dm cache policy mq: fix promotions to occur as expected

Micro benchmarks that repeatedly issued IO to a single block were
failing to cause a promotion from the origin device to the cache.  Fix
this by not updating the stats during map() if -EWOULDBLOCK will be
returned.

The mq policy will only update stats, consider migration, etc, once per
tick period (a unit of time established between dm-cache core and the
policies).

When the IO thread calls the policy's map method, if it would like to
migrate the associated block it returns -EWOULDBLOCK, the IO then gets
handed over to a worker thread which handles the migration.  The worker
thread calls map again, to check the migration is still needed (avoids a
race among other things).  *BUT*, before this fix, if we were still in
the same tick period the stats were already updated by the previous map
call -- so the migration would no longer be requested.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-cache-policy-mq.c

@@ -730,15 +730,18 @@ static int pre_cache_entry_found(struct mq_policy *mq, struct entry *e,
 	int r = 0;
 	bool updated = updated_this_tick(mq, e);
 
-	requeue_and_update_tick(mq, e);
-
 	if ((!discarded_oblock && updated) ||
-	    !should_promote(mq, e, discarded_oblock, data_dir))
+	    !should_promote(mq, e, discarded_oblock, data_dir)) {
+		requeue_and_update_tick(mq, e);
 		result->op = POLICY_MISS;
-	else if (!can_migrate)
+
+	} else if (!can_migrate)
 		r = -EWOULDBLOCK;
-	else
+
+	else {
+		requeue_and_update_tick(mq, e);
 		r = pre_cache_to_cache(mq, e, result);
+	}
 
 	return r;
 }