Re: [PATCH v4] io_uring: reduce latency by reissueing the operation

[Pavel Begunkov <asml.silence@gmail.com>]

On 6/22/21 1:17 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().

Looks good

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>

> Detecting the situation and reissuing the operation is much faster
> than going ahead and push the operation to the io-wq.
> 
> Performance improvement testing has been performed with:
> Single thread, 1 TCP connection receiving a 5 Mbps stream, no sqpoll.
> 
> 4 measurements have been taken:
> 1. The time it takes to process a read request when data is already available
> 2. The time it takes to process by calling twice io_issue_sqe() after vfs_poll() indicated that data was available
> 3. The time it takes to execute io_queue_async_work()
> 4. The time it takes to complete a read request asynchronously
> 
> 2.25% of all the read operations did use the new path.
> 
> ready data (baseline)
> avg	3657.94182918628
> min	580
> max	20098
> stddev	1213.15975908162
> 
> reissue	completion
> average	7882.67567567568
> min	2316
> max	28811
> stddev	1982.79172973284
> 
> insert io-wq time
> average	8983.82276995305
> min	3324
> max	87816
> stddev	2551.60056552038
> 
> async time completion
> average	24670.4758861127
> min	10758
> max	102612
> stddev	3483.92416873804
> 
> Conclusion:
> On average reissuing the sqe with the patch code is 1.1uSec faster and
> in the worse case scenario 59uSec faster than placing the request on
> io-wq
> 
> On average completion time by reissuing the sqe with the patch code is
> 16.79uSec faster and in the worse case scenario 73.8uSec faster than
> async completion.
> 
> 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