block, bfq: remove direct switch to an entity in higher class

If the function bfq_update_next_in_service is invoked as a consequence
of the activation or requeueing of an entity, say E, and finds out
that E belongs to a higher-priority class than that of the current
next-in-service entity, then it sets next_in_service directly to
E. But this may lead to anomalous schedules, because E may happen not
be eligible for service, because its virtual start time is higher than
the system virtual time for its service tree.

This commit addresses this issue by simply removing this direct
switch.
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Tested-by: Lee Tibbert <lee.tibbert@gmail.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-wf2q.c

@@ -86,9 +86,8 @@ static bool bfq_update_next_in_service(struct bfq_sched_data *sd,
 	 * or repositiong of an entity that does not coincide with
 	 * sd->next_in_service, then a full lookup in the active tree
 	 * can be avoided. In fact, it is enough to check whether the
-	 * just-modified entity has a higher priority than
-	 * sd->next_in_service, or, even if it has the same priority
-	 * as sd->next_in_service, is eligible and has a lower virtual
+	 * just-modified entity has the same priority as
+	 * sd->next_in_service, is eligible and has a lower virtual
 	 * finish time than sd->next_in_service. If this compound
 	 * condition holds, then the new entity becomes the new
 	 * next_in_service. Otherwise no change is needed.
@@ -104,9 +103,8 @@ static bool bfq_update_next_in_service(struct bfq_sched_data *sd,
 
 		/*
 		 * If there is already a next_in_service candidate
-		 * entity, then compare class priorities or timestamps
-		 * to decide whether to replace sd->service_tree with
-		 * new_entity.
+		 * entity, then compare timestamps to decide whether
+		 * to replace sd->service_tree with new_entity.
 		 */
 		if (next_in_service) {
 			unsigned int new_entity_class_idx =
@@ -114,10 +112,6 @@ static bool bfq_update_next_in_service(struct bfq_sched_data *sd,
 			struct bfq_service_tree *st =
 				sd->service_tree + new_entity_class_idx;
 
-			/*
-			 * For efficiency, evaluate the most likely
-			 * sub-condition first.
-			 */
 			replace_next =
 				(new_entity_class_idx ==
 				 bfq_class_idx(next_in_service)
@@ -125,10 +119,7 @@ static bool bfq_update_next_in_service(struct bfq_sched_data *sd,
 				 !bfq_gt(new_entity->start, st->vtime)
 				 &&
 				 bfq_gt(next_in_service->finish,
-					new_entity->finish))
-				||
-				new_entity_class_idx <
-				bfq_class_idx(next_in_service);
+					new_entity->finish));
 		}
 
 		if (replace_next)