Re: [RFC] io_uring: stop only support read/write for ctx with IORING_SETUP_IOPOLL

[Jens Axboe <axboe@kernel.dk>]

On 11/6/19 5:00 AM, yangerkun wrote:
> 
> 
> On 2019/11/4 22:01, Jens Axboe wrote:
>> On 11/4/19 4:46 AM, yangerkun wrote:
>>>
>>>
>>> On 2019/11/4 18:09, Bob Liu wrote:
>>>> On 11/4/19 4:56 PM, yangerkun wrote:
>>>>> There is no problem to support other type request for the ctx with
>>>>> IORING_SETUP_IOPOLL.
>>>>
>>>> Could you describe the benefit of doing this?
>>>
>>> Hi,
>>>
>>> I am trying to replace libaio with io_uring in InnoDB/MariaDB(which
>>> build on xfs/nvme). And in order to simulate the timeout mechanism
>>> like io_getevents, firstly, to use the poll function of io_uring's fd
>>> has been selected and it really did work. But while trying to enable
>>> IORING_SETUP_IOPOLL since xfs has iopoll function interface, the
>>> mechanism will fail since io_uring_poll does check the cq.head between
>>> cached_cq_tail, which will not update until we call io_uring_enter and
>>> do the poll. So, instead, I decide to use timeout requests in
>>> io_uring but will return -EINVAL since we enable IORING_SETUP_IOPOLL
>>> at the same time. I think this combination is a normal scene so as
>>> the other combination descibed in this patch. I am not sure does it a
>>> good solution for this problem, and maybe there exists some better way.
>>
>> I think we can support timeouts pretty easily with SETUP_IOPOLL, but we
>> can't mix and match everything. Pretty sure I've written at length about
>> that before, but the tldr is that for purely polled commands, we won't
>> have an IRQ event. That's the case for nvme if it's misconfigured, but
>> for an optimal setup where nvme is loaded with poll queues, there will
>> never be an interrupt for the command. This means that we can never wait
>> in io_cqring_wait(), we must always call the iopoll poller, because if
>> we wait we might very well be waiting for events that will never happen
>> unless we actively poll for them.
>>
>> This could be supported if we accounted requests, but I don't want to
>> add that kind of overhead. Same with the lock+irqdisable you had to add
>> for this, it's not acceptable overhead.
>>
>> Sounds like you just need timeout support for polling? If so, then that
> 
> Hi,
> 
> Yeah, the thing I need add is the timeout support for polling.
> 
>> is supportable as we know that these events will trigger an async event
>> when they happen. Either that, or it triggers when we poll for
>> completions. So it's safe to support, and we can definitely do that.
> 
> I am a little confuse. What you describe is once enable SETUP_IOPOLL
> and there is a async call of io_timeout_fn, we should not call
> io_cqring_fill_event directly, but leave io_iopoll_check to do this?
> Or, the parallel running for io_iopoll_check may trigger some problem
> since they can going to io_cqring_fill_event.

Maybe that wasn't quite clear, what I'm trying to say is that there are
two outcomes with IORING_OP_TIMEOUT:

1) The number of events specified in the timeout is met. This happens
   through the normal poll command checks when we complete commands.
2) The timer fires. When this happens, we just increment ->cq_timeouts.
   You'd have to make a note of that in the poll loop just like we do in
   cqring_wait() to know to abort if that value is different from when
   we started the loop.

All that's needed to support timeouts with IORING_SETUP_IOPOLL is to
have that ->cq_timeouts check in place. With that, the restriction could
be lifted.

-- 
Jens Axboe