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

[Jens Axboe <axboe@kernel.dk>]

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.

>>> diff --git a/src/include/liburing.h b/src/include/liburing.h
>>> index 0505a4f..6176a63 100644
>>> --- a/src/include/liburing.h
>>> +++ b/src/include/liburing.h
>>> @@ -56,6 +56,7 @@ struct io_uring {
>>>  	struct io_uring_sq sq;
>>>  	struct io_uring_cq cq;
>>>  	unsigned flags;
>>> +	unsigned features;
>>>  	int ring_fd;
>>>  };
>>
>> This breaks the API, as it changes the size of the ring...
>>
> 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.

-- 
Jens Axboe