Re: [PATCH for-next 5/7] io_uring: remove ->flush_cqes optimisation

[Jens Axboe <axboe@kernel.dk>]

On 6/19/22 5:26 AM, Pavel Begunkov wrote:
> It's not clear how widely used IOSQE_CQE_SKIP_SUCCESS is, and how often
> ->flush_cqes flag prevents from completion being flushed. Sometimes it's
> high level of concurrency that enables it at least for one CQE, but
> sometimes it doesn't save much because nobody waiting on the CQ.
> 
> Remove ->flush_cqes flag and the optimisation, it should benefit the
> normal use case. Note, that there is no spurious eventfd problem with
> that as checks for spuriousness were incorporated into
> io_eventfd_signal().

Would be note to quantify, which should be pretty easy. Eg run a nop
workload, then run the same but with CQE_SKIP_SUCCESS set. That'd take
it to the extreme, and I do think it'd be nice to have an understanding
of how big the gap could potentially be.

With luck, it doesn't really matter. Always nice to kill stuff like
this, if it isn't that impactful.

-- 
Jens Axboe