[PATCH 2/2] block: attempt direct issue of plug list

[Jens Axboe <axboe@xxxxxxxxx>]

If we have just one queue type in the plug list, then we can extend our
direct issue to cover a full plug list as well.

Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
 block/blk-core.c       |  1 +
 block/blk-mq.c         | 60 ++++++++++++++++++++++++++++++++++++++++++
 include/linux/blkdev.h |  1 +
 3 files changed, 62 insertions(+)

diff --git a/block/blk-core.c b/block/blk-core.c
index 14d20909f61a..e6ad5b51d0c3 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1555,6 +1555,7 @@ void blk_start_plug_nr_ios(struct blk_plug *plug, unsigned short nr_ios)
 	plug->nr_ios = min_t(unsigned short, nr_ios, BLK_MAX_REQUEST_COUNT);
 	plug->rq_count = 0;
 	plug->multiple_queues = false;
+	plug->has_elevator = false;
 	plug->nowait = false;
 	INIT_LIST_HEAD(&plug->cb_list);
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 620233b85af2..d0fe86b46d1b 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2147,6 +2147,58 @@ void blk_mq_insert_requests(struct blk_mq_hw_ctx *hctx, struct blk_mq_ctx *ctx,
 	spin_unlock(&ctx->lock);
 }
 
+static void blk_mq_commit_rqs(struct blk_mq_hw_ctx *hctx, int *queued,
+			      bool from_schedule)
+{
+	if (hctx->queue->mq_ops->commit_rqs) {
+		trace_block_unplug(hctx->queue, *queued, !from_schedule);
+		hctx->queue->mq_ops->commit_rqs(hctx);
+	}
+	*queued = 0;
+}
+
+static void blk_mq_plug_issue_direct(struct blk_plug *plug, bool from_schedule)
+{
+	struct blk_mq_hw_ctx *hctx = NULL;
+	struct request *rq;
+	int queued = 0;
+	int errors = 0;
+
+	while ((rq = rq_list_pop(&plug->mq_list))) {
+		bool last = rq_list_empty(plug->mq_list);
+		blk_status_t ret;
+
+		if (hctx != rq->mq_hctx) {
+			if (hctx)
+				blk_mq_commit_rqs(hctx, &queued, from_schedule);
+			hctx = rq->mq_hctx;
+		}
+
+		ret = blk_mq_request_issue_directly(rq, last);
+		switch (ret) {
+		case BLK_STS_OK:
+			queued++;
+			break;
+		case BLK_STS_RESOURCE:
+		case BLK_STS_DEV_RESOURCE:
+			blk_mq_request_bypass_insert(rq, false, last);
+			blk_mq_commit_rqs(hctx, &queued, from_schedule);
+			return;
+		default:
+			blk_mq_end_request(rq, ret);
+			errors++;
+			break;
+		}
+	}
+
+	/*
+	 * If we didn't flush the entire list, we could have told the driver
+	 * there was more coming, but that turned out to be a lie.
+	 */
+	if (errors)
+		blk_mq_commit_rqs(hctx, &queued, from_schedule);
+}
+
 void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 {
 	struct blk_mq_hw_ctx *this_hctx;
@@ -2158,6 +2210,12 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 		return;
 	plug->rq_count = 0;
 
+	if (!plug->multiple_queues && !plug->has_elevator) {
+		blk_mq_plug_issue_direct(plug, from_schedule);
+		if (rq_list_empty(plug->mq_list))
+			return;
+	}
+
 	this_hctx = NULL;
 	this_ctx = NULL;
 	depth = 0;
@@ -2374,6 +2432,8 @@ static void blk_add_rq_to_plug(struct blk_plug *plug, struct request *rq)
 		if (nxt && nxt->q != rq->q)
 			plug->multiple_queues = true;
 	}
+	if (!plug->has_elevator && (rq->rq_flags & RQF_ELV))
+		plug->has_elevator = true;
 	rq->rq_next = NULL;
 	rq_list_add(&plug->mq_list, rq);
 	plug->rq_count++;
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 80668e316eea..2e93682f8f68 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -720,6 +720,7 @@ struct blk_plug {
 	unsigned short rq_count;
 
 	bool multiple_queues;
+	bool has_elevator;
 	bool nowait;
 
 	struct list_head cb_list; /* md requires an unplug callback */
-- 
2.33.1