--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -625,12 +625,13 @@ void bfq_pos_tree_add_move(struct bfq_da
 }
 
 /*
- * Tell whether there are active queues or groups with differentiated weights.
+ * Tell whether there are active queues with different weights or
+ * active groups.
  */
-static bool bfq_differentiated_weights(struct bfq_data *bfqd)
+static bool bfq_varied_queue_weights_or_active_groups(struct bfq_data *bfqd)
 {
 	/*
-	 * For weights to differ, at least one of the trees must contain
+	 * For queue weights to differ, queue_weights_tree must contain
 	 * at least two nodes.
 	 */
 	return (!RB_EMPTY_ROOT(&bfqd->queue_weights_tree) &&
@@ -638,9 +639,7 @@ static bool bfq_differentiated_weights(s
 		 bfqd->queue_weights_tree.rb_node->rb_right)
 #ifdef CONFIG_BFQ_GROUP_IOSCHED
 	       ) ||
-	       (!RB_EMPTY_ROOT(&bfqd->group_weights_tree) &&
-		(bfqd->group_weights_tree.rb_node->rb_left ||
-		 bfqd->group_weights_tree.rb_node->rb_right)
+		(bfqd->num_active_groups > 0
 #endif
 	       );
 }
@@ -658,26 +657,25 @@ static bool bfq_differentiated_weights(s
  * 3) all active groups at the same level in the groups tree have the same
  *    number of children.
  *
- * Unfortunately, keeping the necessary state for evaluating exactly the
- * above symmetry conditions would be quite complex and time-consuming.
- * Therefore this function evaluates, instead, the following stronger
- * sub-conditions, for which it is much easier to maintain the needed
- * state:
+ * Unfortunately, keeping the necessary state for evaluating exactly
+ * the last two symmetry sub-conditions above would be quite complex
+ * and time consuming.  Therefore this function evaluates, instead,
+ * only the following stronger two sub-conditions, for which it is
+ * much easier to maintain the needed state:
  * 1) all active queues have the same weight,
- * 2) all active groups have the same weight,
- * 3) all active groups have at most one active child each.
- * In particular, the last two conditions are always true if hierarchical
- * support and the cgroups interface are not enabled, thus no state needs
- * to be maintained in this case.
+ * 2) there are no active groups.
+ * In particular, the last condition is always true if hierarchical
+ * support or the cgroups interface are not enabled, thus no state
+ * needs to be maintained in this case.
  */
 static bool bfq_symmetric_scenario(struct bfq_data *bfqd)
 {
-	return !bfq_differentiated_weights(bfqd);
+	return !bfq_varied_queue_weights_or_active_groups(bfqd);
 }
 
 /*
  * If the weight-counter tree passed as input contains no counter for
- * the weight of the input entity, then add that counter; otherwise just
+ * the weight of the input queue, then add that counter; otherwise just
  * increment the existing counter.
  *
  * Note that weight-counter trees contain few nodes in mostly symmetric
@@ -688,25 +686,25 @@ static bool bfq_symmetric_scenario(struc
  * In most scenarios, the rate at which nodes are created/destroyed
  * should be low too.
  */
-void bfq_weights_tree_add(struct bfq_data *bfqd, struct bfq_entity *entity,
+void bfq_weights_tree_add(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 			  struct rb_root *root)
 {
+	struct bfq_entity *entity = &bfqq->entity;
 	struct rb_node **new = &(root->rb_node), *parent = NULL;
 
 	/*
-	 * Do not insert if the entity is already associated with a
+	 * Do not insert if the queue is already associated with a
 	 * counter, which happens if:
-	 *   1) the entity is associated with a queue,
-	 *   2) a request arrival has caused the queue to become both
+	 *   1) a request arrival has caused the queue to become both
 	 *      non-weight-raised, and hence change its weight, and
 	 *      backlogged; in this respect, each of the two events
 	 *      causes an invocation of this function,
-	 *   3) this is the invocation of this function caused by the
+	 *   2) this is the invocation of this function caused by the
 	 *      second event. This second invocation is actually useless,
 	 *      and we handle this fact by exiting immediately. More
 	 *      efficient or clearer solutions might possibly be adopted.
 	 */
-	if (entity->weight_counter)
+	if (bfqq->weight_counter)
 		return;
 
 	while (*new) {
@@ -716,7 +714,7 @@ void bfq_weights_tree_add(struct bfq_dat
 		parent = *new;
 
 		if (entity->weight == __counter->weight) {
-			entity->weight_counter = __counter;
+			bfqq->weight_counter = __counter;
 			goto inc_counter;
 		}
 		if (entity->weight < __counter->weight)
@@ -725,66 +723,67 @@ void bfq_weights_tree_add(struct bfq_dat
 			new = &((*new)->rb_right);
 	}
 
-	entity->weight_counter = kzalloc(sizeof(struct bfq_weight_counter),
-					 GFP_ATOMIC);
+	bfqq->weight_counter = kzalloc(sizeof(struct bfq_weight_counter),
+				       GFP_ATOMIC);
 
 	/*
 	 * In the unlucky event of an allocation failure, we just
-	 * exit. This will cause the weight of entity to not be
-	 * considered in bfq_differentiated_weights, which, in its
-	 * turn, causes the scenario to be deemed wrongly symmetric in
-	 * case entity's weight would have been the only weight making
-	 * the scenario asymmetric. On the bright side, no unbalance
-	 * will however occur when entity becomes inactive again (the
-	 * invocation of this function is triggered by an activation
-	 * of entity). In fact, bfq_weights_tree_remove does nothing
-	 * if !entity->weight_counter.
+	 * exit. This will cause the weight of queue to not be
+	 * considered in bfq_varied_queue_weights_or_active_groups,
+	 * which, in its turn, causes the scenario to be deemed
+	 * wrongly symmetric in case bfqq's weight would have been
+	 * the only weight making the scenario asymmetric.  On the
+	 * bright side, no unbalance will however occur when bfqq
+	 * becomes inactive again (the invocation of this function
+	 * is triggered by an activation of queue).  In fact,
+	 * bfq_weights_tree_remove does nothing if
+	 * !bfqq->weight_counter.
 	 */
-	if (unlikely(!entity->weight_counter))
+	if (unlikely(!bfqq->weight_counter))
 		return;
 
-	entity->weight_counter->weight = entity->weight;
-	rb_link_node(&entity->weight_counter->weights_node, parent, new);
-	rb_insert_color(&entity->weight_counter->weights_node, root);
+	bfqq->weight_counter->weight = entity->weight;
+	rb_link_node(&bfqq->weight_counter->weights_node, parent, new);
+	rb_insert_color(&bfqq->weight_counter->weights_node, root);
 
 inc_counter:
-	entity->weight_counter->num_active++;
+	bfqq->weight_counter->num_active++;
 }
 
 /*
- * Decrement the weight counter associated with the entity, and, if the
+ * Decrement the weight counter associated with the queue, and, if the
  * counter reaches 0, remove the counter from the tree.
  * See the comments to the function bfq_weights_tree_add() for considerations
  * about overhead.
  */
 void __bfq_weights_tree_remove(struct bfq_data *bfqd,
-			       struct bfq_entity *entity,
+			       struct bfq_queue *bfqq,
 			       struct rb_root *root)
 {
-	if (!entity->weight_counter)
+	if (!bfqq->weight_counter)
 		return;
 
-	entity->weight_counter->num_active--;
-	if (entity->weight_counter->num_active > 0)
+	bfqq->weight_counter->num_active--;
+	if (bfqq->weight_counter->num_active > 0)
 		goto reset_entity_pointer;
 
-	rb_erase(&entity->weight_counter->weights_node, root);
-	kfree(entity->weight_counter);
+	rb_erase(&bfqq->weight_counter->weights_node, root);
+	kfree(bfqq->weight_counter);
 
 reset_entity_pointer:
-	entity->weight_counter = NULL;
+	bfqq->weight_counter = NULL;
 }
 
 /*
- * Invoke __bfq_weights_tree_remove on bfqq and all its inactive
- * parent entities.
+ * Invoke __bfq_weights_tree_remove on bfqq and decrement the number
+ * of active groups for each queue's inactive parent entity.
  */
 void bfq_weights_tree_remove(struct bfq_data *bfqd,
 			     struct bfq_queue *bfqq)
 {
 	struct bfq_entity *entity = bfqq->entity.parent;
 
-	__bfq_weights_tree_remove(bfqd, &bfqq->entity,
+	__bfq_weights_tree_remove(bfqd, bfqq,
 				  &bfqd->queue_weights_tree);
 
 	for_each_entity(entity) {
@@ -798,17 +797,13 @@ void bfq_weights_tree_remove(struct bfq_
 			 * next_in_service for details on why
 			 * in_service_entity must be checked too).
 			 *
-			 * As a consequence, the weight of entity is
-			 * not to be removed. In addition, if entity
-			 * is active, then its parent entities are
-			 * active as well, and thus their weights are
-			 * not to be removed either. In the end, this
-			 * loop must stop here.
+			 * As a consequence, its parent entities are
+			 * active as well, and thus this loop must
+			 * stop here.
 			 */
 			break;
 		}
-		__bfq_weights_tree_remove(bfqd, entity,
-					  &bfqd->group_weights_tree);
+		bfqd->num_active_groups--;
 	}
 }
 
@@ -3521,9 +3516,11 @@ static bool bfq_better_to_idle(struct bf
 	 * symmetric scenario where:
 	 * (i)  each of these processes must get the same throughput as
 	 *      the others;
-	 * (ii) all these processes have the same I/O pattern
-		(either sequential or random).
-	 * In fact, in such a scenario, the drive will tend to treat
+	 * (ii) the I/O of each process has the same properties, in
+	 *      terms of locality (sequential or random), direction
+	 *      (reads or writes), request sizes, greediness
+	 *      (from I/O-bound to sporadic), and so on.
+	 * In fact, in such a scenario, the drive tends to treat
 	 * the requests of each of these processes in about the same
 	 * way as the requests of the others, and thus to provide
 	 * each of these processes with about the same throughput
@@ -3532,18 +3529,50 @@ static bool bfq_better_to_idle(struct bf
 	 * certainly needed to guarantee that bfqq receives its
 	 * assigned fraction of the device throughput (see [1] for
 	 * details).
+	 * The problem is that idling may significantly reduce
+	 * throughput with certain combinations of types of I/O and
+	 * devices. An important example is sync random I/O, on flash
+	 * storage with command queueing. So, unless bfqq falls in the
+	 * above cases where idling also boosts throughput, it would
+	 * be important to check conditions (i) and (ii) accurately,
+	 * so as to avoid idling when not strictly needed for service
+	 * guarantees.
+	 *
+	 * Unfortunately, it is extremely difficult to thoroughly
+	 * check condition (ii). And, in case there are active groups,
+	 * it becomes very difficult to check condition (i) too. In
+	 * fact, if there are active groups, then, for condition (i)
+	 * to become false, it is enough that an active group contains
+	 * more active processes or sub-groups than some other active
+	 * group. We address this issue with the following bi-modal
+	 * behavior, implemented in the function
+	 * bfq_symmetric_scenario().
 	 *
-	 * We address this issue by controlling, actually, only the
-	 * symmetry sub-condition (i), i.e., provided that
-	 * sub-condition (i) holds, idling is not performed,
-	 * regardless of whether sub-condition (ii) holds. In other
-	 * words, only if sub-condition (i) holds, then idling is
+	 * If there are active groups, then the scenario is tagged as
+	 * asymmetric, conservatively, without checking any of the
+	 * conditions (i) and (ii). So the device is idled for bfqq.
+	 * This behavior matches also the fact that groups are created
+	 * exactly if controlling I/O (to preserve bandwidth and
+	 * latency guarantees) is a primary concern.
+	 *
+	 * On the opposite end, if there are no active groups, then
+	 * only condition (i) is actually controlled, i.e., provided
+	 * that condition (i) holds, idling is not performed,
+	 * regardless of whether condition (ii) holds. In other words,
+	 * only if condition (i) does not hold, then idling is
 	 * allowed, and the device tends to be prevented from queueing
-	 * many requests, possibly of several processes. The reason
-	 * for not controlling also sub-condition (ii) is that we
-	 * exploit preemption to preserve guarantees in case of
-	 * symmetric scenarios, even if (ii) does not hold, as
-	 * explained in the next two paragraphs.
+	 * many requests, possibly of several processes. Since there
+	 * are no active groups, then, to control condition (i) it is
+	 * enough to check whether all active queues have the same
+	 * weight.
+	 *
+	 * Not checking condition (ii) evidently exposes bfqq to the
+	 * risk of getting less throughput than its fair share.
+	 * However, for queues with the same weight, a further
+	 * mechanism, preemption, mitigates or even eliminates this
+	 * problem. And it does so without consequences on overall
+	 * throughput. This mechanism and its benefits are explained
+	 * in the next three paragraphs.
 	 *
 	 * Even if a queue, say Q, is expired when it remains idle, Q
 	 * can still preempt the new in-service queue if the next
@@ -3557,11 +3586,7 @@ static bool bfq_better_to_idle(struct bf
 	 * idling allows the internal queues of the device to contain
 	 * many requests, and thus to reorder requests, we can rather
 	 * safely assume that the internal scheduler still preserves a
-	 * minimum of mid-term fairness. The motivation for using
-	 * preemption instead of idling is that, by not idling,
-	 * service guarantees are preserved without minimally
-	 * sacrificing throughput. In other words, both a high
-	 * throughput and its desired distribution are obtained.
+	 * minimum of mid-term fairness.
 	 *
 	 * More precisely, this preemption-based, idleless approach
 	 * provides fairness in terms of IOPS, and not sectors per
@@ -3580,27 +3605,27 @@ static bool bfq_better_to_idle(struct bf
 	 * 1024/8 times as high as the service received by the other
 	 * queue.
 	 *
-	 * On the other hand, device idling is performed, and thus
-	 * pure sector-domain guarantees are provided, for the
-	 * following queues, which are likely to need stronger
-	 * throughput guarantees: weight-raised queues, and queues
-	 * with a higher weight than other queues. When such queues
-	 * are active, sub-condition (i) is false, which triggers
-	 * device idling.
+	 * The motivation for using preemption instead of idling (for
+	 * queues with the same weight) is that, by not idling,
+	 * service guarantees are preserved (completely or at least in
+	 * part) without minimally sacrificing throughput. And, if
+	 * there is no active group, then the primary expectation for
+	 * this device is probably a high throughput.
 	 *
-	 * According to the above considerations, the next variable is
-	 * true (only) if sub-condition (i) holds. To compute the
-	 * value of this variable, we not only use the return value of
-	 * the function bfq_symmetric_scenario(), but also check
-	 * whether bfqq is being weight-raised, because
-	 * bfq_symmetric_scenario() does not take into account also
-	 * weight-raised queues (see comments on
-	 * bfq_weights_tree_add()). In particular, if bfqq is being
-	 * weight-raised, it is important to idle only if there are
-	 * other, non-weight-raised queues that may steal throughput
-	 * to bfqq. Actually, we should be even more precise, and
-	 * differentiate between interactive weight raising and
-	 * soft real-time weight raising.
+	 * We are now left only with explaining the additional
+	 * compound condition that is checked below for deciding
+	 * whether the scenario is asymmetric. To explain this
+	 * compound condition, we need to add that the function
+	 * bfq_symmetric_scenario checks the weights of only
+	 * non-weight-raised queues, for efficiency reasons (see
+	 * comments on bfq_weights_tree_add()). Then the fact that
+	 * bfqq is weight-raised is checked explicitly here. More
+	 * precisely, the compound condition below takes into account
+	 * also the fact that, even if bfqq is being weight-raised,
+	 * the scenario is still symmetric if all active queues happen
+	 * to be weight-raised. Actually, we should be even more
+	 * precise here, and differentiate between interactive weight
+	 * raising and soft real-time weight raising.
 	 *
 	 * As a side note, it is worth considering that the above
 	 * device-idling countermeasures may however fail in the
@@ -5422,7 +5447,7 @@ static int bfq_init_queue(struct request
 	bfqd->idle_slice_timer.function = bfq_idle_slice_timer;
 
 	bfqd->queue_weights_tree = RB_ROOT;
-	bfqd->group_weights_tree = RB_ROOT;
+	bfqd->num_active_groups = 0;
 
 	INIT_LIST_HEAD(&bfqd->active_list);
 	INIT_LIST_HEAD(&bfqd->idle_list);
--- a/block/bfq-iosched.h
+++ b/block/bfq-iosched.h
@@ -108,15 +108,14 @@ struct bfq_sched_data {
 };
 
 /**
- * struct bfq_weight_counter - counter of the number of all active entities
+ * struct bfq_weight_counter - counter of the number of all active queues
  *                             with a given weight.
  */
 struct bfq_weight_counter {
-	unsigned int weight; /* weight of the entities this counter refers to */
-	unsigned int num_active; /* nr of active entities with this weight */
+	unsigned int weight; /* weight of the queues this counter refers to */
+	unsigned int num_active; /* nr of active queues with this weight */
 	/*
-	 * Weights tree member (see bfq_data's @queue_weights_tree and
-	 * @group_weights_tree)
+	 * Weights tree member (see bfq_data's @queue_weights_tree)
 	 */
 	struct rb_node weights_node;
 };
@@ -151,8 +150,6 @@ struct bfq_weight_counter {
 struct bfq_entity {
 	/* service_tree member */
 	struct rb_node rb_node;
-	/* pointer to the weight counter associated with this entity */
-	struct bfq_weight_counter *weight_counter;
 
 	/*
 	 * Flag, true if the entity is on a tree (either the active or
@@ -266,6 +263,9 @@ struct bfq_queue {
 	/* entity representing this queue in the scheduler */
 	struct bfq_entity entity;
 
+	/* pointer to the weight counter associated with this entity */
+	struct bfq_weight_counter *weight_counter;
+
 	/* maximum budget allowed from the feedback mechanism */
 	int max_budget;
 	/* budget expiration (in jiffies) */
@@ -449,14 +449,9 @@ struct bfq_data {
 	 */
 	struct rb_root queue_weights_tree;
 	/*
-	 * rbtree of non-queue @bfq_entity weight counters, sorted by
-	 * weight. Used to keep track of whether all @bfq_groups have
-	 * the same weight. The tree contains one counter for each
-	 * distinct weight associated to some active @bfq_group (see
-	 * the comments to the functions bfq_weights_tree_[add|remove]
-	 * for further details).
+	 * number of groups with requests still waiting for completion
 	 */
-	struct rb_root group_weights_tree;
+	unsigned int num_active_groups;
 
 	/*
 	 * Number of bfq_queues containing requests (including the
@@ -854,10 +849,10 @@ struct bfq_queue *bic_to_bfqq(struct bfq
 void bic_set_bfqq(struct bfq_io_cq *bic, struct bfq_queue *bfqq, bool is_sync);
 struct bfq_data *bic_to_bfqd(struct bfq_io_cq *bic);
 void bfq_pos_tree_add_move(struct bfq_data *bfqd, struct bfq_queue *bfqq);
-void bfq_weights_tree_add(struct bfq_data *bfqd, struct bfq_entity *entity,
+void bfq_weights_tree_add(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 			  struct rb_root *root);
 void __bfq_weights_tree_remove(struct bfq_data *bfqd,
-			       struct bfq_entity *entity,
+			       struct bfq_queue *bfqq,
 			       struct rb_root *root);
 void bfq_weights_tree_remove(struct bfq_data *bfqd,
 			     struct bfq_queue *bfqq);
--- a/block/bfq-wf2q.c
+++ b/block/bfq-wf2q.c
@@ -788,25 +788,29 @@ __bfq_entity_update_weight_prio(struct b
 		new_weight = entity->orig_weight *
 			     (bfqq ? bfqq->wr_coeff : 1);
 		/*
-		 * If the weight of the entity changes, remove the entity
-		 * from its old weight counter (if there is a counter
-		 * associated with the entity), and add it to the counter
-		 * associated with its new weight.
+		 * If the weight of the entity changes, and the entity is a
+		 * queue, remove the entity from its old weight counter (if
+		 * there is a counter associated with the entity).
 		 */
 		if (prev_weight != new_weight) {
-			root = bfqq ? &bfqd->queue_weights_tree :
-				      &bfqd->group_weights_tree;
-			__bfq_weights_tree_remove(bfqd, entity, root);
+			if (bfqq) {
+				root = &bfqd->queue_weights_tree;
+				__bfq_weights_tree_remove(bfqd, bfqq, root);
+			} else
+				bfqd->num_active_groups--;
 		}
 		entity->weight = new_weight;
 		/*
-		 * Add the entity to its weights tree only if it is
-		 * not associated with a weight-raised queue.
+		 * Add the entity, if it is not a weight-raised queue,
+		 * to the counter associated with its new weight.
 		 */
-		if (prev_weight != new_weight &&
-		    (bfqq ? bfqq->wr_coeff == 1 : 1))
-			/* If we get here, root has been initialized. */
-			bfq_weights_tree_add(bfqd, entity, root);
+		if (prev_weight != new_weight) {
+			if (bfqq && bfqq->wr_coeff == 1) {
+				/* If we get here, root has been initialized. */
+				bfq_weights_tree_add(bfqd, bfqq, root);
+			} else
+				bfqd->num_active_groups++;
+		}
 
 		new_st->wsum += entity->weight;
 
@@ -1012,9 +1016,9 @@ static void __bfq_activate_entity(struct
 	if (!bfq_entity_to_bfqq(entity)) { /* bfq_group */
 		struct bfq_group *bfqg =
 			container_of(entity, struct bfq_group, entity);
+		struct bfq_data *bfqd = bfqg->bfqd;
 
-		bfq_weights_tree_add(bfqg->bfqd, entity,
-				     &bfqd->group_weights_tree);
+		bfqd->num_active_groups++;
 	}
 #endif
 
@@ -1692,7 +1696,7 @@ void bfq_add_bfqq_busy(struct bfq_data *
 
 	if (!bfqq->dispatched)
 		if (bfqq->wr_coeff == 1)
-			bfq_weights_tree_add(bfqd, &bfqq->entity,
+			bfq_weights_tree_add(bfqd, bfqq,
 					     &bfqd->queue_weights_tree);
 
 	if (bfqq->wr_coeff > 1)
