduling, a means of coordinated task
	  selection across SMT siblings. When enabled -- see
	  prctl(PR_SCHED_CORE) -- task selection ensures that all SMT siblings
	  will execute a task from the same 'core group', forcing idle when no
	  matching task is found.

	  Use of this feature includes:
	   - mitigation of some (not all) SMT side channels;
	   - limiting SMT interference to improve determinism and/or performance.

	  SCHED_CORE is default disabled. When it is enabled and unused,
	  which is the likely usage by Linux distributions, there should
	  be no measurable impact on performance.