Re: [PATCH 6/8] io_uring/net: support multishot for send

[Jens Axboe <axboe@kernel.dk>]

On 2/26/24 7:02 AM, Dylan Yudaken wrote:
> On Mon, Feb 26, 2024 at 1:38?PM Jens Axboe <axboe@kernel.dk> wrote:
>>
>> On 2/26/24 3:47 AM, Dylan Yudaken wrote:
>>> On Sun, Feb 25, 2024 at 12:46?AM Jens Axboe <axboe@kernel.dk> wrote:
>>>>
>>>> This works very much like the receive side, except for sends. The idea
>>>> is that an application can fill outgoing buffers in a provided buffer
>>>> group, and then arm a single send that will service them all. For now
>>>> this variant just terminates when we are out of buffers to send, and
>>>> hence the application needs to re-arm it if IORING_CQE_F_MORE isn't
>>>> set, as per usual for multishot requests.
>>>>
>>>
>>> This feels to me a lot like just using OP_SEND with MSG_WAITALL as
>>> described, unless I'm missing something?
>>
>> How so? MSG_WAITALL is "send X amount of data, and if it's a short send,
>> try again" where multishot is "send data from this buffer group, and
>> keep sending data until it's empty". Hence it's the mirror of multishot
>> on the receive side. Unless I'm misunderstanding you somehow, not sure
>> it'd be smart to add special meaning to MSG_WAITALL with provided
>> buffers.
>>
> 
> _If_ you have the data upfront these are very similar, and only differ
> in that the multishot approach will give you more granular progress
> updates. My point was that this might not be a valuable API to people
> for only this use case.

Not sure I agree, it feels like attributing a different meaning to
MSG_WAITALL if you use a provided buffer vs if you don't. And that to me
would seem to be confusing. Particularly when we have multishot on the
receive side, and this is identical, just for sends. Receives will keep
receiving as long as there are buffers in the provided group to receive
into, and sends will keep sending for the same condition. Either one
will terminate if we run out of buffers.

If you make MSG_WAITALL be that for provided buffers + send, then that
behaves differently than MSG_WAITALL with receive, and MSG_WAITALL with
send _without_ provided buffers. I don't think overloading an existing
flag for this purposes is a good idea, particularly when we already have
the existing semantics for multishot on the receive side.

> You do make a good point about MSG_WAITALL though - multishot send
> doesn't really make sense to me without MSG_WAITALL semantics. I
> cannot imagine a useful use case where the first buffer being
> partially sent will still want the second buffer sent.

Right, and I need to tweak that. Maybe we require MSG_WAITALL, or we
make it implied for multishot send. Currently the code doesn't deal with
that.

Maybe if MSG_WAITALL isn't set and we get a short send we don't set
CQE_F_MORE and we just stop. If it is set, then we go through the usual
retry logic. That would make it identical to MSG_WAITALL send without
multishot, which again is something I like in that we don't have
different behaviors depending on which mode we are using.

>>> I actually could imagine it being useful for the previous patches' use
>>> case of queuing up sends and keeping ordering,
>>> and I think the API is more obvious (rather than the second CQE
>>> sending the first CQE's data). So maybe it's worth only
>>> keeping one approach?
>>
>> And here you totally lost me :-)
> 
> I am suggesting here that you don't really need to support buffer
> lists on send without multishot.

That is certainly true, but I also don't see a reason _not_ to support
it. Again mostly because this is how receive and everything else works.
The app is free to issue a single SQE for send without multishot, and
pick the first buffer and send it.

> It's a slightly confusing API (to me) that you queue PushBuffer(A),
> Send(A), PushBuffer(B), Send(B)
> and get back Res(B), Res(A) which are in fact in order A->B.

Now I'm confused again. If you do do the above sequence, the first CQE
posted would be Res(A), and then Res(B)?

> Instead you could queue up PushBuffer(A), Send(Multishot),
> PushBuffer(B), and get back Res(Multishot), Res(Multishot)
> which are in order A -> B.

There should be no difference in ordering of the posted completion
between the two.

-- 
Jens Axboe