[PATCH 21/25] target: fix backend plugging

[Mike Christie <michael.christie@oracle.com>]

target_core_iblock is plugging and unplugging on every command and this
is causing perf issues for drivers that prefer batched cmds. With the
last 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. The next 2 patches add support for iblock and
tcmu device plugging.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
---
 drivers/target/target_core_transport.c | 43 +++++++++++++++++++++++++-
 include/target/target_core_backend.h   |  2 ++
 include/target/target_core_base.h      |  4 +++
 3 files changed, 48 insertions(+), 1 deletion(-)

diff --git a/drivers/target/target_core_transport.c b/drivers/target/target_core_transport.c
index d782d3a0f9d5..45bb5253461a 100644
--- a/drivers/target/target_core_transport.c
+++ b/drivers/target/target_core_transport.c
@@ -1796,10 +1796,42 @@ 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);
@@ -1808,8 +1840,17 @@ 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);
 }
 
 /**
diff --git a/include/target/target_core_backend.h b/include/target/target_core_backend.h
index 6336780d83a7..aa5f83e55501 100644
--- a/include/target/target_core_backend.h
+++ b/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);
diff --git a/include/target/target_core_base.h b/include/target/target_core_base.h
index 5e6703ca102d..b8e0a3250bd0 100644
--- a/include/target/target_core_base.h
+++ b/include/target/target_core_base.h
@@ -769,6 +769,10 @@ 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;
-- 
2.25.1