Re: [RFC 4/4] io_uring: implement futex wait

From: Pavel Begunkov
Date: Tue Jun 01 2021 - 12:29:36 EST

On 6/1/21 5:01 PM, Jens Axboe wrote:
> On 6/1/21 9:58 AM, Pavel Begunkov wrote:
>> On 6/1/21 4:45 PM, Jens Axboe wrote:
>>> On 6/1/21 8:58 AM, Pavel Begunkov wrote:
>>>> Add futex wait requests, those always go through io-wq for simplicity.
>>> Not a huge fan of that, I think this should tap into the waitqueue
>>> instead and just rely on the wakeup callback to trigger the event. That
>>> would be a lot more efficient than punting to io-wq, both in terms of
>>> latency on trigger, but also for efficiency if the app is waiting on a
>>> lot of futexes.
>> Yes, that would be preferable, but looks futexes don't use
>> waitqueues but some manual enqueuing into a plist_node, see
>> futex_wait_queue_me() or mark_wake_futex().
>> Did I miss it somewhere?
> Yes, we'd need to augment that with a callback. I do think that's going

Yeah, that was the first idea, but it's also more intrusive for the
futex codebase. Can be piled on top for next revision of patches.

A question to futex maintainers, how much resistance to merging
something like that I may expect?

> to be necessary, I don't see the io-wq solution working well outside of
> the most basic of use cases. And even for that, it won't be particularly
> efficient for single waits.

Pavel Begunkov