Re: [PATCHSET 0/2] io_uring support for linked timeouts

Pavel Begunkov
2019-11-15T17:12:16Z

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>>>> I just think we need to make sure the ground rules are sane. I'm going
>>>> to write a few test cases to make sure we do the right thing.
>>>>
>>>
>> Ok, let me try to state some rules to discuss:
>
>> 1. REQ -> LINK_TIMEOUT
>> is a valid use case
>
> Yes
>
>> 2. timeout is set at the moment of starting execution of operation.
>> e.g. REQ1, REQ2|DRAIN -> LINK_TIMEOUT
>>
>> Timer is set at the moment, when everything is drained and we
>> sending REQ. i.e. after completion of REQ1
>
> Right, the timeout is prepped before REQ2 is started, armed when it is
> started (if not already done). The prep + arm split is important to
> ensure that a short timeout doesn't even find REQ2.
I've got (and seen the patch) for prep + arm split. Could a
submission block/take a long time? If so, it's probably not what
user would want.

e.g. WRITE -> LINK_TIMEOUT (1s)
- submit write (blocking, takes 2s)
- and only after this 2s have a chance to arm the timeout.

>
>> 3. REQ1 -> LINK_TIMEOUT1 -> REQ2 -> LINK_TIMEOUT2
>>
>> is valid, and LINK_TIMEOUT2 will be set, at the moment of
>> start of REQ2's execution. It also mean, that if
>> LINK_TIMEOUT1 fires, it will cancel REQ1, and REQ2
>> with LINK_TIMEOUT2 (with proper return values)
>
> That's not valid with the patches I sent. It could be, but we'd need to
> fix that bit.
>
It should almost work, if we move linked timeout init/arm code
into __io_submit_sqe(). There is also a problem, which it'll solve:

If a request is deferred, it will skip timeout initialisation,
because io_req_defer() happens before __io_queue_sqe().
io_wq_submit_work() won't initialise/arm the timeout as well,
as it use __io_submit_sqe() directly. So
- rule 2. doesn't work
- free_req() calls io_link_cancel_timeout() for an non-inititialised
timeout

The case I keep in mind is:
read file -> SEND (+LINK_TIMEOUT)
-> RECV (+LINK_TIMEOUT) -> write file ...

We don't care how long file read/write would take,
but would want to limit execution time for network operations.

>> 4. REQ1, LINK_TIMEOUT
>> is invalid, fail it
>
> Correct
>
>> 5. LINK_TIMEOUT1 -> LINK_TIMEOUT2
>> Fail first, link-fail (aka cancelled) for the second one
>
> Correct
>
>> 6. REQ1 -> LINK_TIMEOUT1 -> LINK_TIMEOUT2
>> execute REQ1+LINK_TIMEOUT1, and then fail LINK_TIMEOUT2 as
>> invalid. Also, LINK_TIMEOUT2 could be just cancelled
>> (e.g. if fail_links for REQ1)
>
> Given case 5, why would this one be legal?
>
This one is different if we conceptually consider
REQ + following LINK_TIMEOUT as a single operation with timeout.
If so, it can be said that (REQ1 -> LINK_TIMEOUT1) is valid
pair, but LINK_TIMEOUT2 is a following single link timeout,
that's more like in 4.

7. If we decide to not implement 3., what's about the case below?
REQ1 -> REQ2 -> LINK_TIMEOUT

--
Pavel Begunkov

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