Re: [PATCH RESEND] io_uring: a small optimization for REQ_F_DRAIN_LINK

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

On 11/22/2019 9:01 AM, Jackie Liu wrote:
> From: Jackie Liu <liuyun01@kylinos.cn>
> 
> We don't need to set drain_next every time, make a small judgment
> and add unlikely, it looks like there will be a little optimization.
> 
Not sure about that. It's 1 CMP + 1 SETcc/STORE, which works pretty fast
as @drain_next is hot (especially after read) and there is no write-read
dependency close. For yours, there is likely always 3 CMPs in the way.

Did you benchmarked it somehow or compared assembly?

> Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
> ---
>  resend that patch, because reject by mail-list.
> 
>  fs/io_uring.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 013e5ed..f4ec44a 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2938,12 +2938,14 @@ static void __io_queue_sqe(struct io_kiocb *req)
>  static void io_queue_sqe(struct io_kiocb *req)
>  {
>  	int ret;
> +	bool drain_link = req->flags & REQ_F_DRAIN_LINK;
>  
> -	if (unlikely(req->ctx->drain_next)) {
> +	if (unlikely(req->ctx->drain_next && !drain_link)) {
>  		req->flags |= REQ_F_IO_DRAIN;
>  		req->ctx->drain_next = false;
> +	} else if (unlikely(drain_link)) {
> +		req->ctx->drain_next = true;
>  	}
> -	req->ctx->drain_next = (req->flags & REQ_F_DRAIN_LINK);
>  
>  	ret = io_req_defer(req);
>  	if (ret) {
> 

-- 
Pavel Begunkov