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

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Date:   Wed, 13 Oct 2021 19:25:34 +0800
From:   Yu Kuai <yukuai3@...wei.com>
To:     <paolo.valente@...aro.org>, <axboe@...nel.dk>
CC:     <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <yukuai3@...wei.com>, <yi.zhang@...wei.com>
Subject: [PATCH v3 2/2] block, bfq: do not idle if only one cgroup is activated

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@...wei.com>
---
 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