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    Subject[PATCH 0/6] Drain remote per-cpu directly v3
    Date
    Changelog since v2
    o More conversions from page->lru to page->[pcp_list|buddy_list]
    o Additional test results in changelogs

    Changelog since v1
    o Fix unsafe RT locking scheme
    o Use spin_trylock on UP PREEMPT_RT

    This series has the same intent as Nicolas' series "mm/page_alloc: Remote
    per-cpu lists drain support" -- avoid interference of a high priority
    task due to a workqueue item draining per-cpu page lists. While many
    workloads can tolerate a brief interruption, it may be cause a real-time
    task runnning on a NOHZ_FULL CPU to miss a deadline and at minimum,
    the draining in non-deterministic.

    Currently an IRQ-safe local_lock protects the page allocator per-cpu lists.
    The local_lock on its own prevents migration and the IRQ disabling protects
    from corruption due to an interrupt arriving while a page allocation is
    in progress. The locking is inherently unsafe for remote access unless
    the CPU is hot-removed.

    This series adjusts the locking. A spinlock is added to struct
    per_cpu_pages to protect the list contents while local_lock_irq continues
    to prevent migration and IRQ reentry. This allows a remote CPU to safely
    drain a remote per-cpu list.

    This series is a partial series. Follow-on work should allow the
    local_irq_save to be converted to a local_irq to avoid IRQs being
    disabled/enabled in most cases. Consequently, there are some TODO comments
    highlighting the places that would change if local_irq was used. However,
    there are enough corner cases that it deserves a series on its own
    separated by one kernel release and the priority right now is to avoid
    interference of high priority tasks.

    Patch 1 is a cosmetic patch to clarify when page->lru is storing buddy pages
    and when it is storing per-cpu pages.

    Patch 2 shrinks per_cpu_pages to make room for a spin lock. Strictly speaking
    this is not necessary but it avoids per_cpu_pages consuming another
    cache line.

    Patch 3 is a preparation patch to avoid code duplication.

    Patch 4 is a simple micro-optimisation that improves code flow necessary for
    a later patch to avoid code duplication.

    Patch 5 uses a spin_lock to protect the per_cpu_pages contents while still
    relying on local_lock to prevent migration, stabilise the pcp
    lookup and prevent IRQ reentrancy.

    Patch 6 remote drains per-cpu pages directly instead of using a workqueue.

    include/linux/mm_types.h | 5 +
    include/linux/mmzone.h | 12 +-
    mm/page_alloc.c | 348 +++++++++++++++++++++++++--------------
    3 files changed, 233 insertions(+), 132 deletions(-)

    --
    2.34.1

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