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    SubjectRe: io_uring vs CPU hotplug, was Re: [PATCH 5/9] blk-mq: don't set data->ctx and data->hctx in blk_mq_alloc_request_hctx
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    On 5/20/20 2:03 AM, Christoph Hellwig wrote:
    > On Wed, May 20, 2020 at 11:04:24AM +0800, Ming Lei wrote:
    >> On Wed, May 20, 2020 at 09:18:23AM +0800, Ming Lei wrote:
    >>> On Tue, May 19, 2020 at 05:30:00PM +0200, Christoph Hellwig wrote:
    >>>> On Tue, May 19, 2020 at 09:54:20AM +0800, Ming Lei wrote:
    >>>>> As Thomas clarified, workqueue hasn't such issue any more, and only other
    >>>>> per CPU kthreads can run until the CPU clears the online bit.
    >>>>>
    >>>>> So the question is if IO can be submitted from such kernel context?
    >>>>
    >>>> What other per-CPU kthreads even exist?
    >>>
    >>> I don't know, so expose to wider audiences.
    >>
    >> One user is io uring with IORING_SETUP_SQPOLL & IORING_SETUP_SQ_AFF, see
    >> io_sq_offload_start(), and it is a IO submission kthread.
    >
    > As far as I can tell that code is buggy, as it still needs to migrate
    > the thread away when the cpu is offlined. This isn't a per-cpu kthread
    > in the sene of having one for each CPU.
    >
    > Jens?

    It just uses kthread_create_on_cpu(), nothing home grown. Pretty sure
    they just break affinity if that CPU goes offline.

    --
    Jens Axboe

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