Re: [PATCH] io_uring: export cq overflow status to userspace

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

...
>>>
>>> To fix this issue, export cq overflow status to userspace, then
>>> helper functions() in liburing, such as io_uring_peek_cqe, can be
>>> aware of this cq overflow and do flush accordingly.
>>
>> Is there any way we can accomplish the same without exporting
>> another set of flags? Would it be enough for the SQPOLl thread to set
>> IORING_SQ_NEED_WAKEUP if we're in overflow condition? That should
>> result in the app entering the kernel when it's flushed the user CQ
>> side, and then the sqthread could attempt to flush the pending
>> events as well.
>>
>> Something like this, totally untested...
> 
> OK, took a closer look at this, it's a generic thing, not just
> SQPOLL related. My bad!
> 
> Anyway, my suggestion would be to add IORING_SQ_CQ_OVERFLOW to the
> existing flags, and then make a liburing change almost identical to
> what you had.

How about CQ being full as an indicator that flush is required?
With a properly set CQ size there shouldn't be much false positives.


-- 
Pavel Begunkov