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    Subject[RFC PATCH v5 0/4] scheduler: expose the topology of clusters and add cluster scheduler
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    ARM64 server chip Kunpeng 920 has 6 or 8 clusters in each NUMA node, and each
    cluster has 4 cpus. All clusters share L3 cache data while each cluster has
    local L3 tag. On the other hand, each cluster will share some internal system
    bus. This means cache is much more affine inside one cluster than across
    clusters.

    +-----------------------------------+ +---------+
    | +------+ +------+ +---------------------------+ |
    | | CPU0 | | cpu1 | | +-----------+ | |
    | +------+ +------+ | | | | |
    | +----+ L3 | | |
    | +------+ +------+ cluster | | tag | | |
    | | CPU2 | | CPU3 | | | | | |
    | +------+ +------+ | +-----------+ | |
    | | | |
    +-----------------------------------+ | |
    +-----------------------------------+ | |
    | +------+ +------+ +--------------------------+ |
    | | | | | | +-----------+ | |
    | +------+ +------+ | | | | |
    | | | L3 | | |
    | +------+ +------+ +----+ tag | | |
    | | | | | | | | | |
    | +------+ +------+ | +-----------+ | |
    | | | |
    +-----------------------------------+ | L3 |
    | data |
    +-----------------------------------+ | |
    | +------+ +------+ | +-----------+ | |
    | | | | | | | | | |
    | +------+ +------+ +----+ L3 | | |
    | | | tag | | |
    | +------+ +------+ | | | | |
    | | | | | ++ +-----------+ | |
    | +------+ +------+ |---------------------------+ |
    +-----------------------------------| | |
    +-----------------------------------| | |
    | +------+ +------+ +---------------------------+ |
    | | | | | | +-----------+ | |
    | +------+ +------+ | | | | |
    | +----+ L3 | | |
    | +------+ +------+ | | tag | | |
    | | | | | | | | | |
    | +------+ +------+ | +-----------+ | |
    | | | |
    +-----------------------------------+ | |
    +-----------------------------------+ | |
    | +------+ +------+ +--------------------------+ |
    | | | | | | +-----------+ | |
    | +------+ +------+ | | | | |


    There is a similar need for clustering in x86. Some x86 cores could share L2 caches
    that is similar to the cluster in Kupeng 920 (e.g. on Jacobsville there are 6 clusters
    of 4 Atom cores, each cluster sharing a separate L2, and 24 cores sharing L3).

    Having a sched_domain for clusters will bring two aspects of improvement:
    1. spreading unrelated tasks among clusters, which decreases the contention of resources
    and improve the throughput.
    unrelated tasks might be put randomly without cluster sched_domain:
    +-------------------+ +-----------------+
    | +----+ +----+ | | |
    | |task| |task| | | |
    | |1 | |2 | | | |
    | +----+ +----+ | | |
    | | | |
    | cluster1 | | cluster2 |
    +-------------------+ +-----------------+

    but with cluster sched_domain, they are likely to spread due to LB:
    +-------------------+ +-----------------+
    | +----+ | | +----+ |
    | |task| | | |task| |
    | |1 | | | |2 | |
    | +----+ | | +----+ |
    | | | |
    | cluster1 | | cluster2 |
    +-------------------+ +-----------------+

    2. gathering related tasks within a cluster, which improves the cache affinity of tasks
    talking with each other.
    Without cluster sched_domain, related tasks might be put randomly. In case task1-8 have
    relationship as below:
    Task1 wakes up task4
    Task2 wakes up task5
    Task3 wakes up task6
    Task4 wakes up task7
    With the tuning of select_idle_cpu() to scan local cluster first, those tasks might
    get a chance to be gathered like:
    +---------------------------+ +----------------------+
    | +----+ +-----+ | | +----+ +-----+ |
    | |task| |task | | | |task| |task | |
    | |1 | | 4 | | | |2 | |5 | |
    | +----+ +-----+ | | +----+ +-----+ |
    | | | |
    | cluster1 | | cluster2 |
    | | | |
    | | | |
    | +-----+ +------+ | | +-----+ +------+ |
    | |task | | task | | | |task | |task | |
    | |3 | | 6 | | | |4 | |8 | |
    | +-----+ +------+ | | +-----+ +------+ |
    +---------------------------+ +----------------------+
    Otherwise, the result might be:
    +---------------------------+ +----------------------+
    | +----+ +-----+ | | +----+ +-----+ |
    | |task| |task | | | |task| |task | |
    | |1 | | 2 | | | |5 | |6 | |
    | +----+ +-----+ | | +----+ +-----+ |
    | | | |
    | cluster1 | | cluster2 |
    | | | |
    | | | |
    | +-----+ +------+ | | +-----+ +------+ |
    | |task | | task | | | |task | |task | |
    | |3 | | 4 | | | |7 | |8 | |
    | +-----+ +------+ | | +-----+ +------+ |
    +---------------------------+ +----------------------+

    -v5:
    * split "add scheduler level for clusters" into two patches to evaluate the
    impact of spreading and gathering separately;
    * add a tracepoint of select_idle_cpu for debug purpose; add bcc script in
    commit log;
    * add cluster_id = -1 in reset_cpu_topology()
    * rebased to tip/sched/core

    -v4:
    * rebased to tip/sched/core with the latest unified code of select_idle_cpu
    * added Tim's patch for x86 Jacobsville
    * also added benchmark data of spreading unrelated tasks
    * avoided the iteration of sched_domain by moving to static_key(addressing
    Vincent's comment
    * used acpi_cpu_id for acpi_find_processor_node(addressing Masa's comment)

    Barry Song (2):
    scheduler: add scheduler level for clusters
    scheduler: scan idle cpu in cluster before scanning the whole llc

    Jonathan Cameron (1):
    topology: Represent clusters of CPUs within a die

    Tim Chen (1):
    scheduler: Add cluster scheduler level for x86

    Documentation/admin-guide/cputopology.rst | 26 +++++++++++--
    arch/arm64/Kconfig | 7 ++++
    arch/arm64/kernel/topology.c | 2 +
    arch/x86/Kconfig | 8 ++++
    arch/x86/include/asm/smp.h | 7 ++++
    arch/x86/include/asm/topology.h | 1 +
    arch/x86/kernel/cpu/cacheinfo.c | 1 +
    arch/x86/kernel/cpu/common.c | 3 ++
    arch/x86/kernel/smpboot.c | 43 ++++++++++++++++++++-
    drivers/acpi/pptt.c | 63 +++++++++++++++++++++++++++++++
    drivers/base/arch_topology.c | 15 ++++++++
    drivers/base/topology.c | 10 +++++
    include/linux/acpi.h | 5 +++
    include/linux/arch_topology.h | 5 +++
    include/linux/sched/cluster.h | 19 ++++++++++
    include/linux/sched/topology.h | 7 ++++
    include/linux/topology.h | 13 +++++++
    include/trace/events/sched.h | 22 +++++++++++
    kernel/sched/core.c | 20 ++++++++++
    kernel/sched/fair.c | 36 +++++++++++++++++-
    kernel/sched/sched.h | 1 +
    kernel/sched/topology.c | 5 +++
    22 files changed, 313 insertions(+), 6 deletions(-)
    create mode 100644 include/linux/sched/cluster.h

    --
    1.8.3.1

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