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

[Pavel Begunkov <asml.silence@gmail.com>]

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On 15/11/2019 12:40, Pavel Begunkov wrote:
>>> Finally got to this patch. I think, find it adding too many edge cases
>>> and it isn't integrated consistently into what we have now. I would love
>>> to hear your vision, but I'd try to implement them in such a way, that it
>>> doesn't need to modify the framework, at least for some particular case.
>>> In other words, as opcodes could have been added from the outside with a
>>> function table.
>>
>> I agree, it could do with a bit of cleanup. Incrementals would be
>> appreciated!
>>
>>> Also, it's not so consistent with the userspace API as well.
>>>
>>> 1. If we specified drain for the timeout, should its start be delayed
>>> until then? I would prefer so.
>>>
>>> E.g. send_msg + drained linked_timeout, which would set a timeout from the
>>> start of the send.
>>
>> What cases would that apply to, what would the timeout even do in this
>> case? The point of the linked timeout is to abort the previous command.
>> Maybe I'm not following what you mean here.
>>
> Hmm, got it a bit wrong with defer. io_queue_link_head() can defer it
> without setting timeout. However, it seems that io_wq_submit_work()
> won't set a timer, as it uses __io_submit_sqe(), but not
> __io_queue_sqe(), which handles all this with linked timeouts.
> 
> Indeed, maybe it be, that you wanted to place it in __io_submit_sqe?
> 
>>> 2. Why it could be only the second one in a link? May we want to cancel
>>> from a certain point?
>>> e.g. "op1 -> op2 -> timeout -> op3" cancels op2 and op3
>>
>> Logically it need not be the second, it just has to follow another
>> request. Is there a bug there?
>>
> __io_queue_sqe looks only for the second one in a link. Other linked
> timeouts will be ignored, if I get the code right.
> 
> Also linking may (or __may not__) be an issue. As you remember, the head
> is linked through link_list, and all following with list.
> i.e. req_head.link_list <-> req.list <-> req.list <-> req.list
> 
> free_req() (last time I saw it), expects that timeout's previous request
> is linked with link_list. If a timeout can fire in the middle of a link
> (during execution), this could be not the case. But it depends on when
> we set an timeout.
> 
> BTW, personally I'd link them all through link_list. E.g. may get rid of
> splicing in free_req(). I'll try to make it later.
> 
>>> 3. It's a bit strange, that the timeout affects a request from the left,
>>> and after as an consequence cancels everything on the right (i.e. chain).
>>> Could we place it in the head? So it would affect all requests on the right
>>> from it.
>>
>> But that's how links work, though. If you keep linking, then everything
>> that depends on X will fail, if X itself isn't succesful.
>>
> Right. That's about what userspace API would be saner. To place timeout
> on the left of a request, or on the right, with the same resulting effect.
> 
> Let put this question away until the others are clear.
> 
>>> 4. I'd prefer to handle it as a new generic command and setting a timer
>>> in __io_submit_sqe().
>>>
>>> I believe we can do it more gracefully, and at the same moment giving
>>> more freedom to the user. What do you think?
>>
>> 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

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

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)

4. REQ1, LINK_TIMEOUT
is invalid, fail it

5. LINK_TIMEOUT1 -> LINK_TIMEOUT2
Fail first, link-fail (aka cancelled) for the second one

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)

-- 
Pavel Begunkov


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