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Date:   Thu, 21 May 2020 10:13:59 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        John Garry <john.garry@...wei.com>,
        Bart Van Assche <bvanassche@....org>,
        Hannes Reinecke <hare@...e.com>, io-uring@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: 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

Ming Lei <ming.lei@...hat.com> writes:
> On Thu, May 21, 2020 at 12:14:18AM +0200, Thomas Gleixner wrote:
>> When the CPU is finally offlined, i.e. the CPU cleared the online bit in
>> the online mask is definitely too late simply because it still runs on
>> that outgoing CPU _after_ the hardware queue is shut down and drained.
>
> IMO, the patch in Christoph's blk-mq-hotplug.2 still works for percpu
> kthread.
>
> It is just not optimal in the retrying, but it should be fine. When the
> percpu kthread is scheduled on the CPU to be offlined:
>
> - if the kthread doesn't observe the INACTIVE flag, the allocated request
> will be drained.
>
> - otherwise, the kthread just retries and retries to allocate & release,
> and sooner or later, its time slice is consumed, and migrated out, and the
> cpu hotplug handler will get chance to run and move on, then the cpu is
> shutdown.

1) This is based on the assumption that the kthread is in the SCHED_OTHER
   scheduling class. Is that really a valid assumption?

2) What happens in the following scenario:

   unplug

     mq_offline
       set_ctx_inactive()
       drain_io()
       
   io_kthread()
       try_queue()
       wait_on_ctx()

   Can this happen and if so what will wake up that thread?

I'm not familiar enough with that code to answer #2, but this really
wants to be properly described and documented.

Thanks,

        tglx

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