[RFC PATCH 2/8] bfq: limit the IO depth of idle_class to 1

[brookxu]

From: Chunguang Xu <brookxu@xxxxxxxxxxx>

From: Chunguang Xu <brookxu@xxxxxxxxxxx>

The IO depth of idle_class is limited to 1, so that it can
avoid introducing a larger tail latency under a device with
a larger IO depth. Although limiting the IO depth may reduce
the performance of idle_class, it is generally not a big
problem, because idle_class usually does not have strict
performance requirements.

Signed-off-by: Chunguang Xu <brookxu@xxxxxxxxxxx>
---
 block/bfq-iosched.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index ec482e6641ff..ea9d7f6f4e3d 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -4808,6 +4808,17 @@ static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
 	if (!bfqq)
 		goto exit;
 
+	/*
+	 * Here, the IO depth of idle_class is limited to 1, so that it
+	 * can avoid introducing a larger tail latency under a device
+	 * with a larger IO depth. Although limiting the IO depth may
+	 * reduce the performance of idle_class, it is generally not a
+	 * big problem, because idle_class usually does not have strict
+	 * performance requirements.
+	 */
+	if (bfq_class_idle(bfqq) && bfqq->dispatched)
+		goto exit;
+
 	rq = bfq_dispatch_rq_from_bfqq(bfqd, bfqq);
 
 	if (rq) {
-- 
2.30.0