Re: [PATCH] f2fs: skip direct I/O iostat work when disabled

[Jaegeuk Kim <jaegeuk@kernel.org>]

On 05/27, Wenjie Qi wrote:
> F2FS iostat is optional and is disabled by default, but the direct
> I/O submit path still allocates and binds a bio_iostat_ctx, updates
> the submit timestamp, and replaces bi_end_io for every DIO bio even
> when sbi->iostat_enable is false.
> 
> The end_io path also calls f2fs_update_iostat(), which returns
> immediately when iostat is disabled. This adds avoidable per-bio
> overhead to the default direct I/O hot path.
> 
> Skip DIO iostat context setup and the direct read/write byte updates
> when iostat is disabled. If iostat is enabled through sysfs before
> submission, the existing context allocation and latency accounting path
> is still used.
> 
> QEMU benchmark on a 1GiB F2FS virtio-blk image, with iostat_enable=0,
> 4KiB O_DIRECT I/O over a 64MiB file, 50000 iterations per run:
> 
>                          baseline     patched
>   direct_read median    65264.50 ns  55470.95 ns
>   direct_read recheck   65553.75 ns  55470.95 ns
>   direct_write median   68054.62 ns  56309.44 ns
>   direct_write recheck  66873.51 ns  56309.44 ns
> 
> Signed-off-by: Wenjie Qi <qiwenjie@xiaomi.com>
> ---
>  fs/f2fs/file.c   | 9 +++++++--
>  fs/f2fs/iostat.h | 6 ++++++
>  2 files changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> index 6edf0105dbc8..3ad8bd660b33 100644
> --- a/fs/f2fs/file.c
> +++ b/fs/f2fs/file.c
> @@ -4799,6 +4799,9 @@ static void f2fs_dio_iostat_start(struct f2fs_sb_info *sbi, struct bio *bio)
>  {
>  	void *bi_private = bio->bi_private;
>  
> +	if (!f2fs_iostat_enabled(sbi))
> +		return;
> +
>  	iostat_alloc_and_bind_ctx(sbi, bio, bi_private);
>  	iostat_update_submit_ctx(bio, DATA);
>  	bio->bi_end_io = f2fs_dio_end_bio;
> @@ -4816,7 +4819,8 @@ static int f2fs_dio_read_end_io(struct kiocb *iocb, ssize_t size, int error,
>  	dec_page_count(sbi, F2FS_DIO_READ);
>  	if (error)
>  		return error;
> -	f2fs_update_iostat(sbi, NULL, APP_DIRECT_READ_IO, size);
> +	if (f2fs_iostat_enabled(sbi))
> +		f2fs_update_iostat(sbi, NULL, APP_DIRECT_READ_IO, size);

f2fs_update_iostat() checks sbi->iostat_enable?

>  	return 0;
>  }
>  
> @@ -5097,7 +5101,8 @@ static int f2fs_dio_write_end_io(struct kiocb *iocb, ssize_t size, int error,
>  	if (error)
>  		return error;
>  	f2fs_update_time(sbi, REQ_TIME);
> -	f2fs_update_iostat(sbi, NULL, APP_DIRECT_IO, size);
> +	if (f2fs_iostat_enabled(sbi))
> +		f2fs_update_iostat(sbi, NULL, APP_DIRECT_IO, size);
>  	return 0;
>  }
>  
> diff --git a/fs/f2fs/iostat.h b/fs/f2fs/iostat.h
> index 2025225b5bed..d3ef787575be 100644
> --- a/fs/f2fs/iostat.h
> +++ b/fs/f2fs/iostat.h
> @@ -44,6 +44,11 @@ struct bio_iostat_ctx {
>  	struct bio_post_read_ctx *post_read_ctx;
>  };
>  
> +static inline bool f2fs_iostat_enabled(struct f2fs_sb_info *sbi)
> +{
> +	return sbi->iostat_enable;
> +}
> +
>  static inline void iostat_update_submit_ctx(struct bio *bio,
>  			enum page_type type)
>  {
> @@ -72,6 +77,7 @@ static inline void f2fs_update_iostat(struct f2fs_sb_info *sbi, struct inode *in
>  		enum iostat_type type, unsigned long long io_bytes) {}
>  static inline void f2fs_update_read_folio_count(struct f2fs_sb_info *sbi,
>  		struct folio *folio) {}
> +static inline bool f2fs_iostat_enabled(struct f2fs_sb_info *sbi) { return false; }
>  static inline void iostat_update_and_unbind_ctx(struct bio *bio) {}
>  static inline void iostat_alloc_and_bind_ctx(struct f2fs_sb_info *sbi,
>  		struct bio *bio, struct bio_post_read_ctx *ctx) {}
> -- 
> 2.43.0
>