Re: [PATCH 3/7] io_uring: don't wait when under-submitting

[Pavel Begunkov <asml.silence@gmail.com>]

On 12/18/2019 4:02 PM, Jens Axboe wrote:
> On 12/18/19 2:38 AM, Pavel Begunkov wrote:
>> On 12/18/2019 3:06 AM, Jens Axboe wrote:
>>> On 12/17/19 4:55 PM, Jann Horn wrote:
>>>> On Tue, Dec 17, 2019 at 11:54 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>>> There is no reliable way to submit and wait in a single syscall, as
>>>>> io_submit_sqes() may under-consume sqes (in case of an early error).
>>>>> Then it will wait for not-yet-submitted requests, deadlocking the user
>>>>> in most cases.
>>>>>
>>>>> In such cases adjust min_complete, so it won't wait for more than
>>>>> what have been submitted in the current io_uring_enter() call. It
>>>>> may be less than total in-flight, but that up to a user to handle.
>>>>>
>>>>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>> [...]
>>>>>         if (flags & IORING_ENTER_GETEVENTS) {
>>>>>                 unsigned nr_events = 0;
>>>>>
>>>>>                 min_complete = min(min_complete, ctx->cq_entries);
>>>>> +               if (submitted != to_submit)
>>>>> +                       min_complete = min(min_complete, (u32)submitted);
>>>>
>>>> Hm. Let's say someone submits two requests, first an ACCEPT request
>>>> that might stall indefinitely and then a WRITE to a file on disk that
>>>> is expected to complete quickly; and the caller uses min_complete=1
>>>> because they want to wait for the WRITE op. But now the submission of
>>>> the WRITE fails, io_uring_enter() computes min_complete=min(1, 1)=1,
>>>> and it blocks on the ACCEPT op. That would be bad, right?
>>>>
>>>> If the usecase I described is valid, I think it might make more sense
>>>> to do something like this:
>>>>
>>>> u32 missing_submissions = to_submit - submitted;
>>>> min_complete = min(min_complete, ctx->cq_entries);
>>>> if ((flags & IORING_ENTER_GETEVENTS) && missing_submissions < min_complete) {
>>>>   min_complete -= missing_submissions;
>>>>   [...]
>>>> }
>>>>
>>>> In other words: If we do a partially successful submission, only wait
>>>> as long as we know that userspace definitely wants us to wait for one
>>>> of the pending requests; and once we can't tell whether userspace
>>>> intended to wait longer, return to userspace and let the user decide.
>>>>
>>>> Or it might make sense to just ignore IORING_ENTER_GETEVENTS
>>>> completely in the partial submission case, in case userspace wants to
>>>> immediately react to the failed request by writing out an error
>>>> message to a socket or whatever. This case probably isn't
>>>> performance-critical, right? And it would simplify things a bit.
>>>
>>> That's a good point, and Pavel's first patch actually did that. I
>>> didn't consider the different request type case, which might be
>>> uncommon but definitely valid.
>>>
>>> Probably the safest bet here is just to not wait at all if we fail
>>> submitting all of them. This isn't a fast path, there was an error
>>> somehow which meant we didn't submit it all. So just return the
>>> submit count (including 0, not -EAGAIN) if we fail submitting,
>>> and ignore IORING_ENTER_GETEVENTS for that case.
>>>
>> I see nothing wrong with -EAGAIN, it's returned only if it can't
>> allocate memory for the first request. If so, can you then just take the
>> v1? It will probably be applied cleanly.
> 
> -EAGAIN for request alloc is fine, but your v1 also returned -EAGAIN if
> someone asked to submit 0, which is a bug. We must return zero for that
> case.
> 
Now I see which -EAGAIN you meant. You're right, I'll resend

> So your v1 without that would work, something ala:
> 
> if (submitted != to_submit)
> 	goto out;
> 
> without the turning 0 into -EAGAIN unconditionally, io_submit_sqes() does
> the right thing there as it is.
> 

-- 
Pavel Begunkov