linux-kernel - [PATCH BUGFIX 3/4] block, bfq: make reparent_leaf_entity actually work only on leaf entities

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Date:   Sat, 21 Mar 2020 10:45:20 +0100
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        bfq-iosched@...glegroups.com, oleksandr@...alenko.name,
        rasibley@...hat.com, vkabatov@...hat.com, xzhou@...hat.com,
        jstancek@...hat.com, Paolo Valente <paolo.valente@...aro.org>,
        cki-project@...hat.com
Subject: [PATCH BUGFIX 3/4] block, bfq: make reparent_leaf_entity actually work only on leaf entities

bfq_reparent_leaf_entity() reparents the input leaf entity (a leaf
entity represents just a bfq_queue in an entity tree). Yet, the input
entity is guaranteed to always be a leaf entity only in two-level
entity trees. In this respect, because of the error fixed by
commit 14afc5936197 ("block, bfq: fix overwrite of bfq_group pointer
in bfq_find_set_group()"), all (wrongly collapsed) entity trees happened
to actually have only two levels. After the latter commit, this does not
hold any longer.

This commit fixes this problem by modifying
bfq_reparent_leaf_entity(), so that it searches an active leaf entity
down the path that stems from the input entity. Such a leaf entity is
guaranteed to exist when bfq_reparent_leaf_entity() is invoked.

Tested-by: cki-project@...hat.com
Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
---
 block/bfq-cgroup.c | 48 ++++++++++++++++++++++++++++++----------------
 1 file changed, 31 insertions(+), 17 deletions(-)

diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
index 72c6151ace96..efb89db7ba24 100644
--- a/block/bfq-cgroup.c
+++ b/block/bfq-cgroup.c
@@ -815,39 +815,53 @@ static void bfq_flush_idle_tree(struct bfq_service_tree *st)
 /**
  * bfq_reparent_leaf_entity - move leaf entity to the root_group.
  * @bfqd: the device data structure with the root group.
- * @entity: the entity to move.
+ * @entity: the entity to move, if entity is a leaf; or the parent entity
+ *	    of an active leaf entity to move, if entity is not a leaf.
  */
 static void bfq_reparent_leaf_entity(struct bfq_data *bfqd,
-				     struct bfq_entity *entity)
+				     struct bfq_entity *entity,
+				     int ioprio_class)
 {
-	struct bfq_queue *bfqq = bfq_entity_to_bfqq(entity);
+	struct bfq_queue *bfqq;
+	struct bfq_entity *child_entity = entity;
+
+	while (child_entity->my_sched_data) { /* leaf not reached yet */
+		struct bfq_sched_data *child_sd = child_entity->my_sched_data;
+		struct bfq_service_tree *child_st = child_sd->service_tree +
+			ioprio_class;
+		struct rb_root *child_active = &child_st->active;
 
+		child_entity = bfq_entity_of(rb_first(child_active));
+
+		if (!child_entity)
+			child_entity = child_sd->in_service_entity;
+	}
+
+	bfqq = bfq_entity_to_bfqq(child_entity);
 	bfq_bfqq_move(bfqd, bfqq, bfqd->root_group);
 }
 
 /**
- * bfq_reparent_active_entities - move to the root group all active
- *                                entities.
+ * bfq_reparent_active_queues - move to the root group all active queues.
  * @bfqd: the device data structure with the root group.
  * @bfqg: the group to move from.
- * @st: the service tree with the entities.
+ * @st: the service tree to start the search from.
  */
-static void bfq_reparent_active_entities(struct bfq_data *bfqd,
-					 struct bfq_group *bfqg,
-					 struct bfq_service_tree *st)
+static void bfq_reparent_active_queues(struct bfq_data *bfqd,
+				       struct bfq_group *bfqg,
+				       struct bfq_service_tree *st,
+				       int ioprio_class)
 {
 	struct rb_root *active = &st->active;
-	struct bfq_entity *entity = NULL;
-
-	if (!RB_EMPTY_ROOT(&st->active))
-		entity = bfq_entity_of(rb_first(active));
+	struct bfq_entity *entity;
 
-	for (; entity ; entity = bfq_entity_of(rb_first(active)))
-		bfq_reparent_leaf_entity(bfqd, entity);
+	while ((entity = bfq_entity_of(rb_first(active))))
+		bfq_reparent_leaf_entity(bfqd, entity, ioprio_class);
 
 	if (bfqg->sched_data.in_service_entity)
 		bfq_reparent_leaf_entity(bfqd,
-			bfqg->sched_data.in_service_entity);
+					 bfqg->sched_data.in_service_entity,
+					 ioprio_class);
 }
 
 /**
@@ -898,7 +912,7 @@ static void bfq_pd_offline(struct blkg_policy_data *pd)
 		 * There is no need to put the sync queues, as the
 		 * scheduler has taken no reference.
 		 */
-		bfq_reparent_active_entities(bfqd, bfqg, st);
+		bfq_reparent_active_queues(bfqd, bfqg, st, i);
 	}
 
 	__bfq_deactivate_entity(entity, false);
-- 
2.20.1