f2fs: avoid wait if IO end up when do_checkpoint for better performance

Previously, do_checkpoint() will call congestion_wait() for waiting the pages
(previous submitted node/meta/data pages) to be written back.
Because congestion_wait() will set a regular period (e.g. HZ / 50 ) for waiting, and
no additional wake up mechanism was introduced if IO ends up before regular period costed.
Yuan Zhong found there is a situation that after the pages have been written back,
but the checkpoint thread still wait for congestion_wait to exit.

So here we store checkpoint task into f2fs_sb when doing checkpoint, it'll wait for IO completes
if there's IO going on, and in the end IO path, wake up checkpoint task when IO ends up.

Thanks to Yuan Zhong's pre work about this problem.
Reported-by: NYuan Zhong <yuan.mark.zhong@samsung.com>
Signed-off-by: NGu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: NJaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/checkpoint.c

@@ -757,8 +757,15 @@ static void do_checkpoint(struct f2fs_sb_info *sbi, bool is_umount)
 	f2fs_put_page(cp_page, 1);
 
 	/* wait for previous submitted node/meta pages writeback */
-	while (get_pages(sbi, F2FS_WRITEBACK))
-		congestion_wait(BLK_RW_ASYNC, HZ / 50);
+	sbi->cp_task = current;
+	while (get_pages(sbi, F2FS_WRITEBACK)) {
+		set_current_state(TASK_UNINTERRUPTIBLE);
+		if (!get_pages(sbi, F2FS_WRITEBACK))
+			break;
+		io_schedule();
+	}
+	__set_current_state(TASK_RUNNING);
+	sbi->cp_task = NULL;
 
 	filemap_fdatawait_range(sbi->node_inode->i_mapping, 0, LONG_MAX);
 	filemap_fdatawait_range(sbi->meta_inode->i_mapping, 0, LONG_MAX);

fs/f2fs/f2fs.h

@@ -362,6 +362,7 @@ struct f2fs_sb_info {
 	struct mutex writepages;		/* mutex for writepages() */
 	int por_doing;				/* recovery is doing or not */
 	int on_build_free_nids;			/* build_free_nids is doing */
+	struct task_struct *cp_task;		/* checkpoint task */
 
 	/* for orphan inode management */
 	struct list_head orphan_inode_list;	/* orphan inode list */

fs/f2fs/segment.c

@@ -597,6 +597,10 @@ static void f2fs_end_io_write(struct bio *bio, int err)
 
 	if (p->is_sync)
 		complete(p->wait);
+
+	if (!get_pages(p->sbi, F2FS_WRITEBACK) && p->sbi->cp_task)
+		wake_up_process(p->sbi->cp_task);
+
 	kfree(p);
 	bio_put(bio);
 }