linux-kernel - Re: [RFC 4/4] io_uring: implement futex wait

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Date:   Tue, 1 Jun 2021 10:01:11 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org
Cc:     Andres Freund <andres@...razel.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 4/4] io_uring: implement futex wait

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