f2fs: wake up discard_thread iff there is a candidate

This patch fixes to avoid needless wake ups.
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -1494,8 +1494,7 @@ void clear_prefree_segments(struct f2fs_sb_info *sbi, struct cp_control *cpc)
 		kmem_cache_free(discard_entry_slab, entry);
 	}
 
-	dcc->discard_wake = 1;
-	wake_up_interruptible_all(&dcc->discard_wait_queue);
+	wake_up_discard_thread(sbi, false);
 }
 
 static int create_discard_cmd_control(struct f2fs_sb_info *sbi)

fs/f2fs/segment.h

@@ -798,3 +798,28 @@ static inline long nr_pages_to_write(struct f2fs_sb_info *sbi, int type,
 	wbc->nr_to_write = desired;
 	return desired - nr_to_write;
 }
+
+static inline void wake_up_discard_thread(struct f2fs_sb_info *sbi, bool force)
+{
+	struct discard_cmd_control *dcc = SM_I(sbi)->dcc_info;
+	bool wakeup = false;
+	int i;
+
+	if (force)
+		goto wake_up;
+
+	mutex_lock(&dcc->cmd_lock);
+	for (i = MAX_PLIST_NUM - 1;
+			i >= 0 && plist_issue(dcc->pend_list_tag[i]); i--) {
+		if (!list_empty(&dcc->pend_list[i])) {
+			wakeup = true;
+			break;
+		}
+	}
+	mutex_unlock(&dcc->cmd_lock);
+	if (!wakeup)
+		return;
+wake_up:
+	dcc->discard_wake = 1;
+	wake_up_interruptible_all(&dcc->discard_wait_queue);
+}

fs/f2fs/sysfs.c

@@ -178,13 +178,9 @@ static ssize_t f2fs_sbi_store(struct f2fs_attr *a,
 	if (!strcmp(a->attr.name, "iostat_enable") && *ui == 0)
 		f2fs_reset_iostat(sbi);
 	if (!strcmp(a->attr.name, "gc_urgent") && t == 1 && sbi->gc_thread) {
-		struct discard_cmd_control *dcc = SM_I(sbi)->dcc_info;
-
 		sbi->gc_thread->gc_wake = 1;
 		wake_up_interruptible_all(&sbi->gc_thread->gc_wait_queue_head);
-
-		dcc->discard_wake = 1;
-		wake_up_interruptible_all(&dcc->discard_wait_queue);
+		wake_up_discard_thread(sbi, true);
 	}
 
 	return count;