linux-kernel - Re: [PATCH liburing v2 0/1] test: add epoll test case

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Date:   Mon, 3 Feb 2020 10:03:07 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-kernel@...r.kernel.org, io-uring@...r.kernel.org
Subject: Re: [PATCH liburing v2 0/1] test: add epoll test case

On Fri, Jan 31, 2020 at 4:39 PM Jens Axboe <axboe@...nel.dk> wrote:
> On 1/31/20 7:29 AM, Stefano Garzarella wrote:
> > Hi Jens,
> > this is a v2 of the epoll test.
> >
> > v1 -> v2:
> >     - if IORING_FEAT_NODROP is not available, avoid to overflow the CQ
> >     - add 2 new tests to test epoll with IORING_FEAT_NODROP
> >     - cleanups
> >
> > There are 4 sub-tests:
> >     1. test_epoll
> >     2. test_epoll_sqpoll
> >     3. test_epoll_nodrop
> >     4. test_epoll_sqpoll_nodrop
> >
> > In the first 2 tests, I try to avoid to queue more requests than we have room
> > for in the CQ ring. These work fine, I have no faults.
>
> Thanks!
>
> > In the tests 3 and 4, if IORING_FEAT_NODROP is supported, I try to submit as
> > much as I can until I get a -EBUSY, but they often fail in this way:
> > the submitter manages to submit everything, the receiver receives all the
> > submitted bytes, but the cleaner loses completion events (I also tried to put a
> > timeout to epoll_wait() in the cleaner to be sure that it is not related to the
> > patch that I send some weeks ago, but the situation doesn't change, it's like
> > there is still overflow in the CQ).
> >
> > Next week I'll try to investigate better which is the problem.
>
> Does it change if you have an io_uring_enter() with GETEVENTS set? I wonder if
> you just pruned the CQ ring but didn't flush the internal side.

Yes, If I use the io_uring_wait_cqe() instead of io_uring_peek_cqe() all
the tests work great, but it is blocking and the epoll_wait() it is used
only the first time.

>
> > I hope my test make sense, otherwise let me know what is wrong.
>
> I'll take a look...

Thanks!

>
> > Anyway, when I was exploring the library, I had a doubt:
> > - in the __io_uring_get_cqe() should we call sys_io_uring_enter() also if
> >   submit and wait_nr are zero, but IORING_SQ_NEED_WAKEUP is set in the
> >   sq.kflags?
>
> It's a submission side thing, the completion side shouldn't care. That
> flag is only relevant if you're submitting IO with SQPOLL. Then it tells
> you that the thread needs to get woken up, which you need io_uring_enter()
> to do. But for just reaping completions and not needing to submit
> anything new, we don't care if the thread is sleeping.

Thank you for clarifying that,
Stefano