[PATCH 5/5] block: convert struct blk_plug callback list to hlists

[Jens Axboe <axboe@xxxxxxxxx>]

We currently use a doubly linked list, which means the head takes up
16 bytes. As any iteration goes over the full list by first splicing it
to an on-stack copy, we never need to remove members from the middle of
the list.

Convert it to an hlist instead, saving 8 bytes in the blk_plug structure.
This also helps save 40 bytes of text in the core block code, tested on
arm64.

Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
 block/blk-core.c       | 26 ++++++++++++++------------
 include/linux/blkdev.h |  4 ++--
 2 files changed, 16 insertions(+), 14 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 902799f71a59..a487881fe2a6 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1080,7 +1080,7 @@ void blk_start_plug_nr_ios(struct blk_plug *plug, unsigned char nr_ios)
 	plug->rq_count = 0;
 	plug->multiple_queues = false;
 	plug->has_elevator = false;
-	INIT_LIST_HEAD(&plug->cb_list);
+	INIT_HLIST_HEAD(&plug->cb_list);
 
 	/*
 	 * Store ordering should not be needed here, since a potential
@@ -1120,16 +1120,18 @@ EXPORT_SYMBOL(blk_start_plug);
 
 static void flush_plug_callbacks(struct blk_plug *plug, bool from_schedule)
 {
-	LIST_HEAD(callbacks);
+	HLIST_HEAD(callbacks);
 
-	while (!list_empty(&plug->cb_list)) {
-		list_splice_init(&plug->cb_list, &callbacks);
+	while (!hlist_empty(&plug->cb_list)) {
+		struct hlist_node *entry, *tmp;
 
-		while (!list_empty(&callbacks)) {
-			struct blk_plug_cb *cb = list_first_entry(&callbacks,
-							  struct blk_plug_cb,
-							  list);
-			list_del(&cb->list);
+		hlist_move_list(&plug->cb_list, &callbacks);
+
+		hlist_for_each_safe(entry, tmp, &callbacks) {
+			struct blk_plug_cb *cb;
+
+			cb = hlist_entry(entry, struct blk_plug_cb, list);
+			hlist_del(&cb->list);
 			cb->callback(cb, from_schedule);
 		}
 	}
@@ -1144,7 +1146,7 @@ struct blk_plug_cb *blk_check_plugged(blk_plug_cb_fn unplug, void *data,
 	if (!plug)
 		return NULL;
 
-	list_for_each_entry(cb, &plug->cb_list, list)
+	hlist_for_each_entry(cb, &plug->cb_list, list)
 		if (cb->callback == unplug && cb->data == data)
 			return cb;
 
@@ -1154,7 +1156,7 @@ struct blk_plug_cb *blk_check_plugged(blk_plug_cb_fn unplug, void *data,
 	if (cb) {
 		cb->data = data;
 		cb->callback = unplug;
-		list_add(&cb->list, &plug->cb_list);
+		hlist_add_head(&cb->list, &plug->cb_list);
 	}
 	return cb;
 }
@@ -1162,7 +1164,7 @@ EXPORT_SYMBOL(blk_check_plugged);
 
 void __blk_flush_plug(struct blk_plug *plug, bool from_schedule)
 {
-	if (!list_empty(&plug->cb_list))
+	if (!hlist_empty(&plug->cb_list))
 		flush_plug_callbacks(plug, from_schedule);
 	blk_mq_flush_plug_list(plug, from_schedule);
 	/*
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 5b17d0e460e4..f339f856e44f 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -950,13 +950,13 @@ struct blk_plug {
 	bool multiple_queues;
 	bool has_elevator;
 
-	struct list_head cb_list; /* md requires an unplug callback */
+	struct hlist_head cb_list; /* md requires an unplug callback */
 };
 
 struct blk_plug_cb;
 typedef void (*blk_plug_cb_fn)(struct blk_plug_cb *, bool);
 struct blk_plug_cb {
-	struct list_head list;
+	struct hlist_node list;
 	blk_plug_cb_fn callback;
 	void *data;
 };
-- 
2.43.0