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    SubjectRe: [PATCH 0/6] Drain remote per-cpu directly v3
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    On Thu, 2022-05-12 at 09:50 +0100, Mel Gorman wrote:
    > 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.

    FWIW tested this against our RT+nohz_full workloads. I can have another go if
    the locking scheme changes.

    Tested-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>

    Thanks,

    --
    Nicolás Sáenz

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