[PATCH v2 2/4] block, bfq: do not idle if only one cgroup is activated

[Yu Kuai <yukuai3@huawei.com>]

If only one group is activated, there is no need to guarantee the same
share of the throughput of queues in the same group.

If CONFIG_BFQ_GROUP_IOSCHED is enabled, there is no need to check
'varied_queue_weights' and 'multiple_classes_busy':
1) num_groups_with_pending_reqs = 0, idle is not needed
2) num_groups_with_pending_reqs = 1
   - if root group have any pending requests, idle is needed
   - if root group is idle, idle is not needed
3) num_groups_with_pending_reqs > 1, idle is needed

Test procedure:
run "fio -numjobs=1 -ioengine=psync -bs=4k -direct=1 -rw=randread..."
multiple times in the same cgroup(not root).

Test result: total bandwidth(Mib/s)
| total jobs | before this patch | after this patch      |
| ---------- | ----------------- | --------------------- |
| 1          | 33.8              | 33.8                  |
| 2          | 33.8              | 65.4 (32.7 each job)  |
| 4          | 33.8              | 106.8 (26.7 each job) |
| 8          | 33.8              | 126.4 (15.8 each job) |

By the way, if I test with "fio -numjobs=1/2/4/8 ...", test result is
the same with or without this patch.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
---
 block/bfq-iosched.c | 35 ++++++++++++++++++++++++++---------
 1 file changed, 26 insertions(+), 9 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 7c6b412f9a9c..a780205a1be4 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -709,7 +709,9 @@ bfq_pos_tree_add_move(struct bfq_data *bfqd, struct bfq_queue *bfqq)
  * much easier to maintain the needed state:
  * 1) all active queues have the same weight,
  * 2) all active queues belong to the same I/O-priority class,
- * 3) there are no active groups.
+ * 3) there are one active group at most(incluing root_group).
+ * If the last condition is false, there is no need to guarantee the,
+ * same share of the throughput of queues in the same group.
  * 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.
@@ -717,7 +719,26 @@ bfq_pos_tree_add_move(struct bfq_data *bfqd, struct bfq_queue *bfqq)
 static bool bfq_asymmetric_scenario(struct bfq_data *bfqd,
 				   struct bfq_queue *bfqq)
 {
-	bool smallest_weight = bfqq &&
+	bool smallest_weight;
+	bool varied_queue_weights;
+	bool multiple_classes_busy;
+
+#ifdef CONFIG_BFQ_GROUP_IOSCHED
+	if (bfqd->num_groups_with_pending_reqs > 1)
+		return true;
+
+	if (bfqd->num_groups_with_pending_reqs &&
+	    bfqd->num_queues_with_pending_reqs_in_root)
+		return true;
+
+	/*
+	 * Reach here means only one group(incluing root group) has pending
+	 * requests, thus it's safe to return.
+	 */
+	return false;
+#endif
+
+	smallest_weight = bfqq &&
 		bfqq->weight_counter &&
 		bfqq->weight_counter ==
 		container_of(
@@ -729,21 +750,17 @@ static bool bfq_asymmetric_scenario(struct bfq_data *bfqd,
 	 * For queue weights to differ, queue_weights_tree must contain
 	 * at least two nodes.
 	 */
-	bool varied_queue_weights = !smallest_weight &&
+	varied_queue_weights = !smallest_weight &&
 		!RB_EMPTY_ROOT(&bfqd->queue_weights_tree.rb_root) &&
 		(bfqd->queue_weights_tree.rb_root.rb_node->rb_left ||
 		 bfqd->queue_weights_tree.rb_root.rb_node->rb_right);
 
-	bool multiple_classes_busy =
+	multiple_classes_busy =
 		(bfqd->busy_queues[0] && bfqd->busy_queues[1]) ||
 		(bfqd->busy_queues[0] && bfqd->busy_queues[2]) ||
 		(bfqd->busy_queues[1] && bfqd->busy_queues[2]);
 
-	return varied_queue_weights || multiple_classes_busy
-#ifdef CONFIG_BFQ_GROUP_IOSCHED
-	       || bfqd->num_groups_with_pending_reqs > 0
-#endif
-		;
+	return varied_queue_weights || multiple_classes_busy;
 }
 
 /*
-- 
2.31.1