On Sat, Oct 16, 2021 at 07:37:41PM -0600, Jens Axboe wrote:

Use a singly linked list for the blk_plug. This saves 8 bytes in the
blk_plug struct, and makes for faster list manipulations than doubly
linked lists. As we don't use the doubly linked lists for anything,
singly linked is just fine.

This patch also does a few other thing, like changing the same_queue_rq
type and adding a has_elevator flag to the plug.  Can you please split
this out and document the changes better?

static void blk_mq_plug_issue_direct(struct blk_plug *plug, bool from_schedule)
 {
+	struct blk_mq_hw_ctx *hctx = NULL;
+	int queued = 0;
+	int errors = 0;
+
+	while (!rq_list_empty(plug->mq_list)) {
+		struct request *rq;
+		blk_status_t ret;
+
+		rq = rq_list_pop(&plug->mq_list);
 
+		if (!hctx)
+			hctx = rq->mq_hctx;
+		else if (hctx != rq->mq_hctx && hctx->queue->mq_ops->commit_rqs) {
+			trace_block_unplug(hctx->queue, queued, !from_schedule);
+			hctx->queue->mq_ops->commit_rqs(hctx);
+			queued = 0;
+			hctx = rq->mq_hctx;
+		}
+
+		ret = blk_mq_request_issue_directly(rq,
+						rq_list_empty(plug->mq_list));
+		if (ret != BLK_STS_OK) {
+			if (ret == BLK_STS_RESOURCE ||
+					ret == BLK_STS_DEV_RESOURCE) {
+				blk_mq_request_bypass_insert(rq, false,
+						rq_list_empty(plug->mq_list));
+				break;
+			}
+			blk_mq_end_request(rq, ret);
+			errors++;
+		} else
+			queued++;
+	}

This all looks a bit messy to me.  I'd suggest reordering this a bit
including a new helper:

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;
	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);
}