}
/*
- * 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) &&
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
);
}
* 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
* 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) {
parent = *new;
if (entity->weight == __counter->weight) {
- entity->weight_counter = __counter;
+ bfqq->weight_counter = __counter;
goto inc_counter;
}
if (entity->weight < __counter->weight)
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) {
* 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--;
}
}
* 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
* 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
* 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
* 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
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);
projects / openwrt / staging / blogic.git / commitdiff