Re: [PATCH v5 2/2] io_uring: avoid unnecessary io_wq_work copy for fast poll feature

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

On 07/06/2020 18:02, Jens Axboe wrote:
> On 6/3/20 7:46 AM, Pavel Begunkov wrote:
>> On 02/06/2020 04:16, Xiaoguang Wang wrote:
>>> hi Jens, Pavel,
>>>
>>> Will you have a look at this V5 version? Or we hold on this patchset, and
>>> do the refactoring work related io_wq_work firstly.
>>
>> It's entirely up to Jens, but frankly, I think it'll bring more bugs than
>> merits in the current state of things.
> 
> Well, I'd really like to reduce the overhead where we can, particularly
> when the overhead just exists to cater to the slow path.
> 
> Planning on taking the next week off and not do too much, but I'll see
> if I can get some testing in with the current patches.
> 

I just think it should not be done at expense of robustness.

e.g. instead of having tons of if's around ->func, we can get rid of
it and issue everything with io_wq_submit_work(). And there are plenty
of pros of doing that:
- freeing some space in io_kiocb (in req.work in particular)
- removing much of stuff with nice negative diffstat
- helping this series
- even safer than now -- can't be screwed with memcpy(req).

Extra switch-lookup in io-wq shouldn't even be noticeable considering
punting overhead. And even though io-wq loses some flexibility, as for
me that's fine as long as there is only 1 user.


And then we can go and fix every other problem until this patch set
looks good.

-- 
Pavel Begunkov