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

From: Jens Axboe
Date: Tue Jun 01 2021 - 12:01:16 EST

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
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.

Jens Axboe