Re: [PATCH v2 0/4] allow to skip CQE posting

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

On 11/24/21 17:57, Jens Axboe wrote:
> On 11/24/21 10:55 AM, Pavel Begunkov wrote:
>> On 11/10/21 16:47, Jens Axboe wrote:
>>> On 11/10/21 9:42 AM, Pavel Begunkov wrote:
>>>> On 11/10/21 16:14, Jens Axboe wrote:
>>>>> On 11/10/21 8:49 AM, Pavel Begunkov wrote:
>>>>>> It's expensive enough to post an CQE, and there are other
>>>>>> reasons to want to ignore them, e.g. for link handling and
>>>>>> it may just be more convenient for the userspace.
>>>>>>
>>>>>> Try to cover most of the use cases with one flag. The overhead
>>>>>> is one "if (cqe->flags & IOSQE_CQE_SKIP_SUCCESS)" check per
>>>>>> requests and a bit bloated req_set_fail(), should be bearable.
>>>>>
>>>>> I like the idea, one thing I'm struggling with is I think a normal use
>>>>> case of this would be fast IO where we still need to know if a
>>>>> completion event has happened, we just don't need to know the details of
>>>>> it since we already know what those details would be if it ends up in
>>>>> success.
>>>>>
>>>>> How about having a skip counter? That would supposedly also allow drain
>>>>> to work, and it could be mapped with the other cq parts to allow the app
>>>>> to see it as well.
>>>>
>>>> It doesn't go through expensive io_cqring_ev_posted(), so the
>>>> userspace can't really wait on it. It can do some linking tricks to
>>>> alleviate that, but I don't see any new capabilities from the current
>>>> approach.
>>>
>>> I'm not talking about waiting, just reading the cqring entry to see how
>>> many were skipped. If you ask for no cqe, by definition there would be
>>> nothing to wait on for you. Though it'd probably be better as an sqring
>>> entry, since we'd be accounting at that time. Only caveat there is then
>>> if the sqe errors and we do end up posting a cqe..
>>>
>>>> Also the locking is a problem, I was thinking about it, mainly hoping
>>>> that I can adjust cq_extra and leave draining, but it didn't appear
>>>> great to me. AFAIK, it's either an atomic, beating the purpose of the
>>>> thing.
>>>
>>> If we do submission side, then the ring mutex would cover it. No need
>>> for any extra locking
>>
>> Jens, let's decide what we're going to do with this feature
> 
> Only weird bit is the drain, but apart from that I think it looks sane.

agree, but I can't find a fix without penalising performance

> Are you going to send a documentation update to liburing as well? Should
> be detailed in terms of what it does and the usability of it.

yeah, and also need to rebase and resend tests

-- 
Pavel Begunkov