Re: [PATCH liburing 1/2] io_uring_enter: add timeout support

[Jens Axboe <axboe@kernel.dk>]

On 7/30/20 9:16 PM, Jiufei Xue wrote:
> 
> 
> On 2020/7/31 上午10:56, Jens Axboe wrote:
>> On 7/30/20 8:12 PM, Jiufei Xue wrote:
>>>
>>>
>>> On 2020/7/30 下午11:28, Jens Axboe wrote:
>>>> On 7/29/20 8:32 PM, Jiufei Xue wrote:
>>>>> Hi Jens,
>>>>>
>>>>> On 2020/7/30 上午1:51, Jens Axboe wrote:
>>>>>> On 7/29/20 4:10 AM, Jiufei Xue wrote:
>>>>>>> Kernel can handle timeout when feature IORING_FEAT_GETEVENTS_TIMEOUT
>>>>>>> supported. Add two new interfaces: io_uring_wait_cqes2(),
>>>>>>> io_uring_wait_cqe_timeout2() for applications to use this feature.
>>>>>>
>>>>>> Why add new new interfaces, when the old ones already pass in the
>>>>>> timeout? Surely they could just use this new feature, instead of the
>>>>>> internal timeout, if it's available?
>>>>>>
>>>>> Applications use the old one may not call io_uring_submit() because
>>>>> io_uring_wait_cqes() will do it. So I considered to add a new one.
>>>>
>>>> Not sure I see how that's a problem - previously, you could not do that
>>>> either, if you were doing separate submit/complete threads. So this
>>>> doesn't really add any new restrictions. The app can check for the
>>>> feature flag to see if it's safe to do so now.
>>>> Yes, new applications can check for the feature flag. What about the existing
>>>
>>> apps? The existing applications which do not separate submit/complete
>>> threads may use io_uring_wait_cqes() or io_uring_wait_cqe_timeout() without
>>> submiting the requests. No one will do that now when the feature is supported.
>>
>> Right, and I feel like I'm missing something here, what's the issue with
>> that? As far as the application is concerned, a different mechanism may be
>> used to achieve the timeout, but it should work in the same way.
>>
>> So please explain this as clearly as you can, as I'm probably missing
>> something...
>> I am sorry for the confusion. Here is an example below: 
> 
> io_uring_get_sqe
> io_uring_prep_nop
> io_uring_wait_cqe_timeout
> 
> If an existing application call io_uring_wait_cqe_timeout() after preparing
> some sqes, the older APIs will submit the requests.
> 
> However, If we reuse the existing APIs and found the feature is supported,
> we will not submit the requests.
> 
> I think we should not change the behaviors for existing APIs.

Then why not just make the sqe-less timeout path flush existing requests,
if it needs to? Seems a lot simpler than adding odd x2 variants, which
won't really be clear.

Chances are, if it's called with sq entries pending, the caller likely
wants those submitted. Either the caller was aware and relying on that
behavior, or the caller is simply buggy and has a case where it doesn't
submit IO before waiting for completions.

Hence I really don't think that's a big deal, and even arguably the right
thing to do.

-- 
Jens Axboe