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