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To:     Jens Axboe <axboe@kernel.dk>, io-uring@vger.kernel.org
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From:   Pavel Begunkov <asml.silence@gmail.com>
Subject: Re: [RFC 4/4] io_uring: implement futex wait
Message-ID: <5ab4c8bd-3e82-e87b-1ae8-3b32ced72009@gmail.com>
Date:   Tue, 1 Jun 2021 17:29:15 +0100
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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