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

[yangerkun <yangerkun@huawei.com>]

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.

Thanks,
Kun.

> 
> But don't mix polled IO with "normal" IO, it just won't work without
> extra additions that I'm not willing to pay the cost of.
> 
>