Re: [PATCH 1/4] blk-mq: introduce BLK_MQ_F_SCHED_USE_HW_TAG

When blk-mq I/O scheduler is used, we need two tags for
submitting one request. One is called scheduler tag for
allocating request and scheduling I/O, another one is called
driver tag, which is used for dispatching IO to hardware/driver.
This way introduces one extra per-queue allocation for both tags
and request pool, and may not be as efficient as case of none
scheduler.

Also currently we put a default per-hctx limit on schedulable
requests, and this limit may be a bottleneck for some devices,
especialy when these devices have a quite big tag space.

This patch introduces BLK_MQ_F_SCHED_USE_HW_TAG so that we can
allow to use hardware/driver tags directly for IO scheduling if
devices's hardware tag space is big enough. Then we can avoid
the extra resource allocation and make IO submission more
efficient.

Signed-off-by: Ming Lei <redacted>
---
 block/blk-mq-sched.c   | 10 +++++++++-
 block/blk-mq.c         | 35 +++++++++++++++++++++++++++++------
 include/linux/blk-mq.h |  1 +
 3 files changed, 39 insertions(+), 7 deletions(-)

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 27c67465f856..45a675f07b8b 100644
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 0168b27469cb..e530bc54f0d9 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -263,9 +263,19 @@ struct request *__blk_mq_alloc_request(struct blk_mq_alloc_data *data,
 				rq->rq_flags = RQF_MQ_INFLIGHT;
 				atomic_inc(&data->hctx->nr_active);
 			}
-			rq->tag = tag;
-			rq->internal_tag = -1;
-			data->hctx->tags->rqs[rq->tag] = rq;
+			data->hctx->tags->rqs[tag] = rq;
+
+			/*
+			 * If we use hw tag for scheduling, postpone setting
+			 * rq->tag in blk_mq_get_driver_tag().
+			 */
+			if (data->hctx->flags & BLK_MQ_F_SCHED_USE_HW_TAG) {
+				rq->tag = -1;
+				rq->internal_tag = tag;
+			} else {
+				rq->tag = tag;
+				rq->internal_tag = -1;
+			}

@@ -893,9 +909,15 @@ bool blk_mq_get_driver_tag(struct request *rq, struct blk_mq_hw_ctx **hctx,
 static void __blk_mq_put_driver_tag(struct blk_mq_hw_ctx *hctx,
 				    struct request *rq)
 {
-	blk_mq_put_tag(hctx, hctx->tags, rq->mq_ctx, rq->tag);
+	unsigned tag = rq->tag;