On 8/29/21 3:40 AM, Jens Axboe wrote: > 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. Perfect. Thanks, Jens -- Pavel Begunkov
