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-rw-r--r--block/bfq-iosched.h51
1 files changed, 49 insertions, 2 deletions
diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
index 77651d817ecd..0b02bf302de0 100644
--- a/block/bfq-iosched.h
+++ b/block/bfq-iosched.h
@@ -196,6 +196,9 @@ struct bfq_entity {
/* flag, set to request a weight, ioprio or ioprio_class change */
int prio_changed;
+
+ /* flag, set if the entity is counted in groups_with_pending_reqs */
+ bool in_groups_with_pending_reqs;
};
struct bfq_group;
@@ -448,10 +451,54 @@ struct bfq_data {
* bfq_weights_tree_[add|remove] for further details).
*/
struct rb_root queue_weights_tree;
+
/*
- * number of groups with requests still waiting for completion
+ * Number of groups with at least one descendant process that
+ * has at least one request waiting for completion. Note that
+ * this accounts for also requests already dispatched, but not
+ * yet completed. Therefore this number of groups may differ
+ * (be larger) than the number of active groups, as a group is
+ * considered active only if its corresponding entity has
+ * descendant queues with at least one request queued. This
+ * number is used to decide whether a scenario is symmetric.
+ * For a detailed explanation see comments on the computation
+ * of the variable asymmetric_scenario in the function
+ * bfq_better_to_idle().
+ *
+ * However, it is hard to compute this number exactly, for
+ * groups with multiple descendant processes. Consider a group
+ * that is inactive, i.e., that has no descendant process with
+ * pending I/O inside BFQ queues. Then suppose that
+ * num_groups_with_pending_reqs is still accounting for this
+ * group, because the group has descendant processes with some
+ * I/O request still in flight. num_groups_with_pending_reqs
+ * should be decremented when the in-flight request of the
+ * last descendant process is finally completed (assuming that
+ * nothing else has changed for the group in the meantime, in
+ * terms of composition of the group and active/inactive state of child
+ * groups and processes). To accomplish this, an additional
+ * pending-request counter must be added to entities, and must
+ * be updated correctly. To avoid this additional field and operations,
+ * we resort to the following tradeoff between simplicity and
+ * accuracy: for an inactive group that is still counted in
+ * num_groups_with_pending_reqs, we decrement
+ * num_groups_with_pending_reqs when the first descendant
+ * process of the group remains with no request waiting for
+ * completion.
+ *
+ * Even this simpler decrement strategy requires a little
+ * carefulness: to avoid multiple decrements, we flag a group,
+ * more precisely an entity representing a group, as still
+ * counted in num_groups_with_pending_reqs when it becomes
+ * inactive. Then, when the first descendant queue of the
+ * entity remains with no request waiting for completion,
+ * num_groups_with_pending_reqs is decremented, and this flag
+ * is reset. After this flag is reset for the entity,
+ * num_groups_with_pending_reqs won't be decremented any
+ * longer in case a new descendant queue of the entity remains
+ * with no request waiting for completion.
*/
- unsigned int num_active_groups;
+ unsigned int num_groups_with_pending_reqs;
/*
* Number of bfq_queues containing requests (including the