[PATCH 3/8] bfq: limit the IO depth of CLASS_IDLE to 1

[brookxu]

From: Chunguang Xu <brookxu@xxxxxxxxxxx>

The IO depth of queues belong to CLASS_IDLE 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 ec482e6..29940ec 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 queues belong to CLASS_IDLE 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) {
-- 
1.8.3.1