f2fs: fix the periodic wakeups of discard thread

Fix the unnecessary periodic wakeups of discard thread that happens under
below two conditions -

1. When f2fs is heavily utilized over 80%, the current discard policy
sets the max sleep timeout of discard thread as 50ms
(DEF_MIN_DISCARD_ISSUE_TIME). But this is set even when there are
no pending discard commands to be issued.

2. In the issue_discard_thread() path when there are no pending discard
commands, it fails to reset the wait_ms to max timeout value.
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -1114,6 +1114,8 @@ static void __init_discard_policy(struct f2fs_sb_info *sbi,
 				struct discard_policy *dpolicy,
 				int discard_type, unsigned int granularity)
 {
+	struct discard_cmd_control *dcc = SM_I(sbi)->dcc_info;
+
 	/* common policy */
 	dpolicy->type = discard_type;
 	dpolicy->sync = true;
@@ -1133,7 +1135,9 @@ static void __init_discard_policy(struct f2fs_sb_info *sbi,
 		dpolicy->ordered = true;
 		if (utilization(sbi) > DEF_DISCARD_URGENT_UTIL) {
 			dpolicy->granularity = 1;
-			dpolicy->max_interval = DEF_MIN_DISCARD_ISSUE_TIME;
+			if (atomic_read(&dcc->discard_cmd_cnt))
+				dpolicy->max_interval =
+					DEF_MIN_DISCARD_ISSUE_TIME;
 		}
 	} else if (discard_type == DPOLICY_FORCE) {
 		dpolicy->min_interval = DEF_MIN_DISCARD_ISSUE_TIME;
@@ -1749,9 +1753,16 @@ static int issue_discard_thread(void *data)
 	set_freezable();
 
 	do {
+		if (sbi->gc_mode == GC_URGENT_HIGH ||
+			!f2fs_available_free_memory(sbi, DISCARD_CACHE))
+			__init_discard_policy(sbi, &dpolicy, DPOLICY_FORCE, 1);
+		else
 			__init_discard_policy(sbi, &dpolicy, DPOLICY_BG,
 						dcc->discard_granularity);
 
+		if (!atomic_read(&dcc->discard_cmd_cnt))
+		       wait_ms = dpolicy.max_interval;
+
 		wait_event_interruptible_timeout(*q,
 				kthread_should_stop() || freezing(current) ||
 				dcc->discard_wake,
@@ -1777,10 +1788,6 @@ static int issue_discard_thread(void *data)
 		if (!atomic_read(&dcc->discard_cmd_cnt))
 			continue;
 
-		if (sbi->gc_mode == GC_URGENT_HIGH ||
-			!f2fs_available_free_memory(sbi, DISCARD_CACHE))
-			__init_discard_policy(sbi, &dpolicy, DPOLICY_FORCE, 1);
-
 		sb_start_intwrite(sbi->sb);
 
 		issued = __issue_discard_cmd(sbi, &dpolicy);