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Kconfig.preempt
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Kconfig.preempt
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# 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
choice
prompt "Preemption Model"
default PREEMPT_NONE
config PREEMPT_NONE
bool "No Forced Preemption (Server)"
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_NO_PREEMPT
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
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 "Preemption behaviour defined on boot"
depends on HAVE_PREEMPT_DYNAMIC && !PREEMPT_RT
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
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’s 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.