md/raid5: prioritize stripes for writeback

In raid5-cache writeback mode, we have two types of stripes to handle.
- stripes which aren't cached yet
- stripes which are cached and flushing out to raid disks

Upperlayer is more sensistive to latency of the first type of stripes
generally. But we only one handle list for all these stripes, where the
two types of stripes are mixed together. When reclaim flushes a lot of
stripes, the first type of stripes could be noticeably delayed. On the
other hand, if the log space is tight, we'd like to handle the second
type of stripes faster and free log space.

This patch destinguishes the two types stripes. They are added into
different handle list. When we try to get a stripe to handl, we prefer
the first type of stripes unless log space is tight.

This should have no impact for !writeback case.

Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/raid5.h

@@ -542,6 +542,7 @@ struct r5worker {
 
 struct r5worker_group {
 	struct list_head handle_list;
+	struct list_head loprio_list;
 	struct r5conf *conf;
 	struct r5worker *workers;
 	int stripes_cnt;
@@ -608,6 +609,7 @@ struct r5conf {
 						  */
 
 	struct list_head	handle_list; /* stripes needing handling */
+	struct list_head	loprio_list; /* low priority stripes */
 	struct list_head	hold_list; /* preread ready stripes */
 	struct list_head	delayed_list; /* stripes that have plugged requests */
 	struct list_head	bitmap_list; /* stripes delaying awaiting bitmap update */