kernel/Kconfig.smp

# Copyright (c) 2024 Intel Corp.
# SPDX-License-Identifier: Apache-2.0
#
menu "SMP Options"

config SMP
	bool "Symmetric multiprocessing support"
	depends on USE_SWITCH
	depends on !ATOMIC_OPERATIONS_C
	help
	  When true, kernel will be built with SMP support, allowing
	  more than one CPU to schedule Zephyr tasks at a time.

config USE_SWITCH
	bool "Use new-style _arch_switch instead of arch_swap"
	depends on USE_SWITCH_SUPPORTED
	help
	  The _arch_switch() API is a lower level context switching
	  primitive than the original arch_swap mechanism.  It is required
	  for an SMP-aware scheduler, or if the architecture does not
	  provide arch_swap.  In uniprocess situations where the
	  architecture provides both, _arch_switch incurs more somewhat
	  overhead and may be slower.

config USE_SWITCH_SUPPORTED
	bool
	help
	  Indicates whether _arch_switch() API is supported by the
	  currently enabled platform. This option should be selected by
	  platforms that implement it.

config SMP_BOOT_DELAY
	bool "Delay booting secondary cores"
	depends on SMP
	help
	  By default Zephyr will boot all available CPUs during start up.
	  Select this option to skip this and allow custom code
	  (architecture/SoC/board/application) to boot secondary CPUs at
	  a later time.

config MP_MAX_NUM_CPUS
	int "Maximum number of CPUs/cores"
	default 1
	range 1 12
	help
	  Maximum number of multiprocessing-capable cores available to the
	  multicpu API and SMP features.

config SCHED_IPI_SUPPORTED
	bool
	help
	  True if the architecture supports a call to arch_sched_broadcast_ipi()
	  to broadcast an interrupt that will call z_sched_ipi() on other CPUs
	  in the system.  Required for k_thread_abort() to operate with
	  reasonable latency (otherwise we might have to wait for the other
	  thread to take an interrupt, which can be arbitrarily far in the
	  future).

config SCHED_IPI_CASCADE
	bool "Use cascading IPIs to correct localized scheduling"
	depends on SCHED_CPU_MASK && !SCHED_CPU_MASK_PIN_ONLY
	default n
	help
	  Threads that are preempted by a local thread (a thread that is
	  restricted by its CPU mask to execute on a subset of all CPUs) may
	  trigger additional IPIs when the preempted thread is of higher
	  priority than a currently executing thread on another CPU. Although
	  these cascading IPIs will ensure that the system will settle upon a
	  valid set of high priority threads, it comes at a performance cost.

config TRACE_SCHED_IPI
	bool "Test IPI"
	help
	  When true, it will add a hook into z_sched_ipi(), in order
	  to check if schedule IPI has called or not, for testing
	  purpose.
	depends on SCHED_IPI_SUPPORTED
	depends on MP_MAX_NUM_CPUS>1

config IPI_OPTIMIZE
	bool "Optimize IPI delivery"
	default n
	depends on SCHED_IPI_SUPPORTED && MP_MAX_NUM_CPUS>1
	help
	  When selected, the kernel will attempt to determine the minimum
	  set of CPUs that need an IPI to trigger a reschedule in response to
	  a thread newly made ready for execution. This increases the
	  computation required at every scheduler operation by a value that is
	  O(N) in the number of CPUs, and in exchange reduces the number of
	  interrupts delivered. Which to choose is going to depend on
	  application behavior. If the architecture also supports directing
	  IPIs to specific CPUs then this has the potential to significantly
	  reduce the number of IPIs (and consequently ISRs) processed by the
	  system as the number of CPUs increases. If not, the only benefit
	  would be to not issue any IPIs if the newly readied thread is of
	  lower priority than all the threads currently executing on other CPUs.

config KERNEL_COHERENCE
	bool "Place all shared data into coherent memory"
	depends on ARCH_HAS_COHERENCE
	default y if SMP && MP_MAX_NUM_CPUS > 1
	select THREAD_STACK_INFO
	help
	  When available and selected, the kernel will build in a mode
	  where all shared data is placed in multiprocessor-coherent
	  (generally "uncached") memory.  Thread stacks will remain
	  cached, as will application memory declared with
	  __incoherent.  This is intended for Zephyr SMP kernels
	  running on cache-incoherent architectures only.  Note that
	  when this is selected, there is an implicit API change that
	  assumes cache coherence to any memory passed to the kernel.
	  Code that creates kernel data structures in uncached regions
	  may fail strangely.  Some assertions exist to catch these
	  mistakes, but not all circumstances can be tested.

config TICKET_SPINLOCKS
	bool "Ticket spinlocks for lock acquisition fairness [EXPERIMENTAL]"
	select EXPERIMENTAL
	help
	  Basic spinlock implementation is based on single
	  atomic variable and doesn't guarantee locking fairness
	  across multiple CPUs. It's even possible that single CPU
	  will win the contention every time which will result
	  in a live-lock.
	  Ticket spinlocks provide a FIFO order of lock acquisition
	  which resolves such unfairness issue at the cost of slightly
	  increased memory footprint.

endmenu