Re: [LIBURING PATCH] sq_ring_needs_enter: check whether there are sqes when SQPOLL is not enabled

[Jens Axboe <axboe@kernel.dk>]

On 4/13/20 9:46 AM, Hrvoje Zeba wrote:
> On Mon, Apr 13, 2020 at 11:29 AM Jens Axboe <axboe@kernel.dk> wrote:
>>
>> On 4/13/20 1:19 AM, Xiaoguang Wang wrote:
>>> Indeed I'm not sure this patch is necessary, robust applications
>>> should not call io_uring_submit when there are not sqes to submmit.
>>> But still try to add this check, I have seen some applications which
>>> call io_uring_submit(), but there are not sqes to submit.
>>
>> Hmm, not sure it's worth complicating the submit path for that case.
>> A high performant application should not call io_uring_submit() if
>> it didn't queue anything new. Is this a common case you've seen?
>>
> 
> My code calls io_uring_submit() even if there are no sqes to submit to
> avoid spinning if there's nothing to do:
> 
> ...
> uint32_t sleep = (gt::user_contexts_waiting() > 0) ? 0 : 1;
> auto res = io_uring_submit_and_wait(&m_io_uring, sleep);

If you're calling with wait, then an sqe will be submitted with a
timeout operation. So that use case is fine. Or waiting in general.
But calling with {0,0} for {submit,wait} would be kind of silly.

-- 
Jens Axboe