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

[Jens Axboe <axboe@kernel.dk>]

On 11/24/21 11:02 AM, Pavel Begunkov wrote:
> 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

I think we're OK as I don't DRAIN is used very much, and as long as it's
adequately documented in terms of them not co-existing and what the error
code is, then if we do find a way to  make them work together we can
relax them in the future.

>> 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

Great thanks.

-- 
Jens Axboe