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Subject: Re: [PATCH liburing v2 0/1] test: add epoll test case
To:     Stefano Garzarella <sgarzare@redhat.com>
Cc:     linux-kernel@vger.kernel.org, io-uring@vger.kernel.org
References: <20200131142943.120459-1-sgarzare@redhat.com>
 <ebc2efdb-4e7f-0db9-ef04-c02aac0b08b1@kernel.dk>
 <CAGxU2F6qvW28=ULNUi-UHethus2bO6VXYX127HOcH_KPToZC-w@mail.gmail.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <ec04cb8f-01e8-6289-2fd4-6dec8a8e2c02@kernel.dk>
Date:   Thu, 6 Feb 2020 12:15:49 -0700
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On 2/6/20 10:33 AM, Stefano Garzarella wrote:
> 
> 
> On Fri, Jan 31, 2020 at 4:39 PM Jens Axboe <axboe@kernel.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.

-- 
Jens Axboe