f2fs: avoid BG_GC in f2fs_balance_fs

If many threads hit has_not_enough_free_secs() in f2fs_balance_fs() at the same
time, all the threads would do FG_GC or BG_GC.
In this critical path, we totally don't need to do BG_GC at all.
Let's avoid that.
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -2183,7 +2183,7 @@ int f2fs_migrate_page(struct address_space *, struct page *, struct page *,
 int start_gc_thread(struct f2fs_sb_info *);
 void stop_gc_thread(struct f2fs_sb_info *);
 block_t start_bidx_of_node(unsigned int, struct inode *);
-int f2fs_gc(struct f2fs_sb_info *, bool);
+int f2fs_gc(struct f2fs_sb_info *, bool, bool);
 void build_gc_manager(struct f2fs_sb_info *);
 
 /*

fs/f2fs/file.c

@@ -1849,7 +1849,7 @@ static int f2fs_ioc_gc(struct file *filp, unsigned long arg)
 		mutex_lock(&sbi->gc_mutex);
 	}
 
-	ret = f2fs_gc(sbi, sync);
+	ret = f2fs_gc(sbi, sync, true);
 out:
 	mnt_drop_write_file(filp);
 	return ret;

fs/f2fs/gc.c

@@ -82,7 +82,7 @@ static int gc_thread_func(void *data)
 		stat_inc_bggc_count(sbi);
 
 		/* if return value is not zero, no victim was selected */
-		if (f2fs_gc(sbi, test_opt(sbi, FORCE_FG_GC)))
+		if (f2fs_gc(sbi, test_opt(sbi, FORCE_FG_GC), true))
 			wait_ms = gc_th->no_gc_sleep_time;
 
 		trace_f2fs_background_gc(sbi->sb, wait_ms,
@@ -909,7 +909,7 @@ static int do_garbage_collect(struct f2fs_sb_info *sbi,
 	return sec_freed;
 }
 
-int f2fs_gc(struct f2fs_sb_info *sbi, bool sync)
+int f2fs_gc(struct f2fs_sb_info *sbi, bool sync, bool background)
 {
 	unsigned int segno;
 	int gc_type = sync ? FG_GC : BG_GC;
@@ -950,6 +950,9 @@ int f2fs_gc(struct f2fs_sb_info *sbi, bool sync)
 			if (ret)
 				goto stop;
 		}
+	} else if (gc_type == BG_GC && !background) {
+		/* f2fs_balance_fs doesn't need to do BG_GC in critical path. */
+		goto stop;
 	}
 
 	if (segno == NULL_SEGNO && !__get_victim(sbi, &segno, gc_type))

fs/f2fs/segment.c

@@ -366,7 +366,7 @@ void f2fs_balance_fs(struct f2fs_sb_info *sbi, bool need)
 	 */
 	if (has_not_enough_free_secs(sbi, 0, 0)) {
 		mutex_lock(&sbi->gc_mutex);
-		f2fs_gc(sbi, false);
+		f2fs_gc(sbi, false, false);
 	}
 }