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From: Jens Axboe <axboe@kernel.dk>
To: Pavel Begunkov <asml.silence@gmail.com>,
	Victor Stewart <v@nametag.social>,
	io-uring <io-uring@vger.kernel.org>
Subject: Re: io_uring_prep_timeout_update on linked timeouts
Date: Sat, 28 Aug 2021 07:43:15 -0600	[thread overview]
Message-ID: <b53e6d69-9591-607b-c391-bf5fed23c1af@kernel.dk> (raw)
In-Reply-To: <04e3c4ab-4e78-805c-bc4f-f9c6d7e85ec1@gmail.com>

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


  reply	other threads:[~2021-08-28 13:43 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-24 22:43 io_uring_prep_timeout_update on linked timeouts Victor Stewart
2021-08-25  1:27 ` Victor Stewart
2021-08-27  1:40   ` Victor Stewart
2021-08-28  3:22     ` Jens Axboe
2021-08-28 13:39       ` Pavel Begunkov
2021-08-28 13:43         ` Jens Axboe [this message]
2021-08-28 21:38           ` Pavel Begunkov
2021-08-29  2:40             ` Jens Axboe
2021-08-31 11:36               ` Victor Stewart
2021-08-31 16:09                 ` Pavel Begunkov
2021-08-31 16:07               ` Pavel Begunkov

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