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

Pavel Begunkov
2019-11-15T09:40:07Z

>> 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.
>

--
Pavel Begunkov