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Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
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References: <cover.1569139018.git.asml.silence@gmail.com>
 <a4996ae7-ac0a-447b-49b2-7e96275aad29@kernel.dk>
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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: <c15b2d54-c722-8fb4-266f-b589c1a21aa5@gmail.com>
Date:   Mon, 23 Sep 2019 19:21:51 +0300
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From: Pavel Begunkov <asml.silence@gmail.com>
To: Ingo Molnar <mingo@kernel.org>, Jens Axboe <axboe@kernel.dk>
Cc: Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
 linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Message-ID: <c15b2d54-c722-8fb4-266f-b589c1a21aa5@gmail.com>
Subject: Re: [PATCH v2 0/2] Optimise io_uring completion waiting
References: <cover.1569139018.git.asml.silence@gmail.com>
 <a4996ae7-ac0a-447b-49b2-7e96275aad29@kernel.dk>
 <20190923083549.GA42487@gmail.com>
In-Reply-To: <20190923083549.GA42487@gmail.com>

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Hi, and thanks for the feedback.

It could be done with @cond indeed, that's how it works for now.
However, this addresses performance issues only.

The problem with wait_event_*() is that, if we have a counter and are
trying to wake up tasks after each increment, it would schedule each
waiting task O(threshold) times just for it to spuriously check @cond
and go back to sleep. All that overhead (memory barriers, registers
save/load, accounting, etc) turned out to be enough for some workloads
to slow down the system.

With this specialisation it still traverses a wait list and makes
indirect calls to the checker callback, but the list supposedly is
fairly  small, so performance there shouldn't be a problem, at least for
now.

Regarding semantics; It should wake a task when a value passed to
wake_up_threshold() is greater or equal then a task's threshold, that is
specified individually for each task in wait_threshold_*().

In pseudo code:
```
def wake_up_threshold(n, wait_queue):
	for waiter in wait_queue:
		waiter.wake_up_if(n >=3D waiter.threshold);
```

Any thoughts how to do it better? Ideas are very welcome.

BTW, this monster is mostly a copy-paste from wait_event_*(),
wait_bit_*(). We could try to extract some common parts from these
three, but that's another topic.


On 23/09/2019 11:35, Ingo Molnar wrote:
>=20
> * Jens Axboe <axboe@kernel.dk> wrote:
>=20
>> On 9/22/19 2:08 AM, Pavel Begunkov (Silence) wrote:
>>> From: Pavel Begunkov <asml.silence@gmail.com>
>>>
>>> There could be a lot of overhead within generic wait_event_*() used f=
or
>>> waiting for large number of completions. The patchset removes much of=

>>> it by using custom wait event (wait_threshold).
>>>
>>> Synthetic test showed ~40% performance boost. (see patch 2)
>>
>> I'm fine with the io_uring side of things, but to queue this up we
>> really need Peter or Ingo to sign off on the core wakeup bits...
>>
>> Peter?
>=20
> I'm not sure an extension is needed for such a special interface, why n=
ot=20
> just put a ->threshold value next to the ctx->wait field and use either=
=20
> the regular wait_event() APIs with the proper condition, or=20
> wait_event_cmd() style APIs if you absolutely need something more compl=
ex=20
> to happen inside?
>=20
> Should result in a much lower linecount and no scheduler changes. :-)
>=20
> Thanks,
>=20
> 	Ingo
>=20

--=20
Yours sincerely,
Pavel Begunkov


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