linux-kernel - 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

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Date:   Thu, 21 May 2020 20:39:16 +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,

Ming Lei <ming.lei@...hat.com> writes:
> On Thu, May 21, 2020 at 10:13:59AM +0200, Thomas Gleixner wrote:
>> Ming Lei <ming.lei@...hat.com> writes:
>> > On Thu, May 21, 2020 at 12:14:18AM +0200, Thomas Gleixner wrote:
>> > - 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?
>
> Given it is unlikely path, we can add msleep() before retrying when INACTIVE bit
> is observed by current thread, and this way can avoid spinning and should work
> for other schedulers.

That should work, but pretty is something else

>> 
>> 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?
>
> drain_io() releases all tag of this hctx, then wait_on_ctx() will be waken up
> after any tag is released.

drain_io() is already done ...

So looking at that thread function:

static int io_sq_thread(void *data)
{
	struct io_ring_ctx *ctx = data;

        while (...) {
              ....
	      to_submit = io_sqring_entries(ctx);

--> preemption

hotplug runs
   mq_offline()
      set_ctx_inactive();
      drain_io();
      finished();

--> thread runs again

      mutex_lock(&ctx->uring_lock);
      ret = io_submit_sqes(ctx, to_submit, NULL, -1, true);
      mutex_unlock(&ctx->uring_lock);

      ....

      if (!to_submit || ret == -EBUSY)
          ...
      	  wait_on_ctx();

Can this happen or did drain_io() already take care of the 'to_submit'
items and the call to io_submit_sqes() turns into a zero action ?

If the above happens then nothing will wake it up because the context
draining is done and finished.

Thanks,

        tglx