[PATCH v3 2/2] block, bfq: do not idle if only one cgroup is activated

[Yu Kuai]

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

Test procedure:
run "fio -numjobs=1 -ioengine=psync -bs=4k -direct=1 -rw=randread..."
multiple times in the same cgroup.

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. This is because bfq_queue can
be merged in this situation.

Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
 block/bfq-iosched.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index d251735383f7..8d94f511bee8 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -709,7 +709,7 @@ 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.
  * 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.
@@ -741,7 +741,7 @@ static bool bfq_asymmetric_scenario(struct bfq_data *bfqd,
 
 	return varied_queue_weights || multiple_classes_busy
 #ifdef CONFIG_BFQ_GROUP_IOSCHED
-	       || bfqd->num_groups_with_pending_reqs > 0
+	       || bfqd->num_groups_with_pending_reqs > 1
 #endif
 		;
 }
-- 
2.31.1