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    SubjectRe: [RFC PATCH 0/3] newidle_balance() latency mitigation
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    On 28/04/20 06:02, Scott Wood wrote:
    > These patches mitigate latency caused by newidle_balance() on large
    > systems, by enabling interrupts when the lock is dropped, and exiting
    > early at various points if an RT task is runnable on the current CPU.
    >
    > When applied to an RT kernel on a 72-core machine (2 threads per core), I
    > saw significant reductions in latency as reported by rteval -- from
    > over 500us to around 160us with hyperthreading disabled, and from
    > over 1400us to around 380us with hyperthreading enabled.
    >
    > This isn't the first time something like this has been tried:
    > https://lore.kernel.org/lkml/20121222003019.433916240@goodmis.org/
    > That attempt ended up being reverted:
    > https://lore.kernel.org/lkml/5122CD9C.9070702@oracle.com/
    >
    > The problem in that case was the failure to keep BH disabled, and the
    > difficulty of fixing that when called from the post_schedule() hook.
    > This patchset uses finish_task_switch() to call newidle_balance(), which
    > enters in non-atomic context so we have full control over what we disable
    > and when.
    >
    > There was a note at the end about wanting further discussion on the matter --
    > does anyone remember if that ever happened and what the conclusion was?
    > Are there any other issues with enabling interrupts here and/or moving
    > the newidle_balance() call?
    >

    Random thought that just occurred to me; in the grand scheme of things,
    with something in the same spirit as task-stealing (i.e. don't bother with
    a full fledged balance at newidle, just pick one spare task somewhere),
    none of this would be required.

    Sadly I don't think anyone has been looking at it any recently.

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