Re: [PATCH] io_uring: extend async work merging

[Nikolay Borisov <nborisov@suse.com>]

On 11.09.19 г. 19:15 ч., Jens Axboe wrote:
> We currently merge async work items if we see a strict sequential hit.
> This helps avoid unnecessary workqueue switches when we don't need
> them. We can extend this merging to cover cases where it's not a strict
> sequential hit, but the IO still fits within the same page. If an
> application is doing multiple requests within the same page, we don't
> want separate workers waiting on the same page to complete IO. It's much
> faster to let the first worker bring in the page, then operate on that
> page from the same worker to complete the next request(s).
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> ---
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 03fcd974fd1d..4bc3ee4ea81f 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -167,7 +167,7 @@ struct async_list {
>  	struct list_head	list;
>  
>  	struct file		*file;
> -	off_t			io_end;
> +	off_t			io_start;
>  	size_t			io_len;
>  };
>  
> @@ -1189,6 +1189,28 @@ static ssize_t io_import_iovec(struct io_ring_ctx *ctx, int rw,
>  	return import_iovec(rw, buf, sqe_len, UIO_FASTIOV, iovec, iter);
>  }
>  
> +static inline bool io_should_merge(struct async_list *al, struct kiocb *kiocb)
> +{
> +	if (al->file == kiocb->ki_filp) {
> +		off_t start, end;
> +
> +		/*
> +		 * Allow merging if we're anywhere in the range of the same
> +		 * page. Generally this happens for sub-page reads or writes,
> +		 * and it's beneficial to allow the first worker to bring the
> +		 * page in and the piggy backed work can then work on the
> +		 * cached page.
> +		 */
> +		start = al->io_start & PAGE_MASK;

nit: round_down(al->io_start, PAGE_SIZE);

> +		end = (al->io_start + al->io_len + PAGE_SIZE - 1) & PAGE_MASK;
nit: round_up(al->io_start+io_len, PAGE_SIZE)

> +		if (kiocb->ki_pos >= start && kiocb->ki_pos <= end)
> +			return true;
> +	}
> +
> +	al->file = NULL;
> +	return false;
> +}
> +
>  /*
>   * Make a note of the last file/offset/direction we punted to async
>   * context. We'll use this information to see if we can piggy back a
> @@ -1200,9 +1222,8 @@ static void io_async_list_note(int rw, struct io_kiocb *req, size_t len)
>  	struct async_list *async_list = &req->ctx->pending_async[rw];
>  	struct kiocb *kiocb = &req->rw;
>  	struct file *filp = kiocb->ki_filp;
> -	off_t io_end = kiocb->ki_pos + len;
>  
> -	if (filp == async_list->file && kiocb->ki_pos == async_list->io_end) {
> +	if (io_should_merge(async_list, kiocb)) {
>  		unsigned long max_bytes;
>  
>  		/* Use 8x RA size as a decent limiter for both reads/writes */
> @@ -1215,17 +1236,16 @@ static void io_async_list_note(int rw, struct io_kiocb *req, size_t len)
>  			req->flags |= REQ_F_SEQ_PREV;
>  			async_list->io_len += len;
>  		} else {
> -			io_end = 0;
> -			async_list->io_len = 0;
> +			async_list->file = NULL;
>  		}
>  	}
>  
>  	/* New file? Reset state. */
>  	if (async_list->file != filp) {
> -		async_list->io_len = 0;
> +		async_list->io_start = kiocb->ki_pos;
> +		async_list->io_len = len;
>  		async_list->file = filp;
>  	}
> -	async_list->io_end = io_end;
>  }
>  
>  static int io_read(struct io_kiocb *req, const struct sqe_submit *s,
> @@ -1994,7 +2014,7 @@ static void io_sq_wq_submit_work(struct work_struct *work)
>   */
>  static bool io_add_to_prev_work(struct async_list *list, struct io_kiocb *req)
>  {
> -	bool ret = false;
> +	bool ret;
>  
>  	if (!list)
>  		return false;
>