scsi: target: core: Fix backend plugging

target_core_iblock is plugging and unplugging on every command and this is
causing perf issues for drivers that prefer batched cmds. With recent
patches we can now take multiple cmds from a fabric driver queue and then
pass them down the backend drivers in a batch. This patch adds this support
by adding 2 callouts to the backend for plugging and unplugging the
device. Subsequent commits will add support for iblock and tcmu device
plugging.

Link: https://lore.kernel.org/r/20210227170006.5077-22-michael.christie@oracle.com
Reviewed-by: Bodo Stroesser <bostroesser@gmail.com>
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/target/target_core_transport.c

@@ -1807,10 +1807,42 @@ void target_submit_cmd(struct se_cmd *se_cmd, struct se_session *se_sess,
 }
 EXPORT_SYMBOL(target_submit_cmd);
 
+
+static struct se_dev_plug *target_plug_device(struct se_device *se_dev)
+{
+	struct se_dev_plug *se_plug;
+
+	if (!se_dev->transport->plug_device)
+		return NULL;
+
+	se_plug = se_dev->transport->plug_device(se_dev);
+	if (!se_plug)
+		return NULL;
+
+	se_plug->se_dev = se_dev;
+	/*
+	 * We have a ref to the lun at this point, but the cmds could
+	 * complete before we unplug, so grab a ref to the se_device so we
+	 * can call back into the backend.
+	 */
+	config_group_get(&se_dev->dev_group);
+	return se_plug;
+}
+
+static void target_unplug_device(struct se_dev_plug *se_plug)
+{
+	struct se_device *se_dev = se_plug->se_dev;
+
+	se_dev->transport->unplug_device(se_plug);
+	config_group_put(&se_dev->dev_group);
+}
+
 void target_queued_submit_work(struct work_struct *work)
 {
 	struct se_cmd_queue *sq = container_of(work, struct se_cmd_queue, work);
 	struct se_cmd *se_cmd, *next_cmd;
+	struct se_dev_plug *se_plug = NULL;
+	struct se_device *se_dev = NULL;
 	struct llist_node *cmd_list;
 
 	cmd_list = llist_del_all(&sq->cmd_list);
@@ -1819,8 +1851,17 @@ void target_queued_submit_work(struct work_struct *work)
 		return;
 
 	cmd_list = llist_reverse_order(cmd_list);
-	llist_for_each_entry_safe(se_cmd, next_cmd, cmd_list, se_cmd_list)
+	llist_for_each_entry_safe(se_cmd, next_cmd, cmd_list, se_cmd_list) {
+		if (!se_dev) {
+			se_dev = se_cmd->se_dev;
+			se_plug = target_plug_device(se_dev);
+		}
+
 		target_submit(se_cmd);
+	}
+
+	if (se_plug)
+		target_unplug_device(se_plug);
 }
 
 /**

include/target/target_core_backend.h

@@ -34,6 +34,8 @@ struct target_backend_ops {
 	int (*configure_device)(struct se_device *);
 	void (*destroy_device)(struct se_device *);
 	void (*free_device)(struct se_device *device);
+	struct se_dev_plug *(*plug_device)(struct se_device *se_dev);
+	void (*unplug_device)(struct se_dev_plug *se_plug);
 
 	ssize_t (*set_configfs_dev_params)(struct se_device *,
 					   const char *, ssize_t);

include/target/target_core_base.h

@@ -770,6 +770,10 @@ struct se_cmd_queue {
 	struct work_struct	work;
 };
 
+struct se_dev_plug {
+	struct se_device	*se_dev;
+};
+
 struct se_device_queue {
 	struct list_head	state_list;
 	spinlock_t		lock;