nvme-multipath: Add visibility for queue-depth io-policy

This patch helps add nvme native multipath visibility for queue-depth
io-policy. It adds a new attribute file named "queue_depth" under
namespace device path node which would print the number of active/
in-flight I/O requests currently queued for the given path.

For instance, if we have a shared namespace accessible from two different
controllers/paths then accessing head block node of the shared namespace
would show the following output:

$ ls -l /sys/block/nvme1n1/multipath/
nvme1c1n1 -> ../../../../../pci052e:78/052e:78:00.0/nvme/nvme1/nvme1c1n1
nvme1c3n1 -> ../../../../../pci058e:78/058e:78:00.0/nvme/nvme3/nvme1c3n1

In the above example, nvme1n1 is head gendisk node created for a shared
namespace and the namespace is accessible from nvme1c1n1 and nvme1c3n1
paths. For queue-depth io-policy we can then refer the "queue_depth"
attribute file created under each namespace path:

$ cat /sys/block/nvme1n1/multipath/nvme1c1n1/queue_depth
518

$cat /sys/block/nvme1n1/multipath/nvme1c3n1/queue_depth
504

>From the above output, we can infer that I/O workload targeted at nvme1n1
uses two paths nvme1c1n1 and nvme1c3n1 and the current queue depth of each
path is 518 and 504 respectively. Reading "queue_depth" file when
configured io-policy is anything but queue-depth would show no output.

Reviewed-by: Sagi Grimberg <[e-mail]>
Reviewed-by: Hannes Reinecke <[e-mail]>
Signed-off-by: Nilay Shroff <[e-mail]>
Signed-off-by: Keith Busch <[e-mail]>

drivers/nvme/host/multipath.c

···
 }
 DEVICE_ATTR_RO(ana_state);
 
+static ssize_t queue_depth_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	struct nvme_ns *ns = nvme_get_ns_from_dev(dev);
+
+	if (ns->head->subsys->iopolicy != NVME_IOPOLICY_QD)
+		return 0;
+
+	return sysfs_emit(buf, "%d\n", atomic_read(&ns->ctrl->nr_active));
+}
+DEVICE_ATTR_RO(queue_depth);
+
 static ssize_t numa_nodes_show(struct device *dev, struct device_attribute *attr,
 		char *buf)
 {

drivers/nvme/host/nvme.h

···
 extern bool multipath;
 extern struct device_attribute dev_attr_ana_grpid;
 extern struct device_attribute dev_attr_ana_state;
+extern struct device_attribute dev_attr_queue_depth;
 extern struct device_attribute dev_attr_numa_nodes;
 extern struct device_attribute subsys_attr_iopolicy;
 

drivers/nvme/host/sysfs.c

···
 #ifdef CONFIG_NVME_MULTIPATH
 	&dev_attr_ana_grpid.attr,
 	&dev_attr_ana_state.attr,
+	&dev_attr_queue_depth.attr,
 	&dev_attr_numa_nodes.attr,
 #endif
 	&dev_attr_io_passthru_err_log_enabled.attr,
···
 		if (!nvme_ctrl_use_ana(nvme_get_ns_from_dev(dev)->ctrl))
 			return 0;
 	}
-	if (a == &dev_attr_numa_nodes.attr) {
+	if (a == &dev_attr_queue_depth.attr || a == &dev_attr_numa_nodes.attr) {
 		if (nvme_disk_is_ns_head(dev_to_disk(dev)))
 			return 0;
 	}