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Subject: Re: [PATCH v3] io_uring: reduce latency by reissueing the operation
To:     Olivier Langlois <olivier@trillion01.com>,
        Jens Axboe <axboe@kernel.dk>, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <4deda7761d61c189f4e2581828f852c8a1acb723.1624303174.git.olivier@trillion01.com>
From:   Pavel Begunkov <asml.silence@gmail.com>
Message-ID: <ab4e6bc7-e5f5-0c0d-9e03-1e490f1ad22a@gmail.com>
Date:   Tue, 22 Jun 2021 12:00:23 +0100
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On 6/21/21 8:22 PM, Olivier Langlois wrote:
> It is quite frequent that when an operation fails and returns EAGAIN,
> the data becomes available between that failure and the call to
> vfs_poll() done by io_arm_poll_handler().
> 
> Detecting the situation and reissuing the operation is much faster
> than going ahead and push the operation to the io-wq.

Looks good to me, will stamp later after testing it out.

 
> Signed-off-by: Olivier Langlois <olivier@trillion01.com>
> ---
>  fs/io_uring.c | 31 ++++++++++++++++++++++---------
>  1 file changed, 22 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index fc8637f591a6..5efa67c2f974 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -5152,7 +5152,13 @@ static __poll_t __io_arm_poll_handler(struct io_kiocb *req,
>  	return mask;
>  }
>  
> -static bool io_arm_poll_handler(struct io_kiocb *req)
> +enum {
> +	IO_APOLL_OK,
> +	IO_APOLL_ABORTED,
> +	IO_APOLL_READY
> +};
> +
> +static int io_arm_poll_handler(struct io_kiocb *req)
>  {
>  	const struct io_op_def *def = &io_op_defs[req->opcode];
>  	struct io_ring_ctx *ctx = req->ctx;
> @@ -5162,22 +5168,22 @@ static bool io_arm_poll_handler(struct io_kiocb *req)
>  	int rw;
>  
>  	if (!req->file || !file_can_poll(req->file))
> -		return false;
> +		return IO_APOLL_ABORTED;
>  	if (req->flags & REQ_F_POLLED)
> -		return false;
> +		return IO_APOLL_ABORTED;
>  	if (def->pollin)
>  		rw = READ;
>  	else if (def->pollout)
>  		rw = WRITE;
>  	else
> -		return false;
> +		return IO_APOLL_ABORTED;
>  	/* if we can't nonblock try, then no point in arming a poll handler */
>  	if (!io_file_supports_async(req, rw))
> -		return false;
> +		return IO_APOLL_ABORTED;
>  
>  	apoll = kmalloc(sizeof(*apoll), GFP_ATOMIC);
>  	if (unlikely(!apoll))
> -		return false;
> +		return IO_APOLL_ABORTED;
>  	apoll->double_poll = NULL;
>  
>  	req->flags |= REQ_F_POLLED;
> @@ -5203,12 +5209,14 @@ static bool io_arm_poll_handler(struct io_kiocb *req)
>  	if (ret || ipt.error) {
>  		io_poll_remove_double(req);
>  		spin_unlock_irq(&ctx->completion_lock);
> -		return false;
> +		if (ret)
> +			return IO_APOLL_READY;
> +		return IO_APOLL_ABORTED;
>  	}
>  	spin_unlock_irq(&ctx->completion_lock);
>  	trace_io_uring_poll_arm(ctx, req, req->opcode, req->user_data,
>  				mask, apoll->poll.events);
> -	return true;
> +	return IO_APOLL_OK;
>  }
>  
>  static bool __io_poll_remove_one(struct io_kiocb *req,
> @@ -6437,6 +6445,7 @@ static void __io_queue_sqe(struct io_kiocb *req)
>  	struct io_kiocb *linked_timeout = io_prep_linked_timeout(req);
>  	int ret;
>  
> +issue_sqe:
>  	ret = io_issue_sqe(req, IO_URING_F_NONBLOCK|IO_URING_F_COMPLETE_DEFER);
>  
>  	/*
> @@ -6456,12 +6465,16 @@ static void __io_queue_sqe(struct io_kiocb *req)
>  			io_put_req(req);
>  		}
>  	} else if (ret == -EAGAIN && !(req->flags & REQ_F_NOWAIT)) {
> -		if (!io_arm_poll_handler(req)) {
> +		switch (io_arm_poll_handler(req)) {
> +		case IO_APOLL_READY:
> +			goto issue_sqe;
> +		case IO_APOLL_ABORTED:
>  			/*
>  			 * Queued up for async execution, worker will release
>  			 * submit reference when the iocb is actually submitted.
>  			 */
>  			io_queue_async_work(req);
> +			break;
>  		}
>  	} else {
>  		io_req_complete_failed(req, ret);
> 

-- 
Pavel Begunkov