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References: <20211008115347.425234-1-valentin.schneider@arm.com> <20211008115347.425234-3-valentin.schneider@arm.com>
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From:   Barry Song <21cnbao@gmail.com>
Date:   Sat, 9 Oct 2021 01:37:29 +1300
Message-ID: <CAGsJ_4wqtcOdsFDzR98PFbjxRyTqzf7P3p3erup84SXESYonYw@mail.gmail.com>
Subject: Re: [PATCH 2/2] sched: Centralize SCHED_{SMT, MC, CLUSTER} definitions
To:     Valentin Schneider <valentin.schneider@arm.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        LAK <linux-arm-kernel@lists.infradead.org>,
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On Sat, Oct 9, 2021 at 12:54 AM Valentin Schneider
<valentin.schneider@arm.com> wrote:
>
> Barry recently introduced a new CONFIG_SCHED_CLUSTER, and discussions
> around that highlighted that every architecture redefines its own help text
> and dependencies for CONFIG_SCHED_SMT and CONFIG_SCHED_MC.
>
> Move the definition of those to scheduler's Kconfig to centralize help text
> and generic dependencies (i.e. SMP). Make them depend on a matching
> ARCH_SUPPORTS_SCHED_* which the architectures can select with the relevant
> architecture-specific dependency.
>
> s390 uses its own topology table (set_sched_topology()) and doesn't seem to
> cope without SCHED_MC or SCHED_SMT, so those remain untogglable.
>

Hi Valentin,
Thanks!
I believe this is a cleaner way for Kconfig itself. But I am not quite sure this
is always beneficial of all platforms. It would be perfect if the patch has no
side effects and doesn't change the existing behaviour. But it has side effects
by changing the default N to Y on a couple of platforms.


> Suggested-by: Barry Song <21cnbao@gmail.com>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> ---
>  arch/arm/Kconfig     | 18 ++----------------
>  arch/arm64/Kconfig   | 26 +++-----------------------
>  arch/ia64/Kconfig    |  9 +--------
>  arch/mips/Kconfig    | 10 +---------
>  arch/parisc/Kconfig  |  9 +--------
>  arch/powerpc/Kconfig |  9 +--------
>  arch/s390/Kconfig    |  8 ++------
>  arch/sh/Kconfig      |  1 +
>  arch/sh/mm/Kconfig   |  9 ---------
>  arch/sparc/Kconfig   | 20 ++------------------
>  arch/x86/Kconfig     | 26 +++-----------------------
>  kernel/sched/Kconfig | 43 +++++++++++++++++++++++++++++++++++++++++++
>  12 files changed, 60 insertions(+), 128 deletions(-)
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index fc196421b2ce..13aac98edf06 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -32,6 +32,8 @@ config ARM
>         select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT if CPU_V7
>         select ARCH_SUPPORTS_ATOMIC_RMW
>         select ARCH_SUPPORTS_HUGETLBFS if ARM_LPAE
> +       select ARCH_SUPPORTS_SCHED_SMT if ARM_CPU_TOPOLOGY
> +       select ARCH_SUPPORTS_SCHED_MC if ARM_CPU_TOPOLOGY
>         select ARCH_USE_BUILTIN_BSWAP
>         select ARCH_USE_CMPXCHG_LOCKREF
>         select ARCH_USE_MEMTEST
> @@ -1166,22 +1168,6 @@ config ARM_CPU_TOPOLOGY
>           affinity between processors which is then used to describe the cpu
>           topology of an ARM System.
>
> -config SCHED_MC
> -       bool "Multi-core scheduler support"
> -       depends on ARM_CPU_TOPOLOGY
> -       help
> -         Multi-core scheduler support improves the CPU scheduler's decision
> -         making when dealing with multi-core CPU chips at a cost of slightly
> -         increased overhead in some places. If unsure say N here.
> -
> -config SCHED_SMT
> -       bool "SMT scheduler support"
> -       depends on ARM_CPU_TOPOLOGY
> -       help
> -         Improves the CPU scheduler's decision making when dealing with
> -         MultiThreading at a cost of slightly increased overhead in some
> -         places. If unsure say N here.
> -
>  config HAVE_ARM_SCU
>         bool
>         help
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index d13677f4731d..8a49dd33f5e3 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -88,6 +88,9 @@ config ARM64
>         select ARCH_SUPPORTS_ATOMIC_RMW
>         select ARCH_SUPPORTS_INT128 if CC_HAS_INT128
>         select ARCH_SUPPORTS_NUMA_BALANCING
> +       select ARCH_SUPPORTS_SCHED_SMT
> +       select ARCH_SUPPORTS_SCHED_MC
> +       select ARCH_SUPPORTS_SCHED_CLUSTER
>         select ARCH_WANT_COMPAT_IPC_PARSE_VERSION if COMPAT
>         select ARCH_WANT_DEFAULT_BPF_JIT
>         select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
> @@ -982,29 +985,6 @@ config CPU_LITTLE_ENDIAN
>
>  endchoice
>
> -config SCHED_MC
> -       bool "Multi-core scheduler support"
> -       help
> -         Multi-core scheduler support improves the CPU scheduler's decision
> -         making when dealing with multi-core CPU chips at a cost of slightly
> -         increased overhead in some places. If unsure say N here.
> -
> -config SCHED_CLUSTER
> -       bool "Cluster scheduler support"
> -       help
> -         Cluster scheduler support improves the CPU scheduler's decision
> -         making when dealing with machines that have clusters of CPUs.
> -         Cluster usually means a couple of CPUs which are placed closely
> -         by sharing mid-level caches, last-level cache tags or internal
> -         busses.
> -
> -config SCHED_SMT
> -       bool "SMT scheduler support"
> -       help
> -         Improves the CPU scheduler's decision making when dealing with
> -         MultiThreading at a cost of slightly increased overhead in some
> -         places. If unsure say N here.
> -
>  config NR_CPUS
>         int "Maximum number of CPUs (2-4096)"
>         range 2 4096
> diff --git a/arch/ia64/Kconfig b/arch/ia64/Kconfig
> index 045792cde481..67f3d84242ae 100644
> --- a/arch/ia64/Kconfig
> +++ b/arch/ia64/Kconfig
> @@ -18,6 +18,7 @@ config IA64
>         select ARCH_ENABLE_MEMORY_HOTPLUG
>         select ARCH_ENABLE_MEMORY_HOTREMOVE
>         select ARCH_SUPPORTS_ACPI
> +       select ARCH_SUPPORTS_SCHED_SMT
>         select ACPI_SYSTEM_POWER_STATES_SUPPORT if ACPI
>         select ARCH_MIGHT_HAVE_ACPI_PDC if ACPI
>         select FORCE_PCI
> @@ -247,14 +248,6 @@ config HOTPLUG_CPU
>           can be controlled through /sys/devices/system/cpu/cpu#.
>           Say N if you want to disable CPU hotplug.
>
> -config SCHED_SMT
> -       bool "SMT scheduler support"
> -       depends on SMP
> -       help
> -         Improves the CPU scheduler's decision making when dealing with
> -         Intel IA64 chips with MultiThreading at a cost of slightly increased
> -         overhead in some places. If unsure say N here.
> -
>  config PERMIT_BSP_REMOVE
>         bool "Support removal of Bootstrap Processor"
>         depends on HOTPLUG_CPU
> diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
> index 771ca53af06d..cc60d440b097 100644
> --- a/arch/mips/Kconfig
> +++ b/arch/mips/Kconfig
> @@ -2373,17 +2373,9 @@ config MIPS_MT_SMP
>  config MIPS_MT
>         bool
>
> -config SCHED_SMT
> -       bool "SMT (multithreading) scheduler support"
> -       depends on SYS_SUPPORTS_SCHED_SMT
> -       default n
> -       help
> -         SMT scheduler support improves the CPU scheduler's decision making
> -         when dealing with MIPS MT enabled cores at a cost of slightly
> -         increased overhead in some places. If unsure say N here.
> -
>  config SYS_SUPPORTS_SCHED_SMT
>         bool
> +       select ARCH_SUPPORTS_SCHED_SMT
>
>  config SYS_SUPPORTS_MULTITHREADING
>         bool
> diff --git a/arch/parisc/Kconfig b/arch/parisc/Kconfig
> index 4742b6f169b7..6aaa962ec2f4 100644
> --- a/arch/parisc/Kconfig
> +++ b/arch/parisc/Kconfig
> @@ -13,6 +13,7 @@ config PARISC
>         select ARCH_NO_SG_CHAIN
>         select ARCH_SUPPORTS_HUGETLBFS if PA20
>         select ARCH_SUPPORTS_MEMORY_FAILURE
> +       select ARCH_SUPPORTS_SCHED_MC if PARISC_CPU_TOPOLOGY && PA8X00
>         select DMA_OPS
>         select RTC_CLASS
>         select RTC_DRV_GENERIC
> @@ -295,14 +296,6 @@ config PARISC_CPU_TOPOLOGY
>         help
>           Support PARISC cpu topology definition.
>
> -config SCHED_MC
> -       bool "Multi-core scheduler support"
> -       depends on PARISC_CPU_TOPOLOGY && PA8X00
> -       help
> -         Multi-core scheduler support improves the CPU scheduler's decision
> -         making when dealing with multi-core CPU chips at a cost of slightly
> -         increased overhead in some places. If unsure say N here.
> -
>  config IRQSTACKS
>         bool "Use separate kernel stacks when processing interrupts"
>         default y
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index ba5b66189358..9f45b92ccac1 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -151,6 +151,7 @@ config PPC
>         select ARCH_STACKWALK
>         select ARCH_SUPPORTS_ATOMIC_RMW
>         select ARCH_SUPPORTS_DEBUG_PAGEALLOC    if PPC32 || PPC_BOOK3S_64
> +       select ARCH_SUPPORTS_SCHED_SMT          if PPC64
>         select ARCH_USE_BUILTIN_BSWAP
>         select ARCH_USE_CMPXCHG_LOCKREF         if PPC64
>         select ARCH_USE_MEMTEST
> @@ -861,14 +862,6 @@ config PPC_PROT_SAO_LPAR
>  config PPC_COPRO_BASE
>         bool
>
> -config SCHED_SMT
> -       bool "SMT (Hyperthreading) scheduler support"
> -       depends on PPC64 && SMP
> -       help
> -         SMT scheduler support improves the CPU scheduler's decision making
> -         when dealing with POWER5 cpus at a cost of slightly increased
> -         overhead in some places. If unsure say N here.
> -
>  config PPC_DENORMALISATION
>         bool "PowerPC denormalisation exception handling"
>         depends on PPC_BOOK3S_64
> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
> index b86de61b8caa..a0b4117cb1fa 100644
> --- a/arch/s390/Kconfig
> +++ b/arch/s390/Kconfig
> @@ -483,12 +483,6 @@ config NODES_SHIFT
>         depends on NUMA
>         default "1"
>
> -config SCHED_SMT
> -       def_bool n
> -
> -config SCHED_MC
> -       def_bool n
> -
>  config SCHED_BOOK
>         def_bool n
>
> @@ -498,6 +492,8 @@ config SCHED_DRAWER
>  config SCHED_TOPOLOGY
>         def_bool y
>         prompt "Topology scheduler support"
> +       select ARCH_SUPPORTS_SCHED_SMT
> +       select ARCH_SUPPORTS_SCHED_MC
>         select SCHED_SMT
>         select SCHED_MC
>         select SCHED_BOOK
> diff --git a/arch/sh/Kconfig b/arch/sh/Kconfig
> index 6904f4bdbf00..7380ee27d252 100644
> --- a/arch/sh/Kconfig
> +++ b/arch/sh/Kconfig
> @@ -13,6 +13,7 @@ config SUPERH
>         select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
>         select ARCH_HIBERNATION_POSSIBLE if MMU
>         select ARCH_MIGHT_HAVE_PC_PARPORT
> +       select ARCH_SUPPORTS_SCHED_MC
>         select ARCH_WANT_IPC_PARSE_VERSION
>         select CPU_NO_EFFICIENT_FFS
>         select DMA_DECLARE_COHERENT
> diff --git a/arch/sh/mm/Kconfig b/arch/sh/mm/Kconfig
> index ba569cfb4368..1d9f7006a72a 100644
> --- a/arch/sh/mm/Kconfig
> +++ b/arch/sh/mm/Kconfig
> @@ -208,15 +208,6 @@ config HUGETLB_PAGE_SIZE_64MB
>
>  endchoice
>
> -config SCHED_MC
> -       bool "Multi-core scheduler support"
> -       depends on SMP
> -       default y
> -       help
> -         Multi-core scheduler support improves the CPU scheduler's decision
> -         making when dealing with multi-core CPU chips at a cost of slightly
> -         increased overhead in some places. If unsure say N here.
> -
>  endmenu
>
>  menu "Cache configuration"
> diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
> index b120ed947f50..a6cf30d37725 100644
> --- a/arch/sparc/Kconfig
> +++ b/arch/sparc/Kconfig
> @@ -86,6 +86,8 @@ config SPARC64
>         select HAVE_ARCH_AUDITSYSCALL
>         select ARCH_SUPPORTS_ATOMIC_RMW
>         select ARCH_SUPPORTS_DEBUG_PAGEALLOC
> +       select ARCH_SUPPORTS_SCHED_SMT
> +       select ARCH_SUPPORTS_SCHED_MC
>         select HAVE_NMI
>         select HAVE_REGS_AND_STACK_ACCESS_API
>         select ARCH_USE_QUEUED_RWLOCKS
> @@ -290,24 +292,6 @@ if SPARC64
>  source "kernel/power/Kconfig"
>  endif
>
> -config SCHED_SMT
> -       bool "SMT (Hyperthreading) scheduler support"
> -       depends on SPARC64 && SMP
> -       default y
> -       help
> -         SMT scheduler support improves the CPU scheduler's decision making
> -         when dealing with SPARC cpus at a cost of slightly increased overhead
> -         in some places. If unsure say N here.
> -
> -config SCHED_MC
> -       bool "Multi-core scheduler support"
> -       depends on SPARC64 && SMP
> -       default y
> -       help
> -         Multi-core scheduler support improves the CPU scheduler's decision
> -         making when dealing with multi-core CPU chips at a cost of slightly
> -         increased overhead in some places. If unsure say N here.
> -
>  config CMDLINE_BOOL
>         bool "Default bootloader kernel arguments"
>         depends on SPARC64
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 349e59b2f0e3..87a91fd33d85 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -107,6 +107,9 @@ config X86
>         select ARCH_SUPPORTS_KMAP_LOCAL_FORCE_MAP       if NR_CPUS <= 4096
>         select ARCH_SUPPORTS_LTO_CLANG
>         select ARCH_SUPPORTS_LTO_CLANG_THIN
> +       select ARCH_SUPPORTS_SCHED_SMT
> +       select ARCH_SUPPORTS_SCHED_MC
> +       select ARCH_SUPPORTS_SCHED_CLUSTER
>         select ARCH_USE_BUILTIN_BSWAP
>         select ARCH_USE_MEMTEST
>         select ARCH_USE_QUEUED_RWLOCKS
> @@ -1001,29 +1004,6 @@ config NR_CPUS
>           This is purely to save memory: each supported CPU adds about 8KB
>           to the kernel image.
>
> -config SCHED_CLUSTER
> -       bool "Cluster scheduler support"
> -       depends on SMP
> -       default y
> -       help
> -         Cluster scheduler support improves the CPU scheduler's decision
> -         making when dealing with machines that have clusters of CPUs.
> -         Cluster usually means a couple of CPUs which are placed closely
> -         by sharing mid-level caches, last-level cache tags or internal
> -         busses.
> -
> -config SCHED_SMT
> -       def_bool y if SMP
> -
> -config SCHED_MC
> -       def_bool y
> -       prompt "Multi-core scheduler support"
> -       depends on SMP
> -       help
> -         Multi-core scheduler support improves the CPU scheduler's decision
> -         making when dealing with multi-core CPU chips at a cost of slightly
> -         increased overhead in some places. If unsure say N here.
> -
>  config SCHED_MC_PRIO
>         bool "CPU core priorities scheduler support"
>         depends on SCHED_MC && CPU_SUP_INTEL
> diff --git a/kernel/sched/Kconfig b/kernel/sched/Kconfig
> index c8b8e12c9c9c..9ceb08f42aeb 100644
> --- a/kernel/sched/Kconfig
> +++ b/kernel/sched/Kconfig
> @@ -131,3 +131,46 @@ config SCHED_CORE
>           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.
> +
> +
> +#
> +# For architectuers that want to enable generic scheduler handling of
> +# different topology levels:
> +#
> +config ARCH_SUPPORTS_SCHED_SMT
> +       bool
> +
> +config ARCH_SUPPORTS_SCHED_MC
> +       bool
> +
> +config ARCH_SUPPORTS_SCHED_CLUSTER
> +       bool
> +
> +config SCHED_SMT
> +       bool "SMT scheduler support"
> +       depends on ARCH_SUPPORTS_SCHED_SMT && SMP
> +       default y
> +       help
> +         Improves the CPU scheduler's decision making when dealing with
> +         MultiThreading at a cost of slightly increased overhead in some
> +         places. If unsure say N here.
> +
> +config SCHED_MC
> +       bool "Multi-core scheduler support"
> +       depends on ARCH_SUPPORTS_SCHED_MC && SMP
> +       default y
> +       help
> +         Multi-core scheduler support improves the CPU scheduler's decision
> +         making when dealing with multi-core CPU chips at a cost of slightly
> +         increased overhead in some places. If unsure say N here.
> +
> +config SCHED_CLUSTER
> +       bool "Cluster scheduler support"
> +       depends on ARCH_SUPPORTS_SCHED_CLUSTER && SMP
> +       default y
> +       help
> +         Cluster scheduler support improves the CPU scheduler's decision
> +         making when dealing with machines that have clusters of CPUs.
> +         Cluster usually means a couple of CPUs which are placed closely
> +         by sharing mid-level caches, last-level cache tags or internal
> +         busses.
> --
> 2.25.1
>

Thanks
barry