Re: [PATCH 5/6] io_uring: implement multishot mode for accept

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

On 9/3/21 12:00 PM, Hao Xu wrote:
> Refactor io_accept() to support multishot mode.

Multishot with the fixed/direct mode sounds weird (considering that
the slot index is specified by userspace), let's forbid them.

io_accept_prep() {
	if (accept->file_slot && (flags & MULTISHOT))
		return -EINVAL;
	...
}

> theoretical analysis:
>   1) when connections come in fast
>     - singleshot:
>               add accept sqe(userpsace) --> accept inline
>                               ^                 |
>                               |-----------------|
>     - multishot:
>              add accept sqe(userspace) --> accept inline
>                                               ^     |
>                                               |--*--|
> 
>     we do accept repeatedly in * place until get EAGAIN
> 
>   2) when connections come in at a low pressure
>     similar thing like 1), we reduce a lot of userspace-kernel context
>     switch and useless vfs_poll()
> 
> tests:
> Did some tests, which goes in this way:
> 
>   server    client(multiple)
>   accept    connect
>   read      write
>   write     read
>   close     close
> 
> Basically, raise up a number of clients(on same machine with server) to
> connect to the server, and then write some data to it, the server will
> write those data back to the client after it receives them, and then
> close the connection after write return. Then the client will read the
> data and then close the connection. Here I test 10000 clients connect
> one server, data size 128 bytes. And each client has a go routine for
> it, so they come to the server in short time.
> test 20 times before/after this patchset, time spent:(unit cycle, which
> is the return value of clock())
> before:
>   1930136+1940725+1907981+1947601+1923812+1928226+1911087+1905897+1941075
>   +1934374+1906614+1912504+1949110+1908790+1909951+1941672+1969525+1934984
>   +1934226+1914385)/20.0 = 1927633.75
> after:
>   1858905+1917104+1895455+1963963+1892706+1889208+1874175+1904753+1874112
>   +1874985+1882706+1884642+1864694+1906508+1916150+1924250+1869060+1889506
>   +1871324+1940803)/20.0 = 1894750.45
> 
> (1927633.75 - 1894750.45) / 1927633.75 = 1.65%
> 
> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
> ---
> 
> not sure if we should cancel it when io_cqring_fill_event() reurn false
> 
>  fs/io_uring.c | 34 +++++++++++++++++++++++++++++-----
>  1 file changed, 29 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index dae7044e0c24..eb81d37dce78 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -4885,16 +4885,18 @@ static int io_accept_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>  
>  static int io_accept(struct io_kiocb *req, unsigned int issue_flags)
>  {
> +	struct io_ring_ctx *ctx = req->ctx;
>  	struct io_accept *accept = &req->accept;
>  	bool force_nonblock = issue_flags & IO_URING_F_NONBLOCK;
>  	unsigned int file_flags = force_nonblock ? O_NONBLOCK : 0;
>  	bool fixed = !!accept->file_slot;
>  	struct file *file;
> -	int ret, fd;
> +	int ret, ret2 = 0, fd;
>  
>  	if (req->file->f_flags & O_NONBLOCK)
>  		req->flags |= REQ_F_NOWAIT;
>  
> +retry:
>  	if (!fixed) {
>  		fd = __get_unused_fd_flags(accept->flags, accept->nofile);
>  		if (unlikely(fd < 0))
> @@ -4906,20 +4908,42 @@ static int io_accept(struct io_kiocb *req, unsigned int issue_flags)
>  		if (!fixed)
>  			put_unused_fd(fd);
>  		ret = PTR_ERR(file);
> -		if (ret == -EAGAIN && force_nonblock)
> -			return -EAGAIN;
> +		if (ret == -EAGAIN && force_nonblock) {
> +			if ((req->flags & (REQ_F_APOLL_MULTISHOT | REQ_F_POLLED)) ==
> +			    (REQ_F_APOLL_MULTISHOT | REQ_F_POLLED))
> +				ret = 0;
> +			return ret;
> +		}
>  		if (ret == -ERESTARTSYS)
>  			ret = -EINTR;
>  		req_set_fail(req);
>  	} else if (!fixed) {
>  		fd_install(fd, file);
>  		ret = fd;
> +		/*
> +		 * if it's in multishot mode, let's return -EAGAIN to make it go
> +		 * into fast poll path
> +		 */
> +		if ((req->flags & REQ_F_APOLL_MULTISHOT) && force_nonblock &&
> +		   !(req->flags & REQ_F_POLLED))
> +			ret2 = -EAGAIN;
>  	} else {
>  		ret = io_install_fixed_file(req, file, issue_flags,
>  					    accept->file_slot - 1);
>  	}
> -	__io_req_complete(req, issue_flags, ret, 0);
> -	return 0;
> +
> +	if (req->flags & REQ_F_APOLL_MULTISHOT) {
> +		spin_lock(&ctx->completion_lock);
> +		if (io_cqring_fill_event(ctx, req->user_data, ret, 0)) {
> +			io_commit_cqring(ctx);
> +			ctx->cq_extra++;
> +		}
> +		spin_unlock(&ctx->completion_lock);
> +		goto retry;
> +	} else {
> +		__io_req_complete(req, issue_flags, ret, 0);
> +	}
> +	return ret2;
>  }
>  
>  static int io_connect_prep_async(struct io_kiocb *req)
> 

-- 
Pavel Begunkov