dm mpath selector: more evenly distribute ties

Move the last used path to the end of the list (least preferred) so that
ties are more evenly distributed.

For example, in case with three paths with one that is slower than
others, the remaining two would be unevenly used if they tie. This is
due to the rotation not being a truely fair distribution.

Illustrated: paths a, b, c, 'c' has 1 outstanding IO, a and b are 'tied'
Three possible rotations:
(a, b, c) -> best path 'a'
(b, c, a) -> best path 'b'
(c, a, b) -> best path 'a'
(a, b, c) -> best path 'a'
(b, c, a) -> best path 'b'
(c, a, b) -> best path 'a'
...

So 'a' is used 2x more than 'b', although they should be used evenly.

With this change, the most recently used path is always the least
preferred, removing this bias resulting in even distribution.
(a, b, c) -> best path 'a'
(b, c, a) -> best path 'b'
(c, a, b) -> best path 'a'
(c, b, a) -> best path 'b'
...
Signed-off-by: Khazhismel Kumykov <khazhy@google.com>
Reviewed-by: Martin Wilck <mwilck@suse.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-queue-length.c

@@ -195,9 +195,6 @@ static struct dm_path *ql_select_path(struct path_selector *ps, size_t nr_bytes)
 	if (list_empty(&s->valid_paths))
 		goto out;
 
-	/* Change preferred (first in list) path to evenly balance. */
-	list_move_tail(s->valid_paths.next, &s->valid_paths);
-
 	list_for_each_entry(pi, &s->valid_paths, list) {
 		if (!best ||
 		    (atomic_read(&pi->qlen) < atomic_read(&best->qlen)))
@@ -210,6 +207,9 @@ static struct dm_path *ql_select_path(struct path_selector *ps, size_t nr_bytes)
 	if (!best)
 		goto out;
 
+	/* Move most recently used to least preferred to evenly balance. */
+	list_move_tail(&best->list, &s->valid_paths);
+
 	ret = best->path;
 out:
 	spin_unlock_irqrestore(&s->lock, flags);

drivers/md/dm-service-time.c

@@ -282,9 +282,6 @@ static struct dm_path *st_select_path(struct path_selector *ps, size_t nr_bytes)
 	if (list_empty(&s->valid_paths))
 		goto out;
 
-	/* Change preferred (first in list) path to evenly balance. */
-	list_move_tail(s->valid_paths.next, &s->valid_paths);
-
 	list_for_each_entry(pi, &s->valid_paths, list)
 		if (!best || (st_compare_load(pi, best, nr_bytes) < 0))
 			best = pi;
@@ -292,6 +289,9 @@ static struct dm_path *st_select_path(struct path_selector *ps, size_t nr_bytes)
 	if (!best)
 		goto out;
 
+	/* Move most recently used to least preferred to evenly balance. */
+	list_move_tail(&best->list, &s->valid_paths);
+
 	ret = best->path;
 out:
 	spin_unlock_irqrestore(&s->lock, flags);