block: change plugging to use a singly linked list

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 yields a bump in default (merging enabled) performance from 7.0
to 7.1M IOPS, and ~7.5M IOPS with merging disabled.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -1550,7 +1550,7 @@ void blk_start_plug_nr_ios(struct blk_plug *plug, unsigned short nr_ios)
 	if (tsk->plug)
 		return;
 
-	INIT_LIST_HEAD(&plug->mq_list);
+	plug->mq_list = NULL;
 	plug->cached_rq = NULL;
 	plug->nr_ios = min_t(unsigned short, nr_ios, BLK_MAX_REQUEST_COUNT);
 	plug->rq_count = 0;
@@ -1640,7 +1640,7 @@ void blk_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 {
 	flush_plug_callbacks(plug, from_schedule);
 
-	if (!list_empty(&plug->mq_list))
+	if (!rq_list_empty(plug->mq_list))
 		blk_mq_flush_plug_list(plug, from_schedule);
 	if (unlikely(!from_schedule && plug->cached_rq))
 		blk_mq_free_plug_rqs(plug);

block/blk-merge.c

@@ -1090,11 +1090,11 @@ bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
 	struct request *rq;
 
 	plug = blk_mq_plug(q, bio);
-	if (!plug || list_empty(&plug->mq_list))
+	if (!plug || rq_list_empty(plug->mq_list))
 		return false;
 
 	/* check the previously added entry for a quick merge attempt */
-	rq = list_last_entry(&plug->mq_list, struct request, queuelist);
+	rq = rq_list_peek(&plug->mq_list);
 	if (rq->q == q) {
 		/*
 		 * Only blk-mq multiple hardware queues case checks the rq in

block/blk-mq.c

@@ -2151,34 +2151,46 @@ void blk_mq_insert_requests(struct blk_mq_hw_ctx *hctx, struct blk_mq_ctx *ctx,
 
 void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 {
+	struct blk_mq_hw_ctx *this_hctx;
+	struct blk_mq_ctx *this_ctx;
+	unsigned int depth;
 	LIST_HEAD(list);
 
-	if (list_empty(&plug->mq_list))
+	if (rq_list_empty(plug->mq_list))
 		return;
-	list_splice_init(&plug->mq_list, &list);
 	plug->rq_count = 0;
 
+	this_hctx = NULL;
+	this_ctx = NULL;
+	depth = 0;
 	do {
-		struct list_head rq_list;
+		struct request *rq;
-		struct request *rq, *head_rq = list_entry_rq(list.next);
+
-		struct list_head *pos = &head_rq->queuelist; /* skip first */
+		rq = rq_list_pop(&plug->mq_list);
-		struct blk_mq_hw_ctx *this_hctx = head_rq->mq_hctx;
+
-		struct blk_mq_ctx *this_ctx = head_rq->mq_ctx;
+		if (!this_hctx) {
-		unsigned int depth = 1;
+			this_hctx = rq->mq_hctx;
+			this_ctx = rq->mq_ctx;
+		} else if (this_hctx != rq->mq_hctx || this_ctx != rq->mq_ctx) {
+			trace_block_unplug(this_hctx->queue, depth,
+						!from_schedule);
+			blk_mq_sched_insert_requests(this_hctx, this_ctx,
+						&list, from_schedule);
+			depth = 0;
+			this_hctx = rq->mq_hctx;
+			this_ctx = rq->mq_ctx;
 
-		list_for_each_continue(pos, &list) {
-			rq = list_entry_rq(pos);
-			BUG_ON(!rq->q);
-			if (rq->mq_hctx != this_hctx || rq->mq_ctx != this_ctx)
-				break;
-			depth++;
 		}
 
-		list_cut_before(&rq_list, &list, pos);
+		list_add(&rq->queuelist, &list);
-		trace_block_unplug(head_rq->q, depth, !from_schedule);
+		depth++;
-		blk_mq_sched_insert_requests(this_hctx, this_ctx, &rq_list,
+	} while (!rq_list_empty(plug->mq_list));
+
+	if (!list_empty(&list)) {
+		trace_block_unplug(this_hctx->queue, depth, !from_schedule);
+		blk_mq_sched_insert_requests(this_hctx, this_ctx, &list,
 						from_schedule);
-	} while(!list_empty(&list));
+	}
 }
 
 static void blk_mq_bio_to_request(struct request *rq, struct bio *bio,
@@ -2358,16 +2370,15 @@ void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx *hctx,
 
 static void blk_add_rq_to_plug(struct blk_plug *plug, struct request *rq)
 {
-	list_add_tail(&rq->queuelist, &plug->mq_list);
+	if (!plug->multiple_queues) {
-	plug->rq_count++;
+		struct request *nxt = rq_list_peek(&plug->mq_list);
-	if (!plug->multiple_queues && !list_is_singular(&plug->mq_list)) {
-		struct request *tmp;
 
-		tmp = list_first_entry(&plug->mq_list, struct request,
+		if (nxt && nxt->q != rq->q)
-						queuelist);
-		if (tmp->q != rq->q)
 			plug->multiple_queues = true;
 	}
+	rq->rq_next = NULL;
+	rq_list_add(&plug->mq_list, rq);
+	plug->rq_count++;
 }
 
 /*
@@ -2479,13 +2490,15 @@ void blk_mq_submit_bio(struct bio *bio)
 		unsigned int request_count = plug->rq_count;
 		struct request *last = NULL;
 
-		if (!request_count)
+		if (!request_count) {
 			trace_block_plug(q);
-		else
+		} else if (!blk_queue_nomerges(q)) {
-			last = list_entry_rq(plug->mq_list.prev);
+			last = rq_list_peek(&plug->mq_list);
+			if (blk_rq_bytes(last) < BLK_PLUG_FLUSH_SIZE)
+				last = NULL;
+		}
 
-		if (request_count >= blk_plug_max_rq_count(plug) || (last &&
+		if (request_count >= blk_plug_max_rq_count(plug) || last) {
-		    blk_rq_bytes(last) >= BLK_PLUG_FLUSH_SIZE)) {
 			blk_flush_plug_list(plug, false);
 			trace_block_plug(q);
 		}
@@ -2505,10 +2518,7 @@ void blk_mq_submit_bio(struct bio *bio)
 		 * the plug list is empty, and same_queue_rq is invalid.
 		 */
 		if (same_queue_rq) {
-			next_rq = list_last_entry(&plug->mq_list,
+			next_rq = rq_list_pop(&plug->mq_list);
-							struct request,
-							queuelist);
-			list_del_init(&next_rq->queuelist);
 			plug->rq_count--;
 		}
 		blk_add_rq_to_plug(plug, rq);

include/linux/blkdev.h

@@ -728,7 +728,7 @@ extern void blk_set_queue_dying(struct request_queue *);
  * schedule() where blk_schedule_flush_plug() is called.
  */
 struct blk_plug {
-	struct list_head mq_list; /* blk-mq requests */
+	struct request *mq_list; /* blk-mq requests */
 
 	/* if ios_left is > 1, we can batch tag/rq allocations */
 	struct request *cached_rq;
@@ -777,8 +777,7 @@ static inline bool blk_needs_flush_plug(struct task_struct *tsk)
 	struct blk_plug *plug = tsk->plug;
 
 	return plug &&
-		 (!list_empty(&plug->mq_list) ||
+		 (plug->mq_list || !list_empty(&plug->cb_list));
-		 !list_empty(&plug->cb_list));
 }
 
 int blkdev_issue_flush(struct block_device *bdev);