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Subject: Re: [RFC 4/4] io_uring: implement futex wait
To:     Jens Axboe <axboe@kernel.dk>, io-uring@vger.kernel.org
Cc:     Andres Freund <andres@anarazel.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Darren Hart <dvhart@infradead.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        linux-kernel@vger.kernel.org
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 <bd824ec8-48af-b554-67a1-7ce20fcf608c@kernel.dk>
From:   Pavel Begunkov <asml.silence@gmail.com>
Message-ID: <409a624c-de75-0ee5-b65f-ee09fff34809@gmail.com>
Date:   Tue, 1 Jun 2021 16:58:22 +0100
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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?

-- 
Pavel Begunkov