[RFC PATCHv3 3/6] nvme: add sysfs attribute adp_ewma_shift

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

By default, the EWMA (Exponentially Weighted Moving Average) shift
value, used for storing latency samples for adaptive iopolicy, is set
to 3. The EWMA is calculated using the following formula:

    ewma = (old * ((1 << ewma_shift) - 1) + new) >> ewma_shift;

The default value of 3 assigns ~87.5% weight to the existing EWMA value
and ~12.5% weight to the new latency sample. This provides a stable
average that smooths out short-term variations.

However, different workloads may require faster or slower adaptation to
changing conditions. This commit introduces a new sysfs attribute,
adp_ewma_shift, allowing users to tune the weighting factor.

For example:
  - adp_ewma_shift = 1 => 50% old, 50% new
  - adp_ewma_shift = 0 => 0% old, 100% new

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 drivers/nvme/host/core.c      | 10 ++++++++-
 drivers/nvme/host/multipath.c | 38 +++++++++++++++++++++++++++++++----
 drivers/nvme/host/nvme.h      |  2 ++
 drivers/nvme/host/sysfs.c     |  1 +
 4 files changed, 46 insertions(+), 5 deletions(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 284a7c9c5d1d..ab09b9724674 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -3240,7 +3240,15 @@ static int nvme_init_subsystem(struct nvme_ctrl *ctrl, struct nvme_id_ctrl *id)
 		return -EINVAL;
 	}
 	nvme_mpath_default_iopolicy(subsys);
-
+#ifdef CONFIG_NVME_MULTIPATH
+	/*
+	 * Default value of emwa_shift is set to 3 so that we can assign ~87.5 %
+	 * weight to the existing ewma and ~12.5% weight to the new latency
+	 * sample. This default could be changed through sysfs. This value is
+	 * used while adding latency sample for adaptive iopolicy.
+	 */
+	subsys->adp_ewma_shift = NVME_DEFAULT_ADP_EWMA_SHIFT;
+#endif
 	subsys->dev.class = &nvme_subsys_class;
 	subsys->dev.release = nvme_release_subsystem;
 	subsys->dev.groups = nvme_subsys_attrs_groups;
diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index b438371b8494..95407c0f2f4b 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -294,10 +294,9 @@ static void nvme_mpath_weight_work(struct work_struct *weight_work)
  * For instance, with EWMA_SHIFT = 3, this assigns 7/8 (~87.5 %) weight to
  * the existing/old ewma and 1/8 (~12.5%) weight to the new sample.
  */
-static inline u64 ewma_update(u64 old, u64 new)
+static inline u64 ewma_update(u64 old, u64 new, int ewma_shift)
 {
-	return (old * ((1 << NVME_DEFAULT_ADP_EWMA_SHIFT) - 1)
-			+ new) >> NVME_DEFAULT_ADP_EWMA_SHIFT;
+	return (old * ((1 << ewma_shift) - 1) + new) >> ewma_shift;
 }
 
 static void nvme_mpath_add_sample(struct request *rq, struct nvme_ns *ns)
@@ -389,7 +388,8 @@ static void nvme_mpath_add_sample(struct request *rq, struct nvme_ns *ns)
 		if (unlikely(!stat->slat_ns))
 			WRITE_ONCE(stat->slat_ns, avg_lat_ns);
 		else {
-			slat_ns = ewma_update(stat->slat_ns, avg_lat_ns);
+			slat_ns = ewma_update(stat->slat_ns, avg_lat_ns,
+				READ_ONCE(head->subsys->adp_ewma_shift));
 			WRITE_ONCE(stat->slat_ns, slat_ns);
 		}
 
@@ -1465,6 +1465,36 @@ static ssize_t nvme_subsys_iopolicy_store(struct device *dev,
 SUBSYS_ATTR_RW(iopolicy, S_IRUGO | S_IWUSR,
 		      nvme_subsys_iopolicy_show, nvme_subsys_iopolicy_store);
 
+static ssize_t nvme_subsys_adp_ewma_shift_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	struct nvme_subsystem *subsys =
+		container_of(dev, struct nvme_subsystem, dev);
+
+	return sysfs_emit(buf, "%d\n", READ_ONCE(subsys->adp_ewma_shift));
+}
+
+static ssize_t nvme_subsys_adp_ewma_shift_store(struct device *dev,
+		struct device_attribute *attr, const char *buf, size_t count)
+{
+	int shift, err;
+	struct nvme_subsystem *subsys =
+		container_of(dev, struct nvme_subsystem, dev);
+
+	err = kstrtoint(buf, 0, &shift);
+	if (err)
+		return -EINVAL;
+
+	if (shift < 0)
+		return -EINVAL;
+
+	WRITE_ONCE(subsys->adp_ewma_shift, shift);
+	return count;
+}
+
+SUBSYS_ATTR_RW(adp_ewma_shift, 0644, nvme_subsys_adp_ewma_shift_show,
+		nvme_subsys_adp_ewma_shift_store);
+
 static ssize_t ana_grpid_show(struct device *dev, struct device_attribute *attr,
 		char *buf)
 {
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index 5baf0232726f..9f5b233c747a 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -450,6 +450,7 @@ struct nvme_subsystem {
 	struct ida		ns_ida;
 #ifdef CONFIG_NVME_MULTIPATH
 	enum nvme_iopolicy	iopolicy;
+	int			adp_ewma_shift; /* used for adaptive iopolicy */
 #endif
 };
 
@@ -1043,6 +1044,7 @@ 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 subsys_attr_iopolicy;
+extern struct device_attribute subsys_attr_adp_ewma_shift;
 
 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 1cbab90ed42e..cf9711961b00 100644
--- a/drivers/nvme/host/sysfs.c
+++ b/drivers/nvme/host/sysfs.c
@@ -917,6 +917,7 @@ static struct attribute *nvme_subsys_attrs[] = {
 	&subsys_attr_subsystype.attr,
 #ifdef CONFIG_NVME_MULTIPATH
 	&subsys_attr_iopolicy.attr,
+	&subsys_attr_adp_ewma_shift.attr,
 #endif
 	NULL,
 };
-- 
2.51.0