Re: io_uring and spurious wake-ups from eventfd

[Mark Papadakis <markuspapadakis@icloud.com>]

Thus sounds perfect!

Thanks Jens

@markpapadakis

> On 8 Jan 2020, at 6:24 PM, Jens Axboe <axboe@kernel.dk> wrote:
> 
> On 1/8/20 12:36 AM, Mark Papadakis wrote:
>> 
>> 
>>>> On 7 Jan 2020, at 10:34 PM, Jens Axboe <axboe@kernel.dk> wrote:
>>> 
>>> On 1/7/20 1:26 PM, Jens Axboe wrote:
>>>> On 1/7/20 8:55 AM, Mark Papadakis wrote:
>>>>> This is perhaps an odd request, but if it’s trivial to implement
>>>>> support for this described feature, it could help others like it ‘d
>>>>> help me (I ‘ve been experimenting with io_uring for some time now).
>>>>> 
>>>>> Being able to register an eventfd with an io_uring context is very
>>>>> handy, if you e.g have some sort of reactor thread multiplexing I/O
>>>>> using epoll etc, where you want to be notified when there are pending
>>>>> CQEs to drain. The problem, such as it is, is that this can result in
>>>>> un-necessary/spurious wake-ups.
>>>>> 
>>>>> If, for example, you are monitoring some sockets for EPOLLIN, and when
>>>>> poll says you have pending bytes to read from their sockets, and said
>>>>> sockets are non-blocking, and for each some reported event you reserve
>>>>> an SQE for preadv() to read that data and then you io_uring_enter to
>>>>> submit the SQEs, because the data is readily available, as soon as
>>>>> io_uring_enter returns, you will have your completions available -
>>>>> which you can process.  The “problem” is that poll will wake up
>>>>> immediately thereafter in the next reactor loop iteration because
>>>>> eventfd was tripped (which is reasonable but un-necessary).
>>>>> 
>>>>> What if there was a flag for io_uring_setup() so that the eventfd
>>>>> would only be tripped for CQEs that were processed asynchronously, or,
>>>>> if that’s non-trivial, only for CQEs that reference file FDs?
>>>>> 
>>>>> That’d help with that spurious wake-up.
>>>> 
>>>> One easy way to do that would be for the application to signal that it
>>>> doesn't want eventfd notifications for certain requests. Like using an
>>>> IOSQE_ flag for that. Then you could set that on the requests you submit
>>>> in response to triggering an eventfd event.
>>> 
>> 
>> 
>> Thanks Jens,
>> 
>> This is great, but perhaps there is a somewhat slightly more optimal
>> way to do this.  Ideally, io_uring should trip the eventfd if there
>> are any new completions available, that haven’t been produced In the
>> context of an io_uring_enter(). That is to say, if any SQEs can be
>> immediately served (because data is readily available in
>> Buffers/caches in the kernel), then their respective CQEs will be
>> produced in the context of that io_uring_enter() that submitted said
>> SQEs(and thus the CQEs can be processed immediately after
>> io_uring_enter() returns).  So, if any CQEs are placed in the
>> respective ring at any other time, but not during an io_uring_enter()
>> call, then it means those completions were produced asynchronously,
>> and thus the eventfd can be tripped, otherwise, there is no need to
>> trip the eventfd at all.
>> 
>> e.g (pseudocode):
>> void produce_completion(cfq_ctx *ctx, const bool in_io_uring_enter_ctx) {
>>        cqe_ring_push(cqe_from_ctx(ctx));
>>        if (false == in_io_uring_enter_ctx && eventfd_registered()) {
>>                trip_iouring_eventfd();
>>        } else {
>>                // don't bother
>>        }
>> }
> 
> I see what you're saying, so essentially only trigger eventfd
> notifications if the completions happen async. That does make a lot of
> sense, and it would be cleaner than having to flag this per request as
> well. I think we'd still need to make that opt-in as it changes the
> behavior of it.
> 
> The best way to do that would be to add IORING_REGISTER_EVENTFD_ASYNC or
> something like that. Does the exact same thing as
> IORING_REGISTER_EVENTFD, but only triggers it if completions happen
> async.
> 
> What do you think?
> 
> -- 
> Jens Axboe
>