[PATCHv3 2/7] nvme: export multipath failover count via sysfs

[Nilay Shroff <nilay@linux.ibm.com>]

When an NVMe command completes with a path-specific error, the NVMe
driver may retry the command on an alternate controller or path if one
is available. These failover events indicate that I/O was redirected
away from the original path.

Currently, the number of times requests are failed over to another
available path is not visible to userspace. Exposing this information
can be useful for diagnosing path health and stability.

Export the multipath failover count attribute named "multipath_failover
_count" via sysfs which is both readable and writable and thus allowing
user to reset the counter. This counter can be consumed by monitoring
tools such as nvme-top to help identify paths that consistently trigger
failovers under load.

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 drivers/nvme/host/multipath.c | 27 +++++++++++++++++++++++++++
 drivers/nvme/host/nvme.h      |  2 ++
 drivers/nvme/host/sysfs.c     |  5 +++++
 3 files changed, 34 insertions(+)

diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 174027d1cc19..c8ae935658a4 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -142,6 +142,7 @@ void nvme_failover_req(struct request *req)
 	struct bio *bio;
 
 	nvme_mpath_clear_current_path(ns);
+	WRITE_ONCE(ns->failover, size_add(READ_ONCE(ns->failover), 1));
 
 	/*
 	 * If we got back an ANA error, we know the controller is alive but not
@@ -1168,6 +1169,32 @@ static ssize_t delayed_removal_secs_store(struct device *dev,
 
 DEVICE_ATTR_RW(delayed_removal_secs);
 
+static ssize_t multipath_failover_count_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	struct nvme_ns *ns = nvme_get_ns_from_dev(dev);
+
+	return sysfs_emit(buf, "%lu\n", READ_ONCE(ns->failover));
+}
+
+static ssize_t multipath_failover_count_store(struct device *dev,
+		struct device_attribute *attr, const char *buf, size_t count)
+{
+	unsigned long failover;
+	int ret;
+	struct nvme_ns *ns = nvme_get_ns_from_dev(dev);
+
+	ret = kstrtoul(buf, 0, &failover);
+	if (ret)
+		return -EINVAL;
+
+	WRITE_ONCE(ns->failover, failover);
+
+	return count;
+}
+
+DEVICE_ATTR_RW(multipath_failover_count);
+
 static int nvme_lookup_ana_group_desc(struct nvme_ctrl *ctrl,
 		struct nvme_ana_group_desc *desc, void *data)
 {
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index 237829cdc151..6307243fd216 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -533,6 +533,7 @@ struct nvme_ns {
 #ifdef CONFIG_NVME_MULTIPATH
 	enum nvme_ana_state ana_state;
 	u32 ana_grpid;
+	size_t failover;
 #endif
 	size_t retries;
 	struct list_head siblings;
@@ -1000,6 +1001,7 @@ 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 dev_attr_delayed_removal_secs;
+extern struct device_attribute dev_attr_multipath_failover_count;
 extern struct device_attribute subsys_attr_iopolicy;
 
 static inline bool nvme_disk_is_ns_head(struct gendisk *disk)
diff --git a/drivers/nvme/host/sysfs.c b/drivers/nvme/host/sysfs.c
index 11e7016954a7..78c3a6f78ef8 100644
--- a/drivers/nvme/host/sysfs.c
+++ b/drivers/nvme/host/sysfs.c
@@ -286,6 +286,7 @@ static struct attribute *nvme_ns_attrs[] = {
 	&dev_attr_queue_depth.attr,
 	&dev_attr_numa_nodes.attr,
 	&dev_attr_delayed_removal_secs.attr,
+	&dev_attr_multipath_failover_count.attr,
 #endif
 	&dev_attr_io_passthru_err_log_enabled.attr,
 	&dev_attr_command_retries.attr,
@@ -333,6 +334,10 @@ static umode_t nvme_ns_attrs_are_visible(struct kobject *kobj,
 		if (!nvme_disk_is_ns_head(disk))
 			return 0;
 	}
+	if (a == &dev_attr_multipath_failover_count.attr) {
+		if (nvme_disk_is_ns_head(dev_to_disk(dev)))
+			return 0;
+	}
 #endif
 	return a->mode;
 }
-- 
2.52.0