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

[Jiufei Xue <jiufei.xue@linux.alibaba.com>]

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.

Thanks,
Jiufei

>>>> Oh, yes, I haven't considering that before. So could I add this feature
>>>> bit to io_uring.flags. Any suggestion?
>>>
>>> Either that, or we add this (and add pad that we can use later) and just
>>> say that for the next release you have to re-compile against the lib.
>>> That will break existing applications, unless they are recompiled... But
>>> it might not be a bad idea to do so, just so we can pad io_uring out a
>>> little bit to provide for future flexibility.
>>>
>> Agree about that. So should we increase the major version after adding the
>> feature flag and some pad?
> 
> I think so, also a good time to think about other cases where that might be
> useful.
> 
> But I think we need to flesh out the API first, as that might impact things.
>