linux-kernel - Re: [PATCH liburing 1/1] test: add epoll test case

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Date:   Mon, 27 Jan 2020 11:46:34 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     io-uring@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH liburing 1/1] test: add epoll test case

On 1/27/20 11:25 AM, Stefano Garzarella wrote:
> On Mon, Jan 27, 2020 at 09:32:43AM -0700, Jens Axboe wrote:
>> On 1/27/20 9:17 AM, Stefano Garzarella wrote:
>>> Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>
>>
>> You're not reaping CQ events, and hence you overflow the ring. Once
>> overflown, an attempt to submit new IO will returns in a -16/-EBUSY
>> return value. This is io_uring telling you that it won't submit more
>> IO until you've emptied the completion ring so io_uring can flush
>> the overflown entries to the ring.
> 
> How can I reaping CQ events? (I was hoping the epoll would help me with that)
> 
> What I'm seeing is that the producer (EPOLLOUT) can fill the SQ without issues,
> the consumer (read()) is receiving all the buffers produced, but the thread
> that frees the buffers (EPOLLIN) is not woken up.
> 
> I tried to set a timeout to the epoll_wait(), but the io_uring_peek_cqe()
> returns -EAGAIN.
> 
> If I'm using a ring with 16 entries, it seems to work better, but
> sometimes I lose events and the thread that frees the buffer doesn't wake up.
> 
> Maybe I'm missing something...

OK, so that helps in terms of understanding the issue you are seeing with
it. I'll take a look at this, but it'll probably be a few days. You can
try and enable tracing, I see events completed just fine. Maybe a race
with your epoll wait and event reaping?

-- 
Jens Axboe