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);
 	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