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

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Date:   Thu, 6 Feb 2020 21:12:03 +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 Thu, Feb 6, 2020 at 8:51 PM Jens Axboe <axboe@...nel.dk> wrote:
>
> On 2/6/20 12:15 PM, Jens Axboe wrote:
> > On 2/6/20 10:33 AM, Stefano Garzarella wrote:
> >>
> >>
> >> 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.
> >>
> >> If I do io_uring_enter() with GETEVENTS set and wait_nr = 0 it solves
> >> the issue, I think because we call io_cqring_events() that flushes the
> >> overflow list.
> >>
> >> At this point, should we call io_cqring_events() (that flushes the
> >> overflow list) in io_uring_poll()?
> >> I mean something like this:
> >>
> >> diff --git a/fs/io_uring.c b/fs/io_uring.c
> >> index 77f22c3da30f..2769451af89a 100644
> >> --- a/fs/io_uring.c
> >> +++ b/fs/io_uring.c
> >> @@ -6301,7 +6301,7 @@ static __poll_t io_uring_poll(struct file *file, poll_table *wait)
> >>         if (READ_ONCE(ctx->rings->sq.tail) - ctx->cached_sq_head !=
> >>             ctx->rings->sq_ring_entries)
> >>                 mask |= EPOLLOUT | EPOLLWRNORM;
> >> -       if (READ_ONCE(ctx->rings->cq.head) != ctx->cached_cq_tail)
> >> +       if (!io_cqring_events(ctx, false))
> >>                 mask |= EPOLLIN | EPOLLRDNORM;
> >>
> >>         return mask;
> >
> > That's not a bad idea, would just have to verify that it is indeed safe
> > to always call the flushing variant from there.
>
> Double checked, and it should be fine. We may be invoked with
> ctx->uring_lock held, but that's fine.
>

Maybe yes, I'll check better and I'll send a patch :-)

Thanks,
Stefano