Re: [RFC PATCH v4 13/16] io_uring: add io_recvzc request

[Jens Axboe <axboe@kernel.dk>]

On 3/15/24 5:52 PM, Pavel Begunkov wrote:
> On 3/15/24 18:38, Jens Axboe wrote:
>> On 3/15/24 11:34 AM, Pavel Begunkov wrote:
>>> On 3/14/24 16:14, Jens Axboe wrote:
>>> [...]
>>>>>>> @@ -1053,6 +1058,85 @@ struct io_zc_rx_ifq *io_zc_verify_sock(struct io_kiocb *req,
>>>>>>>         return ifq;
>>>>>>>     }
>>>>>>>     +int io_recvzc_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>>>>>>> +{
>>>>>>> +    struct io_recvzc *zc = io_kiocb_to_cmd(req, struct io_recvzc);
>>>>>>> +
>>>>>>> +    /* non-iopoll defer_taskrun only */
>>>>>>> +    if (!req->ctx->task_complete)
>>>>>>> +        return -EINVAL;
>>>>>>
>>>>>> What's the reasoning behind this?
>>>>>
>>>>> CQ locking, see the comment a couple lines below
>>>>
>>>> My question here was more towards "is this something we want to do".
>>>> Maybe this is just a temporary work-around and it's nothing to discuss,
>>>> but I'm not sure we want to have opcodes only work on certain ring
>>>> setups.
>>>
>>> I don't think it's that unreasonable restricting it. It's hard to
>>> care about !DEFER_TASKRUN for net workloads, it makes CQE posting a bit
>>
>> I think there's a distinction between "not reasonable to support because
>> it's complicated/impossible to do so", and "we prefer not to support
>> it". I agree, as a developer it's hard to care about !DEFER_TASKRUN for
>> networking workloads, but as a user, they will just setup a default
>> queue until they wise up. And maybe this can be a good thing in that
> 
> They'd still need to find a supported NIC and do all the other
> setup, comparably to that it doesn't add much trouble. And my

Hopefully down the line, it'll work on more NICs, and configuration will
be less of a nightmare than it is now.

> usual argument is that io_uring is a low-level api, it's expected
> that people interacting with it directly are experienced enough,
> expect to spend some time to make it right and likely library
> devs.

Have you seen some of the code that has gone in to libraries for
io_uring support? I have, and I don't think that statement is true at
all for that side.

It should work out of the box even with a naive approach, while the best
approach may require some knowledge. At least I think that's the sanest
stance on that.

>> they'd be nudged toward DEFER_TASKRUN, but I can also see some head
>> scratching when something just returns (the worst of all error codes)
>> -EINVAL when they attempt to use it.
> 
> Yeah, we should try to find a better error code, and the check
> should migrate to ifq registration.

Wasn't really a jab at the code in question, just more that -EINVAL is
the ubiqitious error code for all kinds of things and it's hard to
diagnose in general for a user. You just have to start guessing...

>>> cleaner, and who knows where the single task part would become handy.
>>
>> But you can still take advantage of single task, since you know if
>> that's going to be true or not. It just can't be unconditional.
>>
>>> Thinking about ifq termination, which should better cancel and wait
>>> for all corresponding zc requests, it's should be easier without
>>> parallel threads. E.g. what if another thread is in the enter syscall
>>> using ifq, or running task_work and not cancellable. Then apart
>>> from (non-atomic) refcounting, we'd need to somehow wait for it,
>>> doing wake ups on the zc side, and so on.
>>
>> I don't know, not seeing a lot of strong arguments for making it
>> DEFER_TASKRUN only. My worry is that once we starting doing that, then
>> more will follow. And honestly I think that would be a shame.
>>
>> For ifq termination, surely these things are referenced, and termination
>> would need to wait for the last reference to drop? And if that isn't an
>> expected condition (it should not be), then a percpu ref would suffice.
>> Nobody cares if the teardown side is more expensive, as long as the fast
>> path is efficient.
> 
> You can solve any of that, it's true, the question how much crap
> you'd need to add in hot paths and diffstat wise. Just take a look
> at what a nice function io_recvmsg() is together with its helpers
> like io_recvmsg_multishot().

That is true, and I guess my real question is "what would it look like
if we supported !DEFER_TASKRUN". Which I think is a valid question.

> The biggest concern is optimisations and quirks that we can't
> predict at the moment. DEFER_TASKRUN/SINGLE_ISSUER provide a simpler
> model, I'd rather keep recvzc simple than having tens of conditional
> optimisations with different execution flavours and contexts.
> Especially, since it can be implemented later, wouldn't work the
> other way around.

Yes me too, and I'd hate to have two variants just because of that. But
comparing to eg io_recv() and helpers, it's really not that bad. Hence
my question on how much would it take, and how nasty would it be, to
support !DEFER_TASKRUN.

-- 
Jens Axboe