There is a special case when bfq do not need to idle when more than
one groups is active:

 1) all active queues have the same weight,
 2) all active queues have the same request size.
 3) all active queues belong to the same I/O-priority class,

Each time a request is dispatched, bfq can switch in service queue
safely, since the throughput of each active queue is guaranteed to
be equivalent.

Test procedure:
run "fio -numjobs=1 -ioengine=psync -bs=4k -direct=1 -rw=randread..." in
different 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) |

Signed-off-by: Yu Kuai <redacted>
---
 block/bfq-iosched.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 7df3fc0ef4ef..e5a07bd1fd84 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c

@@ -268,6 +268,16 @@ static struct kmem_cache *bfq_pool;
  */
 #define BFQ_RATE_SHIFT		16
 
+/*
+ * 1) bfq keep dispatching requests with same size for at least one second.
+ * 2) bfq dispatch at lease 1024 requests
+ *
+ * We think bfq are dispatching request with same size if the above two
+ * conditions hold true.
+ */
+#define VARIED_REQUEST_SIZE(bfqd) ((bfqd)->dispatch_count < 1024 ||\
+		time_before(jiffies, (bfqd)->dispatch_time + HZ))
+
 /*
  * When configured for computing the duration of the weight-raising
  * for interactive queues automatically (see the comments at the

@@ -724,7 +734,8 @@ static bool bfq_asymmetric_scenario(struct bfq_data *bfqd,
 	bool multiple_classes_busy;
 
 #ifdef CONFIG_BFQ_GROUP_IOSCHED
-	if (bfqd->num_groups_with_pending_reqs > 1)
+	if (bfqd->num_groups_with_pending_reqs > 1 &&
+	    VARIED_REQUEST_SIZE(bfqd))
 		return true;
 
 	if (bfqd->num_groups_with_pending_reqs &&

-- 
2.31.1