Re: io_uring_prep_timeout_update on linked timeouts

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

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.

> Do you have a test case that doesn't work for you? Always easier to
> reason about a test case.
> 

-- 
Pavel Begunkov