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. -- Jens Axboe
