kernel/Kconfig.preempt

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

config PREEMPT_NONE_BUILD
	bool

config PREEMPT_VOLUNTARY_BUILD
	bool

config PREEMPT_BUILD
	bool
	select PREEMPTION
	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK

config ARCH_HAS_PREEMPT_LAZY
	bool

choice
	prompt "Preemption Model"
	default PREEMPT_LAZY if ARCH_HAS_PREEMPT_LAZY
	default PREEMPT_NONE

config PREEMPT_NONE
	bool "No Forced Preemption (Server)"
	depends on !PREEMPT_RT
	depends on ARCH_NO_PREEMPT
	select PREEMPT_NONE_BUILD if !PREEMPT_DYNAMIC
	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_HAS_PREEMPT_LAZY
	depends on !ARCH_NO_PREEMPT
	depends on !PREEMPT_RT
	select PREEMPT_VOLUNTARY_BUILD if !PREEMPT_DYNAMIC
	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 PREEMPT_BUILD if !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_LAZY
	bool "Scheduler controlled preemption model"
	depends on !ARCH_NO_PREEMPT
	depends on ARCH_HAS_PREEMPT_LAZY
	select PREEMPT_BUILD if !PREEMPT_DYNAMIC
	help
	  This option provides a scheduler driven preemption model that
	  is fundamentally similar to full preemption, but is less
	  eager to preempt SCHED_NORMAL tasks in an attempt to
	  reduce lock holder preemption and recover some of the performance
	  gains seen from using Voluntary preemption.

endchoice

config PREEMPT_RT
	bool "Fully Preemptible Kernel (Real-Time)"
	depends on EXPERT && ARCH_SUPPORTS_RT && !COMPILE_TEST
	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.

config PREEMPT_RT_NEEDS_BH_LOCK
	bool "Enforce softirq synchronisation on PREEMPT_RT"
	depends on PREEMPT_RT
	help
	  Enforce synchronisation across the softirqs context. On PREEMPT_RT
	  the softirq is preemptible. This enforces the same per-CPU BLK
	  semantic non-PREEMPT_RT builds have. This should not be needed
	  because per-CPU locks were added to avoid the per-CPU BKL.

	  This switch provides the old behaviour for testing reasons. Select
	  this if you suspect an error with preemptible softirq and want test
	  the old synchronized behaviour.

config PREEMPT_COUNT
       bool

config HAS_SEPARATE_PREEMPT_RESCHED_BITS
	bool
	depends on PREEMPT_COUNT && 64BIT

config PREEMPTION
       bool
       select PREEMPT_COUNT

config PREEMPT_DYNAMIC
	bool "Preemption behaviour defined on boot"
	depends on HAVE_PREEMPT_DYNAMIC
	select JUMP_LABEL if HAVE_PREEMPT_DYNAMIC_KEY
	select PREEMPT_BUILD
	default y if HAVE_PREEMPT_DYNAMIC_CALL
	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.

config SCHED_CLASS_EXT
	bool "Extensible Scheduling Class"
	depends on BPF_SYSCALL && BPF_JIT && DEBUG_INFO_BTF
	select GENERIC_ALLOCATOR
	select STACKTRACE if STACKTRACE_SUPPORT
	help
	  This option enables a new scheduler class sched_ext (SCX), which
	  allows scheduling policies to be implemented as BPF programs to
	  achieve the following:

	  - Ease of experimentation and exploration: Enabling rapid
	    iteration of new scheduling policies.
	  - Customization: Building application-specific schedulers which
	    implement policies that are not applicable to general-purpose
	    schedulers.
	  - Rapid scheduler deployments: Non-disruptive swap outs of
	    scheduling policies in production environments.

	  sched_ext leverages BPF struct_ops feature to define a structure
	  which exports function callbacks and flags to BPF programs that
	  wish to implement scheduling policies. The struct_ops structure
	  exported by sched_ext is struct sched_ext_ops, and is conceptually
	  similar to struct sched_class.

	  For more information:
	    Documentation/scheduler/sched-ext.rst
	    https://github.com/sched-ext/scx