[PATCH v2 09/18] scsi-multipath: add scsi_mpath_get_nr_active()

Add scsi_mpath_get_nr_active() for calculating the number of active
requests associated with an mpath_device. This is required for queue
depth multipath iopolicy.

For NVMe, this count is per controller. The reason is that many NSes may
be connected to a controller, so congestion should be judged at
controller level.

SCSI has no definition of a controller, but SCSI host is a comparable
concept.

Indeed, many SCSI disks may be connected to the same SCSI host, so it
makes sense to count number of active requests at this point. However,
for a transport like iSCSI Initiator over TCP/IP, we have a separate SCSI
host per SCSI device (so there the count would be same at SCSI device
level).

Signed-off-by: John Garry <[email protected]>
---
 drivers/scsi/scsi_multipath.c | 11 +++++++++++
 include/scsi/scsi_host.h      |  4 ++++
 2 files changed, 15 insertions(+)

diff --git a/drivers/scsi/scsi_multipath.c b/drivers/scsi/scsi_multipath.c
index a3bf95e2a18eb..80f32b940339f 100644
--- a/drivers/scsi/scsi_multipath.c
+++ b/drivers/scsi/scsi_multipath.c
@@ -351,12 +351,23 @@ static bool scsi_mpath_available_path(struct mpath_device *mpath_device)
 	return false;
 }
 
+static int scsi_mpath_get_nr_active(struct mpath_device *mpath_device)
+{
+	struct scsi_mpath_device *scsi_mpath_dev =
+				to_scsi_mpath_device(mpath_device);
+	struct scsi_device *sdev = scsi_mpath_dev->sdev;
+	struct Scsi_Host *shost = sdev->host;
+
+	return atomic_read(&shost->mpath_nr_active);
+}
+
 struct mpath_head_template smpdt = {
 	.is_disabled = scsi_mpath_is_disabled,
 	.is_optimized = scsi_mpath_is_optimized,
 	.available_path = scsi_mpath_available_path,
 	.get_iopolicy = scsi_mpath_get_iopolicy,
 	.clone_bio = scsi_mpath_clone_bio,
+	.get_nr_active = scsi_mpath_get_nr_active,
 };
 
 static struct scsi_mpath_head *scsi_mpath_alloc_head(void)
diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
index f6e12565a81de..979d1e89f0f13 100644
--- a/include/scsi/scsi_host.h
+++ b/include/scsi/scsi_host.h
@@ -752,6 +752,10 @@ struct Scsi_Host {
 	/* Delay for runtime autosuspend */
 	int rpm_autosuspend_delay;
 
+	#ifdef CONFIG_SCSI_MULTIPATH
+	atomic_t mpath_nr_active;
+	#endif
+
 	/*
 	 * We should ensure that this is aligned, both for better performance
 	 * and also because some compilers (m68k) don't automatically force
-- 
2.43.5