fs/writeback: only calculate dirtied_before when b_io is empty

The dirtied_before is only used when b_io is not empty, so only calculate
when b_io is not empty.
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Link: https://lore.kernel.org/r/20240228091958.288260-5-shikemeng@huaweicloud.comReviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/fs-writeback.c

@@ -2105,20 +2105,21 @@ static long wb_writeback(struct bdi_writeback *wb,
 
 		spin_lock(&wb->list_lock);
 
+		trace_writeback_start(wb, work);
+		if (list_empty(&wb->b_io)) {
 			/*
-		 * Kupdate and background works are special and we want to
-		 * include all inodes that need writing. Livelock avoidance is
-		 * handled by these works yielding to any other work so we are
-		 * safe.
+			 * Kupdate and background works are special and we want
+			 * to include all inodes that need writing. Livelock
+			 * avoidance is handled by these works yielding to any
+			 * other work so we are safe.
 			 */
 			if (work->for_kupdate) {
 				dirtied_before = jiffies -
-				msecs_to_jiffies(dirty_expire_interval * 10);
+					msecs_to_jiffies(dirty_expire_interval *
+							 10);
 			} else if (work->for_background)
 				dirtied_before = jiffies;
 
-		trace_writeback_start(wb, work);
-		if (list_empty(&wb->b_io)) {
 			queue_io(wb, work, dirtied_before);
 			queued = true;
 		}