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

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Date:   Mon, 27 Jan 2020 19:25:34 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     io-uring@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH liburing 1/1] test: add epoll test case

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...

Thanks,
Stefano