[PATCH 23/25] target_core_user: add backend plug/unplug callouts

[Mike Christie <michael.christie@xxxxxxxxxx>]

This patch adds plug/unplug callouts for tcmu, so we can avoid the
number of times we switch to userspace. Using this driver with tcm
loop is a common config, and dependng on the nr_hw_queues
(nr_hw_queues=1 performs much better) and fio jobs (lower num jobs
around 4) this patch can increase IOPs by only around 5-10% because
we hit other issues like the big per tcmu device mutex.

Signed-off-by: Mike Christie <michael.christie@xxxxxxxxxx>
Reviewed-by: Bodo Stroesser <bostroesser@xxxxxxxxx> 
---
 drivers/target/target_core_user.c | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/drivers/target/target_core_user.c b/drivers/target/target_core_user.c
index a5991df23581..a433bda56b89 100644
--- a/drivers/target/target_core_user.c
+++ b/drivers/target/target_core_user.c
@@ -111,6 +111,7 @@ struct tcmu_dev {
 	struct kref kref;
 
 	struct se_device se_dev;
+	struct se_dev_plug se_plug;
 
 	char *name;
 	struct se_hba *hba;
@@ -119,6 +120,7 @@ struct tcmu_dev {
 #define TCMU_DEV_BIT_BROKEN 1
 #define TCMU_DEV_BIT_BLOCKED 2
 #define TCMU_DEV_BIT_TMR_NOTIFY 3
+#define TCM_DEV_BIT_PLUGGED 4
 	unsigned long flags;
 
 	struct uio_info uio_info;
@@ -959,6 +961,25 @@ static uint32_t ring_insert_padding(struct tcmu_dev *udev, size_t cmd_size)
 	return cmd_head;
 }
 
+static void tcmu_unplug_device(struct se_dev_plug *se_plug)
+{
+	struct se_device *se_dev = se_plug->se_dev;
+	struct tcmu_dev *udev = TCMU_DEV(se_dev);
+
+	clear_bit(TCM_DEV_BIT_PLUGGED, &udev->flags);
+	uio_event_notify(&udev->uio_info);
+}
+
+static struct se_dev_plug *tcmu_plug_device(struct se_device *se_dev)
+{
+	struct tcmu_dev *udev = TCMU_DEV(se_dev);
+
+	if (!test_and_set_bit(TCM_DEV_BIT_PLUGGED, &udev->flags))
+		return &udev->se_plug;
+
+	return NULL;
+}
+
 /**
  * queue_cmd_ring - queue cmd to ring or internally
  * @tcmu_cmd: cmd to queue
@@ -1086,8 +1107,8 @@ static int queue_cmd_ring(struct tcmu_cmd *tcmu_cmd, sense_reason_t *scsi_err)
 
 	list_add_tail(&tcmu_cmd->queue_entry, &udev->inflight_queue);
 
-	/* TODO: only if FLUSH and FUA? */
-	uio_event_notify(&udev->uio_info);
+	if (!test_bit(TCM_DEV_BIT_PLUGGED, &udev->flags))
+		uio_event_notify(&udev->uio_info);
 
 	return 0;
 
@@ -2840,6 +2861,8 @@ static struct target_backend_ops tcmu_ops = {
 	.configure_device	= tcmu_configure_device,
 	.destroy_device		= tcmu_destroy_device,
 	.free_device		= tcmu_free_device,
+	.unplug_device		= tcmu_unplug_device,
+	.plug_device		= tcmu_plug_device,
 	.parse_cdb		= tcmu_parse_cdb,
 	.tmr_notify		= tcmu_tmr_notify,
 	.set_configfs_dev_params = tcmu_set_configfs_dev_params,
-- 
2.25.1