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From:   Pavel Begunkov <asml.silence@gmail.com>
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Subject: Re: [PATCH v2 0/2] Optimise io_uring completion waiting
Message-ID: <4240453d-2a6a-092f-28a4-523d2f6fc6c1@gmail.com>
Date:   Tue, 24 Sep 2019 12:20:53 +0300
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From: Pavel Begunkov <asml.silence@gmail.com>
To: Jens Axboe <axboe@kernel.dk>, Ingo Molnar <mingo@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
 linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Message-ID: <4240453d-2a6a-092f-28a4-523d2f6fc6c1@gmail.com>
Subject: Re: [PATCH v2 0/2] Optimise io_uring completion waiting
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On 24/09/2019 11:27, Jens Axboe wrote:
> On 9/24/19 2:02 AM, Jens Axboe wrote:
>> On 9/24/19 1:06 AM, Pavel Begunkov wrote:
>>> On 24/09/2019 02:00, Jens Axboe wrote:
>>>>> I think we can do the same thing, just wrapping the waitqueue in a
>>>>> structure with a count in it, on the stack. Got some flight time
>>>>> coming up later today, let me try and cook up a patch.
>>>>
>>>> Totally untested, and sent out 5 min before departure... But somethi=
ng
>>>> like this.
>>> Hmm, reminds me my first version. Basically that's the same thing but=

>>> with macroses inlined. I wanted to make it reusable and self-containe=
d,
>>> though.
>>>
>>> If you don't think it could be useful in other places, sure, we could=
 do
>>> something like that. Is that so?
>>
>> I totally agree it could be useful in other places. Maybe formalized a=
nd
>> used with wake_up_nr() instead of adding a new primitive? Haven't look=
ed
>> into that, I may be talking nonsense.
>>
>> In any case, I did get a chance to test it and it works for me. Here's=

>> the "finished" version, slightly cleaned up and with a comment added
>> for good measure.
>=20
> Notes:
>=20
> This version gets the ordering right, you need exclusive waits to get
> fifo ordering on the waitqueue.
>=20
> Both versions (yours and mine) suffer from the problem of potentially
> waking too many. I don't think this is a real issue, as generally we
> don't do threaded access to the io_urings. But if you had the following=

> tasks wait on the cqring:
>=20
> [min_events =3D 32], [min_events =3D 8], [min_events =3D 8]
>=20
> and we reach the io_cqring_events() =3D=3D threshold, we'll wake all th=
ree.
> I don't see a good solution to this, so I suspect we just live with
> until proven an issue. Both versions are much better than what we have
> now.
>=20
If io_cqring_events() =3D=3D 8, only the last two would be woken up in bo=
th
implementations, as to_wait/threshold is specified per waiter. Isn't it?

Agree with waiting, I don't see a good real-life case for that, that
couldn't be done efficiently in userspace.


--=20
Yours sincerely,
Pavel Begunkov


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