Re: io_uring_prep_timeout_update on linked timeouts

[Victor Stewart <v@nametag.social>]

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.

V