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To:     Jens Axboe <axboe@kernel.dk>, Ingo Molnar <mingo@kernel.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
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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: <38ea2681-dbf0-457a-dcc0-406d10e2572b@gmail.com>
Date:   Tue, 24 Sep 2019 12:21:54 +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: <38ea2681-dbf0-457a-dcc0-406d10e2572b@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>
 <c15b2d54-c722-8fb4-266f-b589c1a21aa5@gmail.com>
 <df612e90-8999-0085-d2d6-4418e044e429@gmail.com>
 <731b2087-7786-5374-68ff-8cba42f0cd68@kernel.dk>
 <759b9b48-1de3-1d43-3e39-9c530bfffaa0@kernel.dk>
 <43244626-9cfd-0c0b-e7a1-878363712ef3@gmail.com>
 <f2608e3d-bb4e-9984-79e8-a2ab4f855c7f@kernel.dk>
In-Reply-To: <f2608e3d-bb4e-9984-79e8-a2ab4f855c7f@kernel.dk>

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On 24/09/2019 11:02, 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 somethin=
g
>>> 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-contained=
,
>> though.
>>
>> If you don't think it could be useful in other places, sure, we could =
do
>> something like that. Is that so?
>=20
> I totally agree it could be useful in other places. Maybe formalized an=
d
> used with wake_up_nr() instead of adding a new primitive? Haven't looke=
d
> into that, I may be talking nonsense.

@nr there is about number of tasks to wake up. AFAIK doesn't solve the
problem.


>=20
> 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
>=20
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index ca7570aca430..14fae454cf75 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2768,6 +2768,42 @@ static int io_ring_submit(struct io_ring_ctx *ct=
x, unsigned int to_submit,
>  	return submit;
>  }
> =20
> +struct io_wait_queue {
> +	struct wait_queue_entry wq;
> +	struct io_ring_ctx *ctx;
> +	struct task_struct *task;
> +	unsigned to_wait;
> +	unsigned nr_timeouts;
> +};
> +
> +static inline bool io_should_wake(struct io_wait_queue *iowq)
> +{
> +	struct io_ring_ctx *ctx =3D iowq->ctx;
> +
> +	/*
> +	 * Wake up if we have enough events, or if a timeout occured since we=

> +	 * started waiting. For timeouts, we always want to return to userspa=
ce,
> +	 * regardless of event count.
> +	 */
> +	return io_cqring_events(ctx->rings) >=3D iowq->to_wait ||
> +			atomic_read(&ctx->cq_timeouts) !=3D iowq->nr_timeouts;
> +}
> +
> +static int io_wake_function(struct wait_queue_entry *curr, unsigned in=
t mode,
> +			    int wake_flags, void *key)
> +{
> +	struct io_wait_queue *iowq =3D container_of(curr, struct io_wait_queu=
e,
> +							wq);
> +
> +	if (io_should_wake(iowq)) {
> +		list_del_init(&curr->entry);
> +		wake_up_process(iowq->task);
> +		return 1;
> +	}
> +
> +	return -1;
> +}
> +
>  /*
>   * Wait until events become available, if we don't already have some. =
The
>   * application must reap them itself, as they reside on the shared cq =
ring.
> @@ -2775,8 +2811,16 @@ static int io_ring_submit(struct io_ring_ctx *ct=
x, unsigned int to_submit,
>  static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
>  			  const sigset_t __user *sig, size_t sigsz)
>  {
> +	struct io_wait_queue iowq =3D {
> +		.wq =3D {
> +			.func	=3D io_wake_function,
> +			.entry	=3D LIST_HEAD_INIT(iowq.wq.entry),
> +		},
> +		.task		=3D current,
> +		.ctx		=3D ctx,
> +		.to_wait	=3D min_events,
> +	};
>  	struct io_rings *rings =3D ctx->rings;
> -	unsigned nr_timeouts;
>  	int ret;
> =20
>  	if (io_cqring_events(rings) >=3D min_events)
> @@ -2795,15 +2839,16 @@ static int io_cqring_wait(struct io_ring_ctx *c=
tx, int min_events,
>  			return ret;
>  	}
> =20
> -	nr_timeouts =3D atomic_read(&ctx->cq_timeouts);
> -	/*
> -	 * Return if we have enough events, or if a timeout occured since
> -	 * we started waiting. For timeouts, we always want to return to
> -	 * userspace.
> -	 */
> -	ret =3D wait_event_interruptible(ctx->wait,
> -				io_cqring_events(rings) >=3D min_events ||
> -				atomic_read(&ctx->cq_timeouts) !=3D nr_timeouts);
> +	iowq.nr_timeouts =3D atomic_read(&ctx->cq_timeouts);
> +	prepare_to_wait_exclusive(&ctx->wait, &iowq.wq, TASK_INTERRUPTIBLE);
> +	do {
> +		if (io_should_wake(&iowq))
> +			break;
> +		schedule();
> +		set_current_state(TASK_INTERRUPTIBLE);
> +	} while (1);
> +	finish_wait(&ctx->wait, &iowq.wq);
> +
>  	restore_saved_sigmask_unless(ret =3D=3D -ERESTARTSYS);
>  	if (ret =3D=3D -ERESTARTSYS)
>  		ret =3D -EINTR;
>=20

--=20
Yours sincerely,
Pavel Begunkov


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