Re: io_uring_prep_timeout_update on linked timeouts

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

On 8/29/21 3:40 AM, Jens Axboe wrote:
> On 8/28/21 3:38 PM, Pavel Begunkov wrote:
>> On 8/28/21 2:43 PM, Jens Axboe wrote:
>>> On 8/28/21 7:39 AM, Pavel Begunkov wrote:
>>>> On 8/28/21 4:22 AM, Jens Axboe wrote:
>>>>> On 8/26/21 7:40 PM, Victor Stewart wrote:
>>>>>> On Wed, Aug 25, 2021 at 2:27 AM Victor Stewart <v@nametag.social> wrote:
>>>>>>>
>>>>>>> On Tue, Aug 24, 2021 at 11:43 PM Victor Stewart <v@nametag.social> wrote:
>>>>>>>>
>>>>>>>> we're able to update timeouts with io_uring_prep_timeout_update
>>>>>>>> without having to cancel
>>>>>>>> and resubmit, has it ever been considered adding this ability to
>>>>>>>> linked timeouts?
>>>>>>>
>>>>>>> whoops turns out this does work. just tested it.
>>>>>>
>>>>>> doesn't work actually. missed that because of a bit of misdirection.
>>>>>> returns -ENOENT.
>>>>>>
>>>>>> the problem with the current way of cancelling then resubmitting
>>>>>> a new a timeout linked op (let's use poll here) is you have 3 situations:
>>>>>>
>>>>>> 1) the poll triggers and you get some positive value. all good.
>>>>>>
>>>>>> 2) the linked timeout triggers and cancels the poll, so the poll
>>>>>> operation returns -ECANCELED.
>>>>>>
>>>>>> 3) you cancel the existing poll op, and submit a new one with
>>>>>> the updated linked timeout. now the original poll op returns
>>>>>> -ECANCELED.
>>>>>>
>>>>>> so solely from looking at the return value of the poll op in 2) and 3)
>>>>>> there is no way to disambiguate them. of course the linked timeout
>>>>>> operation result will allow you to do so, but you'd have to persist state
>>>>>> across cqe processings. you can also track the cancellations and know
>>>>>> to skip the explicitly cancelled ops' cqes (which is what i chose).
>>>>>>
>>>>>> there's also the problem of efficiency. you can imagine in a QUIC
>>>>>> server where you're constantly updating that poll timeout in response
>>>>>> to idle timeout and ACK scheduling, this extra work mounts.
>>>>>>
>>>>>> so i think the ability to update linked timeouts via
>>>>>> io_uring_prep_timeout_update would be fantastic.
>>>>>
>>>>> Hmm, I'll need to dig a bit, but whether it's a linked timeout or not
>>>>> should not matter. It's a timeout, it's queued and updated the same way.
>>>>> And we even check this in some of the liburing tests.
>>>>
>>>> We don't keep linked timeouts in ->timeout_list, so it's not
>>>> supported and has never been. Should be doable, but we need
>>>> to be careful synchronising with the link's head.
>>>
>>> Yeah shoot you are right, I guess that explains the ENOENT. Would be
>>> nice to add, though. Synchronization should not be that different from
>>> dealing with regular timeouts.
>>
>> _Not tested_, but something like below should do. will get it
>> done properly later, but even better if we already have a test
>> case. Victor?
> 
> FWIW, I wrote a simple test case for it, and it seemed to work fine.
> Nothing fancy, just a piped read that would never finish with a linked
> timeout (1s), submit, then submit a ltimeout update that changes it to
> 2s instead. Test runs and update completes first with res == 0 as
> expected, and 2s later the ltimeout completes with -EALREADY (because
> the piped read went async) and the piped read gets canceled.
> 
> That seems to be as expected, and didn't trigger anything odd.

Perfect. Thanks, Jens

-- 
Pavel Begunkov