f2fs: let discard thread wait a little longer if dev is busy

This patch modify discard thread wait policy as below:
	issued       io_interrupted     wait time(ms)
1.        8                 0               50
2.      (0,8)               1               50
3.        0                 1              500 (dev is busy)
4.        0                 0            60000 (no candidates)
Signed-off-by: Yunlei He <heyunlei@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -180,6 +180,7 @@ enum {
 #define DEF_MAX_DISCARD_REQUEST		8	/* issue 8 discards per round */
 #define DEF_MAX_DISCARD_LEN		512	/* Max. 2MB per discard */
 #define DEF_MIN_DISCARD_ISSUE_TIME	50	/* 50 ms, if exists */
+#define DEF_MID_DISCARD_ISSUE_TIME	500	/* 500 ms, if device busy */
 #define DEF_MAX_DISCARD_ISSUE_TIME	60000	/* 60 s, if no candidates */
 #define DEF_DISCARD_URGENT_UTIL		80	/* do more discard over 80% */
 #define DEF_CP_INTERVAL			60	/* 60 secs */
@@ -282,6 +283,7 @@ enum {
 struct discard_policy {
 	int type;			/* type of discard */
 	unsigned int min_interval;	/* used for candidates exist */
+	unsigned int mid_interval;	/* used for device busy */
 	unsigned int max_interval;	/* used for candidates not exist */
 	unsigned int max_requests;	/* # of discards issued per round */
 	unsigned int io_aware_gran;	/* minimum granularity discard not be aware of I/O */

fs/f2fs/segment.c

@@ -937,6 +937,7 @@ static void __init_discard_policy(struct f2fs_sb_info *sbi,
 
 	if (discard_type == DPOLICY_BG) {
 		dpolicy->min_interval = DEF_MIN_DISCARD_ISSUE_TIME;
+		dpolicy->mid_interval = DEF_MID_DISCARD_ISSUE_TIME;
 		dpolicy->max_interval = DEF_MAX_DISCARD_ISSUE_TIME;
 		dpolicy->io_aware = true;
 		dpolicy->sync = false;
@@ -946,6 +947,7 @@ static void __init_discard_policy(struct f2fs_sb_info *sbi,
 		}
 	} else if (discard_type == DPOLICY_FORCE) {
 		dpolicy->min_interval = DEF_MIN_DISCARD_ISSUE_TIME;
+		dpolicy->mid_interval = DEF_MID_DISCARD_ISSUE_TIME;
 		dpolicy->max_interval = DEF_MAX_DISCARD_ISSUE_TIME;
 		dpolicy->io_aware = false;
 	} else if (discard_type == DPOLICY_FSTRIM) {
@@ -1420,9 +1422,11 @@ static int issue_discard_thread(void *data)
 		sb_start_intwrite(sbi->sb);
 
 		issued = __issue_discard_cmd(sbi, &dpolicy);
-		if (issued) {
+		if (issued > 0) {
 			__wait_all_discard_cmd(sbi, &dpolicy);
 			wait_ms = dpolicy.min_interval;
+		} else if (issued == -1){
+			wait_ms = dpolicy.mid_interval;
 		} else {
 			wait_ms = dpolicy.max_interval;
 		}