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From: Thomas Gleixner <[email protected]>
To: Ming Lei <[email protected]>
Cc: Jens Axboe <[email protected]>, Christoph Hellwig <[email protected]>,
	[email protected], [email protected],
	John Garry <[email protected]>,
	Bart Van Assche <[email protected]>,
	Hannes Reinecke <[email protected]>,
	[email protected], Peter Zijlstra <[email protected]>
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
Date: Thu, 21 May 2020 20:39:16 +0200	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <20200521092340.GA751297@T590>

Ming,

Ming Lei <[email protected]> writes:
> On Thu, May 21, 2020 at 10:13:59AM +0200, Thomas Gleixner wrote:
>> Ming Lei <[email protected]> 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

  reply	other threads:[~2020-05-21 18:39 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20200518093155.GB35380@T590>
     [not found] ` <[email protected]>
     [not found]   ` <20200518115454.GA46364@T590>
     [not found]     ` <[email protected]>
     [not found]       ` <20200518141107.GA50374@T590>
     [not found]         ` <[email protected]>
     [not found]           ` <20200519015420.GA70957@T590>
     [not found]             ` <[email protected]>
     [not found]               ` <20200520011823.GA415158@T590>
     [not found]                 ` <20200520030424.GI416136@T590>
2020-05-20  8:03                   ` 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 Christoph Hellwig
2020-05-20 14:45                     ` Jens Axboe
2020-05-20 15:20                       ` Jens Axboe
2020-05-20 15:31                         ` Christoph Hellwig
2020-05-20 19:41                         ` Thomas Gleixner
2020-05-20 20:18                           ` Jens Axboe
2020-05-20 22:14                             ` Thomas Gleixner
2020-05-20 22:40                               ` Jens Axboe
2020-05-21  2:27                               ` Ming Lei
2020-05-21  8:13                                 ` Thomas Gleixner
2020-05-21  9:23                                   ` Ming Lei
2020-05-21 18:39                                     ` Thomas Gleixner [this message]
2020-05-21 18:45                                       ` Jens Axboe
2020-05-21 20:00                                         ` Thomas Gleixner
2020-05-22  1:57                                       ` Ming Lei

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