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a8b76910e4
Commit c597bfddc9 ("sched: Provide Kconfig support for default dynamic
preempt mode") changed the selectable config names for the preemption
model. This means a config file must now select
CONFIG_PREEMPT_BEHAVIOUR=y
rather than
CONFIG_PREEMPT=y
to get a preemptible kernel. This means all arch config files would need to
be updated - right now they'll all end up with the default
CONFIG_PREEMPT_NONE_BEHAVIOUR.
Rather than touch a good hundred of config files, restore usage of
CONFIG_PREEMPT{_NONE, _VOLUNTARY}. Make them configure:
o The build-time preemption model when !PREEMPT_DYNAMIC
o The default boot-time preemption model when PREEMPT_DYNAMIC
Add siblings of those configs with the _BUILD suffix to unconditionally
designate the build-time preemption model (PREEMPT_DYNAMIC is built with
the "highest" preemption model it supports, aka PREEMPT). Downstream
configs should by now all be depending / selected by CONFIG_PREEMPTION
rather than CONFIG_PREEMPT, so only a few sites need patching up.
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Marco Elver <elver@google.com>
Link: https://lore.kernel.org/r/20211110202448.4054153-2-valentin.schneider@arm.com
136 lines
4.7 KiB
Plaintext
136 lines
4.7 KiB
Plaintext
# SPDX-License-Identifier: GPL-2.0-only
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config PREEMPT_NONE_BUILD
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bool
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config PREEMPT_VOLUNTARY_BUILD
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bool
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config PREEMPT_BUILD
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bool
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select PREEMPTION
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select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
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choice
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prompt "Preemption Model"
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default PREEMPT_NONE
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config PREEMPT_NONE
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bool "No Forced Preemption (Server)"
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select PREEMPT_NONE_BUILD if !PREEMPT_DYNAMIC
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help
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This is the traditional Linux preemption model, geared towards
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throughput. It will still provide good latencies most of the
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time, but there are no guarantees and occasional longer delays
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are possible.
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Select this option if you are building a kernel for a server or
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scientific/computation system, or if you want to maximize the
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raw processing power of the kernel, irrespective of scheduling
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latencies.
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config PREEMPT_VOLUNTARY
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bool "Voluntary Kernel Preemption (Desktop)"
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depends on !ARCH_NO_PREEMPT
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select PREEMPT_VOLUNTARY_BUILD if !PREEMPT_DYNAMIC
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help
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This option reduces the latency of the kernel by adding more
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"explicit preemption points" to the kernel code. These new
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preemption points have been selected to reduce the maximum
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latency of rescheduling, providing faster application reactions,
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at the cost of slightly lower throughput.
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This allows reaction to interactive events by allowing a
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low priority process to voluntarily preempt itself even if it
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is in kernel mode executing a system call. This allows
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applications to run more 'smoothly' even when the system is
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under load.
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Select this if you are building a kernel for a desktop system.
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config PREEMPT
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bool "Preemptible Kernel (Low-Latency Desktop)"
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depends on !ARCH_NO_PREEMPT
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select PREEMPT_BUILD
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help
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This option reduces the latency of the kernel by making
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all kernel code (that is not executing in a critical section)
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preemptible. This allows reaction to interactive events by
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permitting a low priority process to be preempted involuntarily
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even if it is in kernel mode executing a system call and would
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otherwise not be about to reach a natural preemption point.
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This allows applications to run more 'smoothly' even when the
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system is under load, at the cost of slightly lower throughput
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and a slight runtime overhead to kernel code.
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Select this if you are building a kernel for a desktop or
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embedded system with latency requirements in the milliseconds
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range.
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config PREEMPT_RT
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bool "Fully Preemptible Kernel (Real-Time)"
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depends on EXPERT && ARCH_SUPPORTS_RT
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select PREEMPTION
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help
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This option turns the kernel into a real-time kernel by replacing
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various locking primitives (spinlocks, rwlocks, etc.) with
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preemptible priority-inheritance aware variants, enforcing
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interrupt threading and introducing mechanisms to break up long
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non-preemptible sections. This makes the kernel, except for very
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low level and critical code paths (entry code, scheduler, low
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level interrupt handling) fully preemptible and brings most
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execution contexts under scheduler control.
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Select this if you are building a kernel for systems which
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require real-time guarantees.
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endchoice
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config PREEMPT_COUNT
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bool
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config PREEMPTION
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bool
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select PREEMPT_COUNT
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config PREEMPT_DYNAMIC
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bool "Preemption behaviour defined on boot"
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depends on HAVE_PREEMPT_DYNAMIC && !PREEMPT_RT
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select PREEMPT_BUILD
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default y
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help
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This option allows to define the preemption model on the kernel
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command line parameter and thus override the default preemption
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model defined during compile time.
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The feature is primarily interesting for Linux distributions which
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provide a pre-built kernel binary to reduce the number of kernel
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flavors they offer while still offering different usecases.
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The runtime overhead is negligible with HAVE_STATIC_CALL_INLINE enabled
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but if runtime patching is not available for the specific architecture
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then the potential overhead should be considered.
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Interesting if you want the same pre-built kernel should be used for
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both Server and Desktop workloads.
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config SCHED_CORE
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bool "Core Scheduling for SMT"
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depends on SCHED_SMT
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help
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This option permits Core Scheduling, a means of coordinated task
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selection across SMT siblings. When enabled -- see
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prctl(PR_SCHED_CORE) -- task selection ensures that all SMT siblings
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will execute a task from the same 'core group', forcing idle when no
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matching task is found.
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Use of this feature includes:
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- mitigation of some (not all) SMT side channels;
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- limiting SMT interference to improve determinism and/or performance.
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SCHED_CORE is default disabled. When it is enabled and unused,
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which is the likely usage by Linux distributions, there should
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be no measurable impact on performance.
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