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. -- Jens Axboe
