[RFC PATCHv2 0/4] nvme-multipath: introduce adaptive I/O policy

[RFC PATCHv2 0/4] nvme-multipath: introduce adaptive I/O policy

[Nilay Shroff]

Hi,

This series introduces a new adaptive I/O policy for NVMe native
multipath. Existing policies such as numa, round-robin, and queue-depth
are static and do not adapt to real-time transport performance. The numa
selects the path closest to the NUMA node of the current CPU, optimizing
memory and path locality, but ignores actual path performance. The
round-robin distributes I/O evenly across all paths, providing fairness
but not performance awareness. The queue-depth reacts to instantaneous
queue occupancy, avoiding heavily loaded paths, but does not account for
actual latency, throughput, or link speed.

The new adaptive policy addresses these gaps selecting paths dynamically
based on measured I/O latency for both PCIe and fabrics. Latency is
derived by passively sampling I/O completions. Each path is assigned a 
weight proportional to its latency score, and I/Os are then forwarded 
accordingly. As condition changes (e.g. latency spikes, bandwidth 
differences), path weights are updated, automatically steering traffic
toward better-performing paths.

Early results show reduced tail latency under mixed workloads and
improved throughput by exploiting higher-speed links more effectively.
For example, with NVMf/TCP using two paths (one throttled with ~30 ms
delay), fio results with random read/write/rw workloads (direct I/O)
showed:

        numa         round-robin   queue-depth  adaptive
        -----------  -----------   -----------  ---------
READ:   50.0 MiB/s   105 MiB/s     230 MiB/s    350 MiB/s
WRITE:  65.9 MiB/s   125 MiB/s     385 MiB/s    446 MiB/s
RW:     R:30.6 MiB/s R:56.5 MiB/s  R:122 MiB/s  R:175 MiB/s
        W:30.7 MiB/s W:56.5 MiB/s  W:122 MiB/s  W:175 MiB/s

This pathcset includes totla 5 patches:
[PATCH 1/4] block: expose blk_stat_{enable,disable}_accounting()
  - Make blk_stat APIs available to block drivers.
  - Needed for per-path latency measurement in adaptive policy.

[PATCH 2/4] nvme-multipath: add adaptive I/O policy
  - Implement path scoring based on latency (EWMA).
  - Distribute I/O proportionally to per-path weights.

[PATCH 3/4] nvme: add generic debugfs support
  - Introduce generic debugfs support for NVMe module

[PATCH 4/4] nvme-multipath: add debugfs attribute for adaptive I/O policy stats
  - Add “adaptive_stat” under per-path and head debugfs directories to
    expose adaptive policy state and statistics.

As ususal, feedback and suggestions are most welcome!

Thanks!

Changes from v1:
  - Ensure that the completion of I/O occurs on the same CPU as the
    submitting I/O CPU (Hannes Reinecke)
  - Remove adapter link speed from the path weight calculation
    (Hannes Reinecke)
  - Add adaptive I/O stat under debugfs instead of current sysfs
    (Hannes Reinecke)  
  - Move path weight calculation to a workqueue from IO completion
    code path

Nilay Shroff (4):
  block: expose blk_stat_{enable,disable}_accounting() to drivers
  nvme-multipath: add support for adaptive I/O policy
  nvme: add generic debugfs support
  nvme-multipath: add debugfs attribute for adaptive I/O policy stats

 block/blk-stat.h              |   4 -
 drivers/nvme/host/Makefile    |   2 +-
 drivers/nvme/host/core.c      |  13 +-
 drivers/nvme/host/debugfs.c   | 239 ++++++++++++++++++++
 drivers/nvme/host/ioctl.c     |   7 +-
 drivers/nvme/host/multipath.c | 400 ++++++++++++++++++++++++++++++++--
 drivers/nvme/host/nvme.h      |  55 ++++-
 drivers/nvme/host/pr.c        |   6 +-
 drivers/nvme/host/sysfs.c     |   2 +-
 include/linux/blk-mq.h        |   4 +
 10 files changed, 705 insertions(+), 27 deletions(-)
 create mode 100644 drivers/nvme/host/debugfs.c

-- 
2.51.0

[RFC PATCHv2 1/4] block: expose blk_stat_{enable,disable}_accounting() to drivers

[Nilay Shroff]

The functions blk_stat_enable_accounting() and
blk_stat_disable_accounting() are currently exported, but their
prototypes are only defined in a private header. Move these prototypes
into a common header so that block drivers can directly use these APIs.

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 block/blk-stat.h       | 4 ----
 include/linux/blk-mq.h | 4 ++++
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/block/blk-stat.h b/block/blk-stat.h
index 9e05bf18d1be..f5d95dd8c0e9 100644
--- a/block/blk-stat.h
+++ b/block/blk-stat.h
@@ -67,10 +67,6 @@ void blk_free_queue_stats(struct blk_queue_stats *);
 
 void blk_stat_add(struct request *rq, u64 now);
 
-/* record time/size info in request but not add a callback */
-void blk_stat_enable_accounting(struct request_queue *q);
-void blk_stat_disable_accounting(struct request_queue *q);
-
 /**
  * blk_stat_alloc_callback() - Allocate a block statistics callback.
  * @timer_fn: Timer callback function.
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index b25d12545f46..f647444643b8 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -735,6 +735,10 @@ int blk_rq_poll(struct request *rq, struct io_comp_batch *iob,
 
 bool blk_mq_queue_inflight(struct request_queue *q);
 
+/* record time/size info in request but not add a callback */
+void blk_stat_enable_accounting(struct request_queue *q);
+void blk_stat_disable_accounting(struct request_queue *q);
+
 enum {
 	/* return when out of requests */
 	BLK_MQ_REQ_NOWAIT	= (__force blk_mq_req_flags_t)(1 << 0),
-- 
2.51.0

[RFC PATCHv2 2/4] nvme-multipath: add support for adaptive I/O policy

[Nilay Shroff]

This commit introduces a new I/O policy named "adaptive". Users can
configure it by writing "adaptive" to "/sys/class/nvme-subsystem/nvme-
subsystemX/iopolicy"

The adaptive policy dynamically distributes I/O based on measured
completion latency. The main idea is to calculate latency for each path,
derive a weight, and then proportionally forward I/O according to those
weights.

To ensure scalability, path latency is measured per-CPU. Each CPU
maintains its own statistics, and I/O forwarding uses these per-CPU
values. Every ~15 seconds, a simple average latency of per-CPU batched
samples are computed and fed into an Exponentially Weighted Moving
Average (EWMA):

avg_latency = div_u64(batch, batch_count);
new_ewma_latency = (prev_ewma_latency * (WEIGHT-1) + avg_latency)/WEIGHT

With WEIGHT = 8, this assigns 7/8 (~87.5%) weight to the previous
latency value and 1/8 (~12.5%) to the most recent latency. This
smoothing reduces jitter, adapts quickly to changing conditions,
avoids storing historical samples, and works well for both low and
high I/O rates. Path weights are then derived from the smoothed (EWMA)
latency as follows (example with two paths A and B):

    path_A_score = NSEC_PER_SEC / path_A_ewma_latency
    path_B_score = NSEC_PER_SEC / path_B_ewma_latency
    total_score  = path_A_score + path_B_score

    path_A_weight = (path_A_score * 100) / total_score
    path_B_weight = (path_B_score * 100) / total_score

where:
  - path_X_ewma_latency is the smoothed latency of a path in nanoseconds
  - NSEC_PER_SEC is used as a scaling factor since valid latencies
    are < 1 second
  - weights are normalized to a 0–100 scale across all paths.

Path credits are refilled based on this weight, with one credit
consumed per I/O. When all credits are consumed, the credits are
refilled again based on the current weight. This ensures that I/O is
distributed across paths proportionally to their calculated weight.

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 drivers/nvme/host/core.c      |  10 +-
 drivers/nvme/host/ioctl.c     |   7 +-
 drivers/nvme/host/multipath.c | 397 ++++++++++++++++++++++++++++++++--
 drivers/nvme/host/nvme.h      |  44 +++-
 drivers/nvme/host/pr.c        |   6 +-
 drivers/nvme/host/sysfs.c     |   2 +-
 6 files changed, 444 insertions(+), 22 deletions(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index fa4181d7de73..c7f21823c137 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -689,6 +689,7 @@ static void nvme_free_ns(struct kref *kref)
 {
 	struct nvme_ns *ns = container_of(kref, struct nvme_ns, kref);
 
+	nvme_free_ns_stat(ns);
 	put_disk(ns->disk);
 	nvme_put_ns_head(ns->head);
 	nvme_put_ctrl(ns->ctrl);
@@ -4137,6 +4138,9 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, struct nvme_ns_info *info)
 	if (nvme_init_ns_head(ns, info))
 		goto out_cleanup_disk;
 
+	if (nvme_alloc_ns_stat(ns))
+		goto out_unlink_ns;
+
 	/*
 	 * If multipathing is enabled, the device name for all disks and not
 	 * just those that represent shared namespaces needs to be based on the
@@ -4161,7 +4165,7 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, struct nvme_ns_info *info)
 	}
 
 	if (nvme_update_ns_info(ns, info))
-		goto out_unlink_ns;
+		goto out_free_ns_stat;
 
 	mutex_lock(&ctrl->namespaces_lock);
 	/*
@@ -4170,7 +4174,7 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, struct nvme_ns_info *info)
 	 */
 	if (test_bit(NVME_CTRL_FROZEN, &ctrl->flags)) {
 		mutex_unlock(&ctrl->namespaces_lock);
-		goto out_unlink_ns;
+		goto out_free_ns_stat;
 	}
 	nvme_ns_add_to_ctrl_list(ns);
 	mutex_unlock(&ctrl->namespaces_lock);
@@ -4201,6 +4205,8 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, struct nvme_ns_info *info)
 	list_del_rcu(&ns->list);
 	mutex_unlock(&ctrl->namespaces_lock);
 	synchronize_srcu(&ctrl->srcu);
+out_free_ns_stat:
+	nvme_free_ns_stat(ns);
  out_unlink_ns:
 	mutex_lock(&ctrl->subsys->lock);
 	list_del_rcu(&ns->siblings);
diff --git a/drivers/nvme/host/ioctl.c b/drivers/nvme/host/ioctl.c
index c212fa952c0f..69d2f72d0e86 100644
--- a/drivers/nvme/host/ioctl.c
+++ b/drivers/nvme/host/ioctl.c
@@ -711,7 +711,7 @@ int nvme_ns_head_ioctl(struct block_device *bdev, blk_mode_t mode,
 		flags |= NVME_IOCTL_PARTITION;
 
 	srcu_idx = srcu_read_lock(&head->srcu);
-	ns = nvme_find_path(head);
+	ns = nvme_find_path(head, open_for_write ? WRITE : READ);
 	if (!ns)
 		goto out_unlock;
 
@@ -742,7 +742,7 @@ long nvme_ns_head_chr_ioctl(struct file *file, unsigned int cmd,
 	int srcu_idx, ret = -EWOULDBLOCK;
 
 	srcu_idx = srcu_read_lock(&head->srcu);
-	ns = nvme_find_path(head);
+	ns = nvme_find_path(head, open_for_write ? WRITE : READ);
 	if (!ns)
 		goto out_unlock;
 
@@ -762,7 +762,8 @@ int nvme_ns_head_chr_uring_cmd(struct io_uring_cmd *ioucmd,
 	struct cdev *cdev = file_inode(ioucmd->file)->i_cdev;
 	struct nvme_ns_head *head = container_of(cdev, struct nvme_ns_head, cdev);
 	int srcu_idx = srcu_read_lock(&head->srcu);
-	struct nvme_ns *ns = nvme_find_path(head);
+	struct nvme_ns *ns = nvme_find_path(head,
+			ioucmd->file->f_mode & FMODE_WRITE ? WRITE : READ);
 	int ret = -EINVAL;
 
 	if (ns)
diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 3da980dc60d9..9ecdaca5e9a0 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -6,6 +6,8 @@
 #include <linux/backing-dev.h>
 #include <linux/moduleparam.h>
 #include <linux/vmalloc.h>
+#include <linux/blk-mq.h>
+#include <linux/math64.h>
 #include <trace/events/block.h>
 #include "nvme.h"
 
@@ -66,9 +68,10 @@ MODULE_PARM_DESC(multipath_always_on,
 	"create multipath node always except for private namespace with non-unique nsid; note that this also implicitly enables native multipath support");
 
 static const char *nvme_iopolicy_names[] = {
-	[NVME_IOPOLICY_NUMA]	= "numa",
-	[NVME_IOPOLICY_RR]	= "round-robin",
-	[NVME_IOPOLICY_QD]      = "queue-depth",
+	[NVME_IOPOLICY_NUMA]	 = "numa",
+	[NVME_IOPOLICY_RR]	 = "round-robin",
+	[NVME_IOPOLICY_QD]       = "queue-depth",
+	[NVME_IOPOLICY_ADAPTIVE] = "adaptive",
 };
 
 static int iopolicy = NVME_IOPOLICY_NUMA;
@@ -83,6 +86,8 @@ static int nvme_set_iopolicy(const char *val, const struct kernel_param *kp)
 		iopolicy = NVME_IOPOLICY_RR;
 	else if (!strncmp(val, "queue-depth", 11))
 		iopolicy = NVME_IOPOLICY_QD;
+	else if (!strncmp(val, "adaptive", 8))
+		iopolicy = NVME_IOPOLICY_ADAPTIVE;
 	else
 		return -EINVAL;
 
@@ -196,6 +201,221 @@ void nvme_mpath_start_request(struct request *rq)
 }
 EXPORT_SYMBOL_GPL(nvme_mpath_start_request);
 
+static void nvme_mpath_weight_work(struct work_struct *weight_work)
+{
+	int cpu, srcu_idx;
+	u32 weight;
+	struct nvme_ns *ns;
+	struct nvme_path_stat *stat;
+	struct nvme_path_work *work = container_of(weight_work,
+			struct nvme_path_work, weight_work);
+	struct nvme_ns_head *head = work->ns->head;
+	int rw = work->rw;
+	u64 total_score = 0;
+
+	cpu = get_cpu();
+
+	srcu_idx = srcu_read_lock(&head->srcu);
+	list_for_each_entry_srcu(ns, &head->list, siblings,
+			srcu_read_lock_held(&head->srcu)) {
+
+		stat = &this_cpu_ptr(ns->info)[rw].stat;
+		if (!READ_ONCE(stat->slat_ns))
+			continue;
+		/*
+		 * Compute the path score as the inverse of smoothed
+		 * latency, scaled by NSEC_PER_SEC. Floating point
+		 * math is unavailable in the kernel, so fixed-point
+		 * scaling is used instead. NSEC_PER_SEC is chosen
+		 * because valid latencies are always < 1 second; longer
+		 * latencies are ignored.
+		 */
+		stat->score = div_u64(NSEC_PER_SEC, READ_ONCE(stat->slat_ns));
+
+		/* Compute total score. */
+		total_score += stat->score;
+	}
+
+	if (!total_score)
+		goto out;
+
+	/*
+	 * After computing the total slatency, we derive per-path weight
+	 * (normalized to the range 0–100). The weight represents the
+	 * relative share of I/O the path should receive.
+	 *
+	 *   - lower smoothed latency -> higher weight
+	 *   - higher smoothed slatency -> lower weight
+	 *
+	 * Next, while forwarding I/O, we assign "credits" to each path
+	 * based on its weight (please also refer nvme_adaptive_path()):
+	 *   - Initially, credits = weight.
+	 *   - Each time an I/O is dispatched on a path, its credits are
+	 *     decremented proportionally.
+	 *   - When a path runs out of credits, it becomes temporarily
+	 *     ineligible until credit is refilled.
+	 *
+	 * I/O distribution is therefore governed by available credits,
+	 * ensuring that over time the proportion of I/O sent to each
+	 * path matches its weight (and thus its performance).
+	 */
+	list_for_each_entry_srcu(ns, &head->list, siblings,
+			srcu_read_lock_held(&head->srcu)) {
+
+		stat = &this_cpu_ptr(ns->info)[rw].stat;
+		weight = div_u64(stat->score * 100, total_score);
+
+		/*
+		 * Ensure the path weight never drops below 1. A weight
+		 * of 0 is used only for newly added paths. During
+		 * bootstrap, a few I/Os are sent to such paths to
+		 * establish an initial weight. Enforcing a minimum
+		 * weight of 1 guarantees that no path is forgotten and
+		 * that each path is probed at least occasionally.
+		 */
+		if (!weight)
+			weight = 1;
+
+		WRITE_ONCE(stat->weight, weight);
+		stat->score = 0;
+	}
+out:
+	srcu_read_unlock(&head->srcu, srcu_idx);
+	put_cpu();
+}
+
+#define NVME_EWMA_SHIFT	3
+static inline u64 ewma_update(u64 old, u64 new)
+{
+	return (old * ((1 << NVME_EWMA_SHIFT) - 1) + new) >> NVME_EWMA_SHIFT;
+}
+
+static void nvme_mpath_add_sample(struct request *rq, struct nvme_ns *ns)
+{
+	int cpu;
+	unsigned int rw;
+	struct nvme_path_info *info;
+	struct nvme_path_stat *stat;
+	u64 now, latency, slat_ns, avg_lat_ns;
+	struct nvme_ns_head *head = ns->head;
+
+	if (list_is_singular(&head->list))
+		return;
+
+	now = ktime_get_ns();
+	latency = now >= rq->io_start_time_ns ? now - rq->io_start_time_ns : 0;
+	if (!latency)
+		return;
+
+	/*
+	 * As completion code path is serialized(i.e. no same completion queue
+	 * update code could run simultaneously on multiple cpu) we can safely
+	 * access per cpu nvme path stat here from another cpu (in case the
+	 * completion cpu is different from submission cpu).
+	 * The only field which could be accessed simultaneously here is the
+	 * path ->weight which may be accessed by this function as well as I/O
+	 * submission path during path selection logic and we protect ->weight
+	 * using READ_ONCE/WRITE_ONCE. Yes this may not be 100% accurate but
+	 * we also don't need to be so accurate here as the path credit would
+	 * be anyways refilled, based on path weight, once path consumes all
+	 * its credits. And we limit path weight/credit max up to 100. Please
+	 * also refer nvme_adaptive_path().
+	 */
+	cpu = blk_mq_rq_cpu(rq);
+	rw = rq_data_dir(rq);
+	info = &per_cpu_ptr(ns->info, cpu)[rw];
+	stat = &info->stat;
+
+	/*
+	 * If latency > ~1s then ignore this sample to prevent EWMA from being
+	 * skewed by pathological outliers (multi-second waits, controller
+	 * timeouts etc.). This keeps path scores representative of normal
+	 * performance and avoids instability from rare spikes. If such high
+	 * latency is real, ANA state reporting or keep-alive error counters
+	 * will mark the path unhealthy and remove it from the head node list,
+	 * so we safely skip such sample here.
+	 */
+	if (unlikely(latency > NSEC_PER_SEC)) {
+		stat->nr_ignored++;
+		return;
+	}
+
+	/*
+	 * Accumulate latency samples and increment the batch count for each
+	 * ~15 second interval. When the interval expires, compute the simple
+	 * average latency over that window, then update the smoothed (EWMA)
+	 * latency. The path weight is recalculated based on this smoothed
+	 * latency.
+	 */
+	stat->batch += latency;
+	stat->batch_count++;
+	stat->nr_samples++;
+
+	if (now > stat->last_weight_ts &&
+			(now - stat->last_weight_ts) >= 15 * NSEC_PER_SEC) {
+
+		stat->last_weight_ts = now;
+
+		/*
+		 * Find simple average latency for the last epoch (~15 sec
+		 * interval).
+		 */
+		avg_lat_ns = div_u64(stat->batch, stat->batch_count);
+
+		/*
+		 * Calculate smooth/EWMA (Exponentially Weighted Moving Average)
+		 * latency. EWMA is preferred over simple average latency
+		 * because it smooths naturally, reduces jitter from sudden
+		 * spikes, and adapts faster to changing conditions. It also
+		 * avoids storing historical samples, and works well for both
+		 * slow and fast I/O rates.
+		 * Formula:
+		 * slat_ns = (prev_slat_ns * (WEIGHT - 1) + (latency)) / WEIGHT
+		 * With WEIGHT = 8, this assigns 7/8 (~87.5 %) weight to the
+		 * existing latency and 1/8 (~12.5%) weight to the new latency.
+		 */
+		if (unlikely(!stat->slat_ns))
+			WRITE_ONCE(stat->slat_ns, avg_lat_ns);
+		else {
+			slat_ns = ewma_update(stat->slat_ns, avg_lat_ns);
+			WRITE_ONCE(stat->slat_ns, slat_ns);
+		}
+
+		stat->batch = stat->batch_count = 0;
+
+		/*
+		 * Defer calculation of the path weight in per-cpu workqueue.
+		 */
+		schedule_work_on(cpu, &info->work.weight_work);
+	}
+}
+
+int nvme_alloc_ns_stat(struct nvme_ns *ns)
+{
+	int rw, cpu;
+	struct nvme_path_work *work;
+	gfp_t gfp = GFP_KERNEL | __GFP_ZERO;
+
+	if (!ns->head->disk)
+		return 0;
+
+	ns->info = __alloc_percpu_gfp(2 * sizeof(struct nvme_path_info),
+			__alignof__(struct nvme_path_info), gfp);
+	if (!ns->info)
+		return -ENOMEM;
+
+	for_each_possible_cpu(cpu) {
+		for (rw = 0; rw < 2; rw++) {
+			work = &per_cpu_ptr(ns->info, cpu)[rw].work;
+			work->ns = ns;
+			work->rw = rw;
+			INIT_WORK(&work->weight_work, nvme_mpath_weight_work);
+		}
+	}
+
+	return 0;
+}
+
 void nvme_mpath_end_request(struct request *rq)
 {
 	struct nvme_ns *ns = rq->q->queuedata;
@@ -203,6 +423,9 @@ void nvme_mpath_end_request(struct request *rq)
 	if (nvme_req(rq)->flags & NVME_MPATH_CNT_ACTIVE)
 		atomic_dec_if_positive(&ns->ctrl->nr_active);
 
+	if (test_bit(NVME_NS_PATH_STAT, &ns->flags))
+		nvme_mpath_add_sample(rq, ns);
+
 	if (!(nvme_req(rq)->flags & NVME_MPATH_IO_STATS))
 		return;
 	bdev_end_io_acct(ns->head->disk->part0, req_op(rq),
@@ -236,6 +459,54 @@ static const char *nvme_ana_state_names[] = {
 	[NVME_ANA_CHANGE]		= "change",
 };
 
+static void nvme_mpath_reset_current_stat(struct nvme_ns *ns)
+{
+	int rw, cpu;
+	struct nvme_path_stat *stat;
+
+	for_each_possible_cpu(cpu) {
+		for (rw = 0; rw < 2; rw++) {
+			stat = &per_cpu_ptr(ns->info, cpu)[rw].stat;
+			memset(stat, 0, sizeof(struct nvme_path_stat));
+		}
+	}
+}
+
+static bool nvme_mpath_set_current_adaptive_path(struct nvme_ns *ns)
+{
+	struct nvme_ns_head *head = ns->head;
+
+	if (!head->disk || head->subsys->iopolicy != NVME_IOPOLICY_ADAPTIVE)
+		return false;
+
+	if (test_and_set_bit(NVME_NS_PATH_STAT, &ns->flags))
+		return false;
+
+	blk_queue_flag_set(QUEUE_FLAG_SAME_FORCE, ns->queue);
+	blk_stat_enable_accounting(ns->queue);
+	return true;
+}
+
+static bool nvme_mpath_clear_current_adaptive_path(struct nvme_ns *ns)
+{
+	int rw, cpu;
+	struct nvme_path_info *info;
+
+	if (!test_and_clear_bit(NVME_NS_PATH_STAT, &ns->flags))
+		return false;
+
+	blk_stat_disable_accounting(ns->queue);
+	blk_queue_flag_clear(QUEUE_FLAG_SAME_FORCE, ns->queue);
+	for_each_online_cpu(cpu) {
+		for (rw = 0; rw < 2; rw++) {
+			info = &per_cpu_ptr(ns->info, cpu)[rw];
+			cancel_work_sync(&info->work.weight_work);
+		}
+	}
+	nvme_mpath_reset_current_stat(ns);
+	return true;
+}
+
 bool nvme_mpath_clear_current_path(struct nvme_ns *ns)
 {
 	struct nvme_ns_head *head = ns->head;
@@ -251,6 +522,8 @@ bool nvme_mpath_clear_current_path(struct nvme_ns *ns)
 			changed = true;
 		}
 	}
+	if (nvme_mpath_clear_current_adaptive_path(ns))
+		changed = true;
 out:
 	return changed;
 }
@@ -269,6 +542,18 @@ void nvme_mpath_clear_ctrl_paths(struct nvme_ctrl *ctrl)
 	srcu_read_unlock(&ctrl->srcu, srcu_idx);
 }
 
+static void nvme_mpath_set_ctrl_paths(struct nvme_ctrl *ctrl)
+{
+	struct nvme_ns *ns;
+	int srcu_idx;
+
+	srcu_idx = srcu_read_lock(&ctrl->srcu);
+	list_for_each_entry_srcu(ns, &ctrl->namespaces, list,
+				srcu_read_lock_held(&ctrl->srcu))
+		nvme_mpath_set_current_adaptive_path(ns);
+	srcu_read_unlock(&ctrl->srcu, srcu_idx);
+}
+
 void nvme_mpath_revalidate_paths(struct nvme_ns *ns)
 {
 	struct nvme_ns_head *head = ns->head;
@@ -281,6 +566,8 @@ void nvme_mpath_revalidate_paths(struct nvme_ns *ns)
 				 srcu_read_lock_held(&head->srcu)) {
 		if (capacity != get_capacity(ns->disk))
 			clear_bit(NVME_NS_READY, &ns->flags);
+
+		nvme_mpath_reset_current_stat(ns);
 	}
 	srcu_read_unlock(&head->srcu, srcu_idx);
 
@@ -405,6 +692,86 @@ static struct nvme_ns *nvme_round_robin_path(struct nvme_ns_head *head)
 	return found;
 }
 
+static inline bool nvme_state_is_live(enum nvme_ana_state state)
+{
+	return state == NVME_ANA_OPTIMIZED || state == NVME_ANA_NONOPTIMIZED;
+}
+
+static struct nvme_ns *nvme_adaptive_path(struct nvme_ns_head *head,
+		unsigned int rw)
+{
+	struct nvme_ns *ns, *found = NULL;
+	struct nvme_path_stat *stat;
+	u32 weight;
+	int refill = 0;
+
+	get_cpu();
+retry:
+	list_for_each_entry_srcu(ns, &head->list, siblings,
+			srcu_read_lock_held(&head->srcu)) {
+
+		if (nvme_path_is_disabled(ns) ||
+				!nvme_state_is_live(ns->ana_state))
+			continue;
+
+		stat = &this_cpu_ptr(ns->info)[rw].stat;
+
+		/*
+		 * When the head path-list is singular we don't calculate the
+		 * only path weight for optimization as we don't need to forward
+		 * I/O to more than one path. The another possibility is whenthe
+		 * path is newly added, we don't know its weight. So we go round
+		 * -robin for each such path and forward I/O to it.Once we start
+		 * getting response for such I/Os, the path weight calculation
+		 * would kick in and then we start using path credit for
+		 * forwarding I/O.
+		 */
+		weight = READ_ONCE(stat->weight);
+		if (unlikely(!weight)) {
+			found = ns;
+			goto out;
+		}
+
+		/*
+		 * To keep path selection logic simple, we don't distinguish
+		 * between ANA optimized and non-optimized states. The non-
+		 * optimized path is expected to have a lower weight, and
+		 * therefore fewer credits. As a result, only a small number of
+		 * I/Os will be forwarded to paths in the non-optimized state.
+		 */
+		if (stat->credit > 0) {
+			--stat->credit;
+			found = ns;
+			goto out;
+		}
+	}
+
+	if (!found && !list_empty(&head->list)) {
+		/*
+		 * Refill credits and retry.
+		 */
+		list_for_each_entry_srcu(ns, &head->list, siblings,
+				srcu_read_lock_held(&head->srcu)) {
+			if (nvme_path_is_disabled(ns) ||
+					!nvme_state_is_live(ns->ana_state))
+				continue;
+
+			stat = &this_cpu_ptr(ns->info)[rw].stat;
+			weight = READ_ONCE(stat->weight);
+			stat->credit = weight;
+			refill = 1;
+		}
+		if (refill)
+			goto retry;
+	}
+out:
+	if (found)
+		stat->sel++;
+
+	put_cpu();
+	return found;
+}
+
 static struct nvme_ns *nvme_queue_depth_path(struct nvme_ns_head *head)
 {
 	struct nvme_ns *best_opt = NULL, *best_nonopt = NULL, *ns;
@@ -461,9 +828,12 @@ static struct nvme_ns *nvme_numa_path(struct nvme_ns_head *head)
 	return ns;
 }
 
-inline struct nvme_ns *nvme_find_path(struct nvme_ns_head *head)
+inline struct nvme_ns *nvme_find_path(struct nvme_ns_head *head,
+		unsigned int rw)
 {
 	switch (READ_ONCE(head->subsys->iopolicy)) {
+	case NVME_IOPOLICY_ADAPTIVE:
+		return nvme_adaptive_path(head, rw);
 	case NVME_IOPOLICY_QD:
 		return nvme_queue_depth_path(head);
 	case NVME_IOPOLICY_RR:
@@ -523,7 +893,7 @@ static void nvme_ns_head_submit_bio(struct bio *bio)
 		return;
 
 	srcu_idx = srcu_read_lock(&head->srcu);
-	ns = nvme_find_path(head);
+	ns = nvme_find_path(head, bio_data_dir(bio));
 	if (likely(ns)) {
 		bio_set_dev(bio, ns->disk->part0);
 		bio->bi_opf |= REQ_NVME_MPATH;
@@ -565,7 +935,7 @@ static int nvme_ns_head_get_unique_id(struct gendisk *disk, u8 id[16],
 	int srcu_idx, ret = -EWOULDBLOCK;
 
 	srcu_idx = srcu_read_lock(&head->srcu);
-	ns = nvme_find_path(head);
+	ns = nvme_find_path(head, READ);
 	if (ns)
 		ret = nvme_ns_get_unique_id(ns, id, type);
 	srcu_read_unlock(&head->srcu, srcu_idx);
@@ -581,7 +951,7 @@ static int nvme_ns_head_report_zones(struct gendisk *disk, sector_t sector,
 	int srcu_idx, ret = -EWOULDBLOCK;
 
 	srcu_idx = srcu_read_lock(&head->srcu);
-	ns = nvme_find_path(head);
+	ns = nvme_find_path(head, READ);
 	if (ns)
 		ret = nvme_ns_report_zones(ns, sector, nr_zones, cb, data);
 	srcu_read_unlock(&head->srcu, srcu_idx);
@@ -807,6 +1177,10 @@ static void nvme_mpath_set_live(struct nvme_ns *ns)
 	}
 	mutex_unlock(&head->lock);
 
+	mutex_lock(&nvme_subsystems_lock);
+	nvme_mpath_set_current_adaptive_path(ns);
+	mutex_unlock(&nvme_subsystems_lock);
+
 	synchronize_srcu(&head->srcu);
 	kblockd_schedule_work(&head->requeue_work);
 }
@@ -855,11 +1229,6 @@ static int nvme_parse_ana_log(struct nvme_ctrl *ctrl, void *data,
 	return 0;
 }
 
-static inline bool nvme_state_is_live(enum nvme_ana_state state)
-{
-	return state == NVME_ANA_OPTIMIZED || state == NVME_ANA_NONOPTIMIZED;
-}
-
 static void nvme_update_ns_ana_state(struct nvme_ana_group_desc *desc,
 		struct nvme_ns *ns)
 {
@@ -1037,10 +1406,12 @@ static void nvme_subsys_iopolicy_update(struct nvme_subsystem *subsys,
 
 	WRITE_ONCE(subsys->iopolicy, iopolicy);
 
-	/* iopolicy changes clear the mpath by design */
+	/* iopolicy changes clear/reset the mpath by design */
 	mutex_lock(&nvme_subsystems_lock);
 	list_for_each_entry(ctrl, &subsys->ctrls, subsys_entry)
 		nvme_mpath_clear_ctrl_paths(ctrl);
+	list_for_each_entry(ctrl, &subsys->ctrls, subsys_entry)
+		nvme_mpath_set_ctrl_paths(ctrl);
 	mutex_unlock(&nvme_subsystems_lock);
 
 	pr_notice("subsysnqn %s iopolicy changed from %s to %s\n",
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index 102fae6a231c..bc87dd80de62 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -421,6 +421,7 @@ enum nvme_iopolicy {
 	NVME_IOPOLICY_NUMA,
 	NVME_IOPOLICY_RR,
 	NVME_IOPOLICY_QD,
+	NVME_IOPOLICY_ADAPTIVE,
 };
 
 struct nvme_subsystem {
@@ -459,6 +460,30 @@ struct nvme_ns_ids {
 	u8	csi;
 };
 
+struct nvme_path_stat {
+	u64 nr_samples;		/* total num of samples processed */
+	u64 nr_ignored;		/* num. of samples ignored */
+	u64 slat_ns;		/* smoothed (ewma) latency in nanoseconds */
+	u64 score;		/* score used for weight calculation */
+	u64 last_weight_ts;	/* timestamp of the last weight calculation */
+	u64 sel;		/* num of times this path is selcted for I/O */
+	u64 batch;		/* accumulated latency sum for current window */
+	u32 batch_count;	/* num of samples accumulated in current window */
+	u32 weight;		/* path weight */
+	u32 credit;		/* path credit for I/O forwarding */
+};
+
+struct nvme_path_work {
+	struct nvme_ns *ns;		/* owning namespace */
+	struct work_struct weight_work;	/* deferred work for weight calculation */
+	int rw;				/* op type : READ or WRITE */
+};
+
+struct nvme_path_info {
+	struct nvme_path_stat stat;	/* path statistics */
+	struct nvme_path_work work;	/* background worker context */
+};
+
 /*
  * Anchor structure for namespaces.  There is one for each namespace in a
  * NVMe subsystem that any of our controllers can see, and the namespace
@@ -534,6 +559,7 @@ struct nvme_ns {
 #ifdef CONFIG_NVME_MULTIPATH
 	enum nvme_ana_state ana_state;
 	u32 ana_grpid;
+	struct nvme_path_info __percpu *info;
 #endif
 	struct list_head siblings;
 	struct kref kref;
@@ -545,6 +571,7 @@ struct nvme_ns {
 #define NVME_NS_FORCE_RO		3
 #define NVME_NS_READY			4
 #define NVME_NS_SYSFS_ATTR_LINK	5
+#define NVME_NS_PATH_STAT		6
 
 	struct cdev		cdev;
 	struct device		cdev_device;
@@ -949,7 +976,7 @@ extern const struct attribute_group *nvme_dev_attr_groups[];
 extern const struct block_device_operations nvme_bdev_ops;
 
 void nvme_delete_ctrl_sync(struct nvme_ctrl *ctrl);
-struct nvme_ns *nvme_find_path(struct nvme_ns_head *head);
+struct nvme_ns *nvme_find_path(struct nvme_ns_head *head, unsigned int rw);
 #ifdef CONFIG_NVME_MULTIPATH
 static inline bool nvme_ctrl_use_ana(struct nvme_ctrl *ctrl)
 {
@@ -978,6 +1005,7 @@ void nvme_mpath_clear_ctrl_paths(struct nvme_ctrl *ctrl);
 void nvme_mpath_remove_disk(struct nvme_ns_head *head);
 void nvme_mpath_start_request(struct request *rq);
 void nvme_mpath_end_request(struct request *rq);
+int nvme_alloc_ns_stat(struct nvme_ns *ns);
 
 static inline void nvme_trace_bio_complete(struct request *req)
 {
@@ -1005,6 +1033,13 @@ static inline bool nvme_mpath_queue_if_no_path(struct nvme_ns_head *head)
 		return true;
 	return false;
 }
+static inline void nvme_free_ns_stat(struct nvme_ns *ns)
+{
+	if (!ns->head->disk)
+		return;
+
+	free_percpu(ns->info);
+}
 #else
 #define multipath false
 static inline bool nvme_ctrl_use_ana(struct nvme_ctrl *ctrl)
@@ -1096,6 +1131,13 @@ static inline bool nvme_mpath_queue_if_no_path(struct nvme_ns_head *head)
 {
 	return false;
 }
+static inline int nvme_alloc_ns_stat(struct nvme_ns *ns)
+{
+	return 0;
+}
+static inline void nvme_free_ns_stat(struct nvme_ns *ns)
+{
+}
 #endif /* CONFIG_NVME_MULTIPATH */
 
 int nvme_ns_get_unique_id(struct nvme_ns *ns, u8 id[16],
diff --git a/drivers/nvme/host/pr.c b/drivers/nvme/host/pr.c
index ca6a74607b13..9f23793dc12f 100644
--- a/drivers/nvme/host/pr.c
+++ b/drivers/nvme/host/pr.c
@@ -53,10 +53,12 @@ static int nvme_send_ns_head_pr_command(struct block_device *bdev,
 		struct nvme_command *c, void *data, unsigned int data_len)
 {
 	struct nvme_ns_head *head = bdev->bd_disk->private_data;
-	int srcu_idx = srcu_read_lock(&head->srcu);
-	struct nvme_ns *ns = nvme_find_path(head);
+	int srcu_idx;
+	struct nvme_ns *ns;
 	int ret = -EWOULDBLOCK;
 
+	srcu_idx = srcu_read_lock(&head->srcu);
+	ns = nvme_find_path(head, nvme_is_write(c) ? WRITE : READ);
 	if (ns) {
 		c->common.nsid = cpu_to_le32(ns->head->ns_id);
 		ret = nvme_submit_sync_cmd(ns->queue, c, data, data_len);
diff --git a/drivers/nvme/host/sysfs.c b/drivers/nvme/host/sysfs.c
index 29430949ce2f..4f9607e9698a 100644
--- a/drivers/nvme/host/sysfs.c
+++ b/drivers/nvme/host/sysfs.c
@@ -194,7 +194,7 @@ static int ns_head_update_nuse(struct nvme_ns_head *head)
 		return 0;
 
 	srcu_idx = srcu_read_lock(&head->srcu);
-	ns = nvme_find_path(head);
+	ns = nvme_find_path(head, READ);
 	if (!ns)
 		goto out_unlock;
 
-- 
2.51.0

[RFC PATCHv2 3/4] nvme: add generic debugfs support

[Nilay Shroff]

Add generic infrastructure for creating and managing debugfs files in
the NVMe module. This introduces helper APIs that allow NVMe drivers to
register and unregister debugfs entries, along with a reusable attribute
structure for defining new debugfs files.

The implementation uses seq_file interfaces to safely expose per-NS and
per-NS-head statistics, while supporting both simple show callbacks and
full seq_operations.

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 drivers/nvme/host/Makefile  |   2 +-
 drivers/nvme/host/debugfs.c | 122 ++++++++++++++++++++++++++++++++++++
 drivers/nvme/host/nvme.h    |  10 +++
 3 files changed, 133 insertions(+), 1 deletion(-)
 create mode 100644 drivers/nvme/host/debugfs.c

diff --git a/drivers/nvme/host/Makefile b/drivers/nvme/host/Makefile
index 6414ec968f99..7962dfc3b2ad 100644
--- a/drivers/nvme/host/Makefile
+++ b/drivers/nvme/host/Makefile
@@ -10,7 +10,7 @@ obj-$(CONFIG_NVME_FC)			+= nvme-fc.o
 obj-$(CONFIG_NVME_TCP)			+= nvme-tcp.o
 obj-$(CONFIG_NVME_APPLE)		+= nvme-apple.o
 
-nvme-core-y				+= core.o ioctl.o sysfs.o pr.o
+nvme-core-y				+= core.o ioctl.o sysfs.o pr.o debugfs.o
 nvme-core-$(CONFIG_NVME_VERBOSE_ERRORS)	+= constants.o
 nvme-core-$(CONFIG_TRACING)		+= trace.o
 nvme-core-$(CONFIG_NVME_MULTIPATH)	+= multipath.o
diff --git a/drivers/nvme/host/debugfs.c b/drivers/nvme/host/debugfs.c
new file mode 100644
index 000000000000..5c441779554f
--- /dev/null
+++ b/drivers/nvme/host/debugfs.c
@@ -0,0 +1,122 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2025 IBM Corporation
+ *	Nilay Shroff <nilay@linux.ibm.com>
+ */
+
+#include <linux/debugfs.h>
+#include <linux/seq_file.h>
+
+#include "nvme.h"
+
+struct nvme_debugfs_attr {
+	const char *name;
+	umode_t mode;
+	int (*show)(void *data, struct seq_file *m);
+	const struct seq_operations *seq_ops;
+};
+
+struct nvme_debugfs_ctx {
+	void *data;
+	struct nvme_debugfs_attr *attr;
+	int srcu_idx;
+};
+
+static int nvme_debugfs_show(struct seq_file *m, void *v)
+{
+	struct nvme_debugfs_ctx *ctx = m->private;
+	void *data = ctx->data;
+	struct nvme_debugfs_attr *attr = ctx->attr;
+
+	return attr->show(data, m);
+}
+
+static int nvme_debugfs_open(struct inode *inode, struct file *file)
+{
+	void *data = inode->i_private;
+	struct nvme_debugfs_attr *attr = debugfs_get_aux(file);
+	struct nvme_debugfs_ctx *ctx;
+	struct seq_file *m;
+	int ret;
+
+	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
+	if (WARN_ON_ONCE(!ctx))
+		return -ENOMEM;
+
+	ctx->data = data;
+	ctx->attr = attr;
+
+	if (attr->seq_ops) {
+		ret = seq_open(file, attr->seq_ops);
+		if (ret) {
+			kfree(ctx);
+			return ret;
+		}
+		m = file->private_data;
+		m->private = ctx;
+		return ret;
+	}
+
+	if (WARN_ON_ONCE(!attr->show)) {
+		kfree(ctx);
+		return -EPERM;
+	}
+
+	return single_open(file, nvme_debugfs_show, ctx);
+}
+
+static int nvme_debugfs_release(struct inode *inode, struct file *file)
+{
+	struct seq_file *m = file->private_data;
+	struct nvme_debugfs_ctx *ctx = m->private;
+	struct nvme_debugfs_attr *attr = ctx->attr;
+	int ret;
+
+	if (attr->seq_ops)
+		ret = seq_release(inode, file);
+	else
+		ret = single_release(inode, file);
+
+	kfree(ctx);
+	return ret;
+}
+
+static const struct file_operations nvme_debugfs_fops = {
+	.owner   = THIS_MODULE,
+	.open    = nvme_debugfs_open,
+	.read    = seq_read,
+	.llseek  = seq_lseek,
+	.release = nvme_debugfs_release,
+};
+
+
+static const struct nvme_debugfs_attr nvme_mpath_debugfs_attrs[] = {
+	{},
+};
+
+static const struct nvme_debugfs_attr nvme_ns_debugfs_attrs[] = {
+	{},
+};
+
+static void nvme_debugfs_create_files(struct request_queue *q,
+		const struct nvme_debugfs_attr *attr, void *data)
+{
+	if (WARN_ON_ONCE(!q->debugfs_dir))
+		return;
+
+	for (; attr->name; attr++)
+		debugfs_create_file_aux(attr->name, attr->mode, q->debugfs_dir,
+				data, (void *)attr, &nvme_debugfs_fops);
+}
+
+void nvme_debugfs_register(struct gendisk *disk)
+{
+	const struct nvme_debugfs_attr *attr;
+
+	if (nvme_disk_is_ns_head(disk))
+		attr = nvme_mpath_debugfs_attrs;
+	else
+		attr = nvme_ns_debugfs_attrs;
+
+	nvme_debugfs_create_files(disk->queue, attr, disk->private_data);
+}
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index bc87dd80de62..fae8e0b34915 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -977,6 +977,16 @@ extern const struct block_device_operations nvme_bdev_ops;
 
 void nvme_delete_ctrl_sync(struct nvme_ctrl *ctrl);
 struct nvme_ns *nvme_find_path(struct nvme_ns_head *head, unsigned int rw);
+void nvme_debugfs_register(struct gendisk *disk);
+static inline void nvme_debugfs_unregister(struct gendisk *disk)
+{
+	/*
+	 * Nothing to do for now. When the request queue is unregistered,
+	 * all files under q->debugfs_dir are recursively deleted.
+	 * This is just a placeholder; the compiler will optimize it out.
+	 */
+}
+
 #ifdef CONFIG_NVME_MULTIPATH
 static inline bool nvme_ctrl_use_ana(struct nvme_ctrl *ctrl)
 {
-- 
2.51.0

[RFC PATCHv2 4/4] nvme-multipath: add debugfs attribute for adaptive I/O policy stat

[Nilay Shroff]

This commit introduces a new debugfs attribute, "adaptive_stat", under
both per-path and head debugfs directories (defined under /sys/kernel/
debug/block/). This attribute provides visibility into the internal
state of the adaptive I/O policy to aid in debugging and performance
analysis.

For per-path entries, "adaptive_stat" reports the corresponding path
statistics such as I/O weight, selection count, processed samples, and
ignored samples.

For head entries, it reports per-CPU statistics for each reachable path,
including I/O weight, path score, smoothed (EWMA) latency, selection
count, processed samples, and ignored samples.

These additions enhance observability of the adaptive I/O path selection
behavior and help diagnose imbalance or instability in multipath
performance.

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 drivers/nvme/host/core.c      |   3 +
 drivers/nvme/host/debugfs.c   | 117 ++++++++++++++++++++++++++++++++++
 drivers/nvme/host/multipath.c |   2 +
 3 files changed, 122 insertions(+)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index c7f21823c137..5db716186ec0 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -4184,6 +4184,8 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, struct nvme_ns_info *info)
 	if (device_add_disk(ctrl->device, ns->disk, nvme_ns_attr_groups))
 		goto out_cleanup_ns_from_list;
 
+	nvme_debugfs_register(ns->disk);
+
 	if (!nvme_ns_head_multipath(ns->head))
 		nvme_add_ns_cdev(ns);
 
@@ -4271,6 +4273,7 @@ static void nvme_ns_remove(struct nvme_ns *ns)
 
 	nvme_mpath_remove_sysfs_link(ns);
 
+	nvme_debugfs_unregister(ns->disk);
 	del_gendisk(ns->disk);
 
 	mutex_lock(&ns->ctrl->namespaces_lock);
diff --git a/drivers/nvme/host/debugfs.c b/drivers/nvme/host/debugfs.c
index 5c441779554f..2e7ebb0199bf 100644
--- a/drivers/nvme/host/debugfs.c
+++ b/drivers/nvme/host/debugfs.c
@@ -89,12 +89,129 @@ static const struct file_operations nvme_debugfs_fops = {
 	.release = nvme_debugfs_release,
 };
 
+static void *nvme_mpath_adp_stat_start(struct seq_file *m, loff_t *pos)
+{
+	struct nvme_ns *ns;
+	struct nvme_debugfs_ctx *ctx = m->private;
+	struct nvme_ns_head *head = ctx->data;
+
+	/* Remember srcu index, so we can unlock later. */
+	ctx->srcu_idx = srcu_read_lock(&head->srcu);
+	ns = list_first_or_null_rcu(&head->list, struct nvme_ns, siblings);
+
+	while (*pos && ns) {
+		ns = list_next_or_null_rcu(&head->list, &ns->siblings,
+				struct nvme_ns, siblings);
+		(*pos)--;
+	}
+
+	return ns;
+}
+
+static void *nvme_mpath_adp_stat_next(struct seq_file *m, void *v, loff_t *pos)
+{
+	struct nvme_ns *ns = v;
+	struct nvme_debugfs_ctx *ctx = m->private;
+	struct nvme_ns_head *head = ctx->data;
+
+	(*pos)++;
+
+	return list_next_or_null_rcu(&head->list, &ns->siblings,
+			struct nvme_ns, siblings);
+}
+
+static void nvme_mpath_adp_stat_stop(struct seq_file *m, void *v)
+{
+	struct nvme_debugfs_ctx *ctx = m->private;
+	struct nvme_ns_head *head = ctx->data;
+	int srcu_idx = ctx->srcu_idx;
+
+	srcu_read_unlock(&head->srcu, srcu_idx);
+}
+
+static int nvme_mpath_adp_stat_show(struct seq_file *m, void *v)
+{
+#ifdef CONFIG_NVME_MULTIPATH
+	int cpu, rw;
+	struct nvme_path_stat *stat;
+	struct nvme_ns *ns = v;
+
+	seq_printf(m, "%s:\n", ns->disk->disk_name);
+	for_each_online_cpu(cpu) {
+		seq_printf(m, "cpu %d : ", cpu);
+		for (rw = 0; rw < 2; rw++) {
+			stat = &per_cpu_ptr(ns->info, cpu)[rw].stat;
+			seq_printf(m, "%u %llu %llu %llu %llu %llu ",
+				stat->weight, stat->score,
+				stat->slat_ns, stat->sel,
+				stat->nr_samples, stat->nr_ignored);
+		}
+		seq_putc(m, '\n');
+	}
+#endif
+	return 0;
+}
+
+static const struct seq_operations nvme_mpath_adp_stat_seq_ops = {
+	.start = nvme_mpath_adp_stat_start,
+	.next  = nvme_mpath_adp_stat_next,
+	.stop  = nvme_mpath_adp_stat_stop,
+	.show  = nvme_mpath_adp_stat_show
+};
 
 static const struct nvme_debugfs_attr nvme_mpath_debugfs_attrs[] = {
+	{"adaptive_stat", 0400, .seq_ops = &nvme_mpath_adp_stat_seq_ops},
 	{},
 };
 
+static void adp_stat_read_all(struct nvme_ns *ns, struct nvme_path_stat *batch)
+{
+#ifdef CONFIG_NVME_MULTIPATH
+	int rw, cpu;
+	u32 ncpu[2] = {0};
+	struct nvme_path_stat *stat;
+
+	for_each_online_cpu(cpu) {
+		for (rw = 0; rw < 2; rw++) {
+			stat = &per_cpu_ptr(ns->info, cpu)[rw].stat;
+			if (stat->weight) {
+				batch[rw].weight += stat->weight;
+				batch[rw].sel += stat->sel;
+				batch[rw].nr_samples += stat->nr_samples;
+				batch[rw].nr_ignored += stat->nr_ignored;
+				ncpu[rw]++;
+			}
+		}
+	}
+
+	for (rw = 0; rw < 2; rw++) {
+		if (!ncpu[rw])
+			continue;
+		batch[rw].weight = DIV_U64_ROUND_CLOSEST(batch[rw].weight,
+				ncpu[rw]);
+	}
+#endif
+}
+static int nvme_ns_adp_stat_show(void *data, struct seq_file *m)
+{
+	struct nvme_path_stat stat[2] = {0};
+	struct nvme_ns *ns = (struct nvme_ns *)data;
+
+	adp_stat_read_all(ns, stat);
+	seq_printf(m, "%u %llu %llu %llu %u %llu %llu %llu\n",
+		stat[READ].weight,
+		stat[READ].sel,
+		stat[READ].nr_samples,
+		stat[READ].nr_ignored,
+		stat[WRITE].weight,
+		stat[WRITE].sel,
+		stat[WRITE].nr_samples,
+		stat[WRITE].nr_ignored);
+	return 0;
+}
+
 static const struct nvme_debugfs_attr nvme_ns_debugfs_attrs[] = {
+	{"adaptive_stat", 0400, nvme_ns_adp_stat_show},
 	{},
 };
 
diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 9ecdaca5e9a0..26495696e24e 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -1059,6 +1059,7 @@ static void nvme_remove_head(struct nvme_ns_head *head)
 
 		nvme_cdev_del(&head->cdev, &head->cdev_device);
 		synchronize_srcu(&head->srcu);
+		nvme_debugfs_unregister(head->disk);
 		del_gendisk(head->disk);
 	}
 	nvme_put_ns_head(head);
@@ -1162,6 +1163,7 @@ static void nvme_mpath_set_live(struct nvme_ns *ns)
 		}
 		nvme_add_ns_head_cdev(head);
 		kblockd_schedule_work(&head->partition_scan_work);
+		nvme_debugfs_register(head->disk);
 	}
 
 	nvme_mpath_add_sysfs_link(ns->head);
-- 
2.51.0

Re: [RFC PATCHv2 1/4] block: expose blk_stat_{enable,disable}_accounting() to drivers

[Hannes Reinecke]

On 10/9/25 12:05, Nilay Shroff wrote:
> The functions blk_stat_enable_accounting() and
> blk_stat_disable_accounting() are currently exported, but their
> prototypes are only defined in a private header. Move these prototypes
> into a common header so that block drivers can directly use these APIs.
> 
> Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
> ---
>   block/blk-stat.h       | 4 ----
>   include/linux/blk-mq.h | 4 ++++
>   2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/block/blk-stat.h b/block/blk-stat.h
> index 9e05bf18d1be..f5d95dd8c0e9 100644
> --- a/block/blk-stat.h
> +++ b/block/blk-stat.h
> @@ -67,10 +67,6 @@ void blk_free_queue_stats(struct blk_queue_stats *);
>   
>   void blk_stat_add(struct request *rq, u64 now);
>   
> -/* record time/size info in request but not add a callback */
> -void blk_stat_enable_accounting(struct request_queue *q);
> -void blk_stat_disable_accounting(struct request_queue *q);
> -
>   /**
>    * blk_stat_alloc_callback() - Allocate a block statistics callback.
>    * @timer_fn: Timer callback function.
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index b25d12545f46..f647444643b8 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -735,6 +735,10 @@ int blk_rq_poll(struct request *rq, struct io_comp_batch *iob,
>   
>   bool blk_mq_queue_inflight(struct request_queue *q);
>   
> +/* record time/size info in request but not add a callback */
> +void blk_stat_enable_accounting(struct request_queue *q);
> +void blk_stat_disable_accounting(struct request_queue *q);
> +
>   enum {
>   	/* return when out of requests */
>   	BLK_MQ_REQ_NOWAIT	= (__force blk_mq_req_flags_t)(1 << 0),

Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich

Re: [RFC PATCHv2 2/4] nvme-multipath: add support for adaptive I/O policy

[Hannes Reinecke]

On 10/9/25 12:05, Nilay Shroff wrote:
> This commit introduces a new I/O policy named "adaptive". Users can
> configure it by writing "adaptive" to "/sys/class/nvme-subsystem/nvme-
> subsystemX/iopolicy"
> 
> The adaptive policy dynamically distributes I/O based on measured
> completion latency. The main idea is to calculate latency for each path,
> derive a weight, and then proportionally forward I/O according to those
> weights.
> 
> To ensure scalability, path latency is measured per-CPU. Each CPU
> maintains its own statistics, and I/O forwarding uses these per-CPU
> values. Every ~15 seconds, a simple average latency of per-CPU batched
> samples are computed and fed into an Exponentially Weighted Moving
> Average (EWMA):
> 
> avg_latency = div_u64(batch, batch_count);
> new_ewma_latency = (prev_ewma_latency * (WEIGHT-1) + avg_latency)/WEIGHT
> 
> With WEIGHT = 8, this assigns 7/8 (~87.5%) weight to the previous
> latency value and 1/8 (~12.5%) to the most recent latency. This
> smoothing reduces jitter, adapts quickly to changing conditions,
> avoids storing historical samples, and works well for both low and
> high I/O rates. Path weights are then derived from the smoothed (EWMA)
> latency as follows (example with two paths A and B):
> 
>      path_A_score = NSEC_PER_SEC / path_A_ewma_latency
>      path_B_score = NSEC_PER_SEC / path_B_ewma_latency
>      total_score  = path_A_score + path_B_score
> 
>      path_A_weight = (path_A_score * 100) / total_score
>      path_B_weight = (path_B_score * 100) / total_score
> 
> where:
>    - path_X_ewma_latency is the smoothed latency of a path in nanoseconds
>    - NSEC_PER_SEC is used as a scaling factor since valid latencies
>      are < 1 second
>    - weights are normalized to a 0–100 scale across all paths.
> 
> Path credits are refilled based on this weight, with one credit
> consumed per I/O. When all credits are consumed, the credits are
> refilled again based on the current weight. This ensures that I/O is
> distributed across paths proportionally to their calculated weight.
> 
> Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
> ---
>   drivers/nvme/host/core.c      |  10 +-
>   drivers/nvme/host/ioctl.c     |   7 +-
>   drivers/nvme/host/multipath.c | 397 ++++++++++++++++++++++++++++++++--
>   drivers/nvme/host/nvme.h      |  44 +++-
>   drivers/nvme/host/pr.c        |   6 +-
>   drivers/nvme/host/sysfs.c     |   2 +-
>   6 files changed, 444 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index fa4181d7de73..c7f21823c137 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -689,6 +689,7 @@ static void nvme_free_ns(struct kref *kref)
>   {
>   	struct nvme_ns *ns = container_of(kref, struct nvme_ns, kref);
>   
> +	nvme_free_ns_stat(ns);
>   	put_disk(ns->disk);
>   	nvme_put_ns_head(ns->head);
>   	nvme_put_ctrl(ns->ctrl);
> @@ -4137,6 +4138,9 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, struct nvme_ns_info *info)
>   	if (nvme_init_ns_head(ns, info))
>   		goto out_cleanup_disk;
>   
> +	if (nvme_alloc_ns_stat(ns))
> +		goto out_unlink_ns;
> +
>   	/*
>   	 * If multipathing is enabled, the device name for all disks and not
>   	 * just those that represent shared namespaces needs to be based on the
> @@ -4161,7 +4165,7 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, struct nvme_ns_info *info)
>   	}
>   
>   	if (nvme_update_ns_info(ns, info))
> -		goto out_unlink_ns;
> +		goto out_free_ns_stat;
>   
>   	mutex_lock(&ctrl->namespaces_lock);
>   	/*
> @@ -4170,7 +4174,7 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, struct nvme_ns_info *info)
>   	 */
>   	if (test_bit(NVME_CTRL_FROZEN, &ctrl->flags)) {
>   		mutex_unlock(&ctrl->namespaces_lock);
> -		goto out_unlink_ns;
> +		goto out_free_ns_stat;
>   	}
>   	nvme_ns_add_to_ctrl_list(ns);
>   	mutex_unlock(&ctrl->namespaces_lock);
> @@ -4201,6 +4205,8 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, struct nvme_ns_info *info)
>   	list_del_rcu(&ns->list);
>   	mutex_unlock(&ctrl->namespaces_lock);
>   	synchronize_srcu(&ctrl->srcu);
> +out_free_ns_stat:
> +	nvme_free_ns_stat(ns);
>    out_unlink_ns:
>   	mutex_lock(&ctrl->subsys->lock);
>   	list_del_rcu(&ns->siblings);
> diff --git a/drivers/nvme/host/ioctl.c b/drivers/nvme/host/ioctl.c
> index c212fa952c0f..69d2f72d0e86 100644
> --- a/drivers/nvme/host/ioctl.c
> +++ b/drivers/nvme/host/ioctl.c
> @@ -711,7 +711,7 @@ int nvme_ns_head_ioctl(struct block_device *bdev, blk_mode_t mode,
>   		flags |= NVME_IOCTL_PARTITION;
>   
>   	srcu_idx = srcu_read_lock(&head->srcu);
> -	ns = nvme_find_path(head);
> +	ns = nvme_find_path(head, open_for_write ? WRITE : READ);
>   	if (!ns)
>   		goto out_unlock;
>   
> @@ -742,7 +742,7 @@ long nvme_ns_head_chr_ioctl(struct file *file, unsigned int cmd,
>   	int srcu_idx, ret = -EWOULDBLOCK;
>   
>   	srcu_idx = srcu_read_lock(&head->srcu);
> -	ns = nvme_find_path(head);
> +	ns = nvme_find_path(head, open_for_write ? WRITE : READ);
>   	if (!ns)
>   		goto out_unlock;
>   
Are we sure that we should account for non-IO commands here, too?
Thing is, the I/O policy should be just that, directing I/O to the
various paths.
But what about commands on the admin queue? Should they be influenced by
the same policy? And can we even define what a 'read' command is?
Wouldn't it be better to pass in a third option here (NONE?) to make it
clear that this is a non-I/O command?

> @@ -762,7 +762,8 @@ int nvme_ns_head_chr_uring_cmd(struct io_uring_cmd *ioucmd,
>   	struct cdev *cdev = file_inode(ioucmd->file)->i_cdev;
>   	struct nvme_ns_head *head = container_of(cdev, struct nvme_ns_head, cdev);
>   	int srcu_idx = srcu_read_lock(&head->srcu);
> -	struct nvme_ns *ns = nvme_find_path(head);
> +	struct nvme_ns *ns = nvme_find_path(head,
> +			ioucmd->file->f_mode & FMODE_WRITE ? WRITE : READ);
>   	int ret = -EINVAL;
>   
>   	if (ns)

See above. I really think that we should be able to pass in a third
option for admin commands.

> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
> index 3da980dc60d9..9ecdaca5e9a0 100644
> --- a/drivers/nvme/host/multipath.c
> +++ b/drivers/nvme/host/multipath.c
> @@ -6,6 +6,8 @@
>   #include <linux/backing-dev.h>
>   #include <linux/moduleparam.h>
>   #include <linux/vmalloc.h>
> +#include <linux/blk-mq.h>
> +#include <linux/math64.h>
>   #include <trace/events/block.h>
>   #include "nvme.h"
>  
Ouch.
#include <math64.h>
always spells disaster :-)> @@ -66,9 +68,10 @@ 
MODULE_PARM_DESC(multipath_always_on,
>   	"create multipath node always except for private namespace with non-unique nsid; note that this also implicitly enables native multipath support");
>   
>   static const char *nvme_iopolicy_names[] = {
> -	[NVME_IOPOLICY_NUMA]	= "numa",
> -	[NVME_IOPOLICY_RR]	= "round-robin",
> -	[NVME_IOPOLICY_QD]      = "queue-depth",
> +	[NVME_IOPOLICY_NUMA]	 = "numa",
> +	[NVME_IOPOLICY_RR]	 = "round-robin",
> +	[NVME_IOPOLICY_QD]       = "queue-depth",
> +	[NVME_IOPOLICY_ADAPTIVE] = "adaptive",
>   };
>   
>   static int iopolicy = NVME_IOPOLICY_NUMA;
> @@ -83,6 +86,8 @@ static int nvme_set_iopolicy(const char *val, const struct kernel_param *kp)
>   		iopolicy = NVME_IOPOLICY_RR;
>   	else if (!strncmp(val, "queue-depth", 11))
>   		iopolicy = NVME_IOPOLICY_QD;
> +	else if (!strncmp(val, "adaptive", 8))
> +		iopolicy = NVME_IOPOLICY_ADAPTIVE;
>   	else
>   		return -EINVAL;
>   
> @@ -196,6 +201,221 @@ void nvme_mpath_start_request(struct request *rq)
>   }
>   EXPORT_SYMBOL_GPL(nvme_mpath_start_request);
>   
> +static void nvme_mpath_weight_work(struct work_struct *weight_work)
> +{
> +	int cpu, srcu_idx;
> +	u32 weight;
> +	struct nvme_ns *ns;
> +	struct nvme_path_stat *stat;
> +	struct nvme_path_work *work = container_of(weight_work,
> +			struct nvme_path_work, weight_work);
> +	struct nvme_ns_head *head = work->ns->head;
> +	int rw = work->rw;
> +	u64 total_score = 0;
> +
> +	cpu = get_cpu();
> +
> +	srcu_idx = srcu_read_lock(&head->srcu);
> +	list_for_each_entry_srcu(ns, &head->list, siblings,
> +			srcu_read_lock_held(&head->srcu)) {
> +
> +		stat = &this_cpu_ptr(ns->info)[rw].stat;
> +		if (!READ_ONCE(stat->slat_ns))
> +			continue;
> +		/*
> +		 * Compute the path score as the inverse of smoothed
> +		 * latency, scaled by NSEC_PER_SEC. Floating point
> +		 * math is unavailable in the kernel, so fixed-point
> +		 * scaling is used instead. NSEC_PER_SEC is chosen
> +		 * because valid latencies are always < 1 second; longer
> +		 * latencies are ignored.
> +		 */
> +		stat->score = div_u64(NSEC_PER_SEC, READ_ONCE(stat->slat_ns));
> +
> +		/* Compute total score. */
> +		total_score += stat->score;
> +	}
> +
> +	if (!total_score)
> +		goto out;
> +
> +	/*
> +	 * After computing the total slatency, we derive per-path weight
> +	 * (normalized to the range 0–100). The weight represents the
> +	 * relative share of I/O the path should receive.

Why do we use the range 0-100?
While this is nice for human consumption, it doesn't lend itself to a
nice computation. Maybe we should select a different scale to allow us
for more efficient computation (0-128? Maybe?)

> +	 *
> +	 *   - lower smoothed latency -> higher weight
> +	 *   - higher smoothed slatency -> lower weight
> +	 *
> +	 * Next, while forwarding I/O, we assign "credits" to each path
> +	 * based on its weight (please also refer nvme_adaptive_path()):
> +	 *   - Initially, credits = weight.
> +	 *   - Each time an I/O is dispatched on a path, its credits are
> +	 *     decremented proportionally.
> +	 *   - When a path runs out of credits, it becomes temporarily
> +	 *     ineligible until credit is refilled.
> +	 *
> +	 * I/O distribution is therefore governed by available credits,
> +	 * ensuring that over time the proportion of I/O sent to each
> +	 * path matches its weight (and thus its performance).
> +	 */
> +	list_for_each_entry_srcu(ns, &head->list, siblings,
> +			srcu_read_lock_held(&head->srcu)) {
> +
> +		stat = &this_cpu_ptr(ns->info)[rw].stat;
> +		weight = div_u64(stat->score * 100, total_score);
> +
> +		/*
> +		 * Ensure the path weight never drops below 1. A weight
> +		 * of 0 is used only for newly added paths. During
> +		 * bootstrap, a few I/Os are sent to such paths to
> +		 * establish an initial weight. Enforcing a minimum
> +		 * weight of 1 guarantees that no path is forgotten and
> +		 * that each path is probed at least occasionally.
> +		 */
> +		if (!weight)
> +			weight = 1;
> +
> +		WRITE_ONCE(stat->weight, weight);
> +		stat->score = 0;
> +	}
> +out:
> +	srcu_read_unlock(&head->srcu, srcu_idx);
> +	put_cpu();
> +}
> +
> +#define NVME_EWMA_SHIFT	3

And we use this value why?

> +static inline u64 ewma_update(u64 old, u64 new)
> +{
> +	return (old * ((1 << NVME_EWMA_SHIFT) - 1) + new) >> NVME_EWMA_SHIFT;
> +}
> +
> +static void nvme_mpath_add_sample(struct request *rq, struct nvme_ns *ns)
> +{
> +	int cpu;
> +	unsigned int rw;
> +	struct nvme_path_info *info;
> +	struct nvme_path_stat *stat;
> +	u64 now, latency, slat_ns, avg_lat_ns;
> +	struct nvme_ns_head *head = ns->head;
> +
> +	if (list_is_singular(&head->list))
> +		return;
> +
> +	now = ktime_get_ns();
> +	latency = now >= rq->io_start_time_ns ? now - rq->io_start_time_ns : 0;
> +	if (!latency)
> +		return;
> +
> +	/*
> +	 * As completion code path is serialized(i.e. no same completion queue
> +	 * update code could run simultaneously on multiple cpu) we can safely
> +	 * access per cpu nvme path stat here from another cpu (in case the
> +	 * completion cpu is different from submission cpu).
> +	 * The only field which could be accessed simultaneously here is the
> +	 * path ->weight which may be accessed by this function as well as I/O
> +	 * submission path during path selection logic and we protect ->weight
> +	 * using READ_ONCE/WRITE_ONCE. Yes this may not be 100% accurate but
> +	 * we also don't need to be so accurate here as the path credit would
> +	 * be anyways refilled, based on path weight, once path consumes all
> +	 * its credits. And we limit path weight/credit max up to 100. Please
> +	 * also refer nvme_adaptive_path().
> +	 */
> +	cpu = blk_mq_rq_cpu(rq);
> +	rw = rq_data_dir(rq);
> +	info = &per_cpu_ptr(ns->info, cpu)[rw];
> +	stat = &info->stat;
> +
Hmm. While the 'SAME_CONP' attribute should help here, I remain
sceptical.
Wouldn't it be possible to look at the hwq map (eg by using something
like blk_mq_map_queue_type()) to figure out the hw context, and then
use the first cpu from that mask?
That way we are guaranteed to always using the same per-cpu ptr.
Might be overkill, though; but it would be good to have some
checks in here in case we do run on the wrong CPU.

> +	/*
> +	 * If latency > ~1s then ignore this sample to prevent EWMA from being
> +	 * skewed by pathological outliers (multi-second waits, controller
> +	 * timeouts etc.). This keeps path scores representative of normal
> +	 * performance and avoids instability from rare spikes. If such high
> +	 * latency is real, ANA state reporting or keep-alive error counters
> +	 * will mark the path unhealthy and remove it from the head node list,
> +	 * so we safely skip such sample here.
> +	 */
> +	if (unlikely(latency > NSEC_PER_SEC)) {
> +		stat->nr_ignored++;
> +		return;
> +	}

I would even go so far as to indicate that EWMA is unusable here.
Can we issue a warning / debug message here to indicate that the path
selector is running suboptimal?

> +
> +	/*
> +	 * Accumulate latency samples and increment the batch count for each
> +	 * ~15 second interval. When the interval expires, compute the simple
> +	 * average latency over that window, then update the smoothed (EWMA)
> +	 * latency. The path weight is recalculated based on this smoothed
> +	 * latency.
> +	 */

Why 15 seconds? Can we make this changeable eg via debugfs?

> +	stat->batch += latency;
> +	stat->batch_count++;
> +	stat->nr_samples++;
> +
> +	if (now > stat->last_weight_ts &&
> +			(now - stat->last_weight_ts) >= 15 * NSEC_PER_SEC) {
> +

At the very least make this a #define. But ideally one should be able
to modify that.

> +		stat->last_weight_ts = now;
> +
> +		/*
> +		 * Find simple average latency for the last epoch (~15 sec
> +		 * interval).
> +		 */
> +		avg_lat_ns = div_u64(stat->batch, stat->batch_count);
> +
> +		/*
> +		 * Calculate smooth/EWMA (Exponentially Weighted Moving Average)
> +		 * latency. EWMA is preferred over simple average latency
> +		 * because it smooths naturally, reduces jitter from sudden
> +		 * spikes, and adapts faster to changing conditions. It also
> +		 * avoids storing historical samples, and works well for both
> +		 * slow and fast I/O rates.
> +		 * Formula:
> +		 * slat_ns = (prev_slat_ns * (WEIGHT - 1) + (latency)) / WEIGHT
> +		 * With WEIGHT = 8, this assigns 7/8 (~87.5 %) weight to the
> +		 * existing latency and 1/8 (~12.5%) weight to the new latency.
> +		 */

Similar comment to the WEIGHT. While 8 might be a nice choice, there is 
nothing indicating it's always the right choice.
Please make it a #define and see if we cannot modify it via debugfs.

> +		if (unlikely(!stat->slat_ns))
> +			WRITE_ONCE(stat->slat_ns, avg_lat_ns);
> +		else {
> +			slat_ns = ewma_update(stat->slat_ns, avg_lat_ns);
> +			WRITE_ONCE(stat->slat_ns, slat_ns);
> +		}
> +
> +		stat->batch = stat->batch_count = 0;
> +
> +		/*
> +		 * Defer calculation of the path weight in per-cpu workqueue.
> +		 */
> +		schedule_work_on(cpu, &info->work.weight_work);
> +	}
> +}
> +
> +int nvme_alloc_ns_stat(struct nvme_ns *ns)
> +{
> +	int rw, cpu;
> +	struct nvme_path_work *work;
> +	gfp_t gfp = GFP_KERNEL | __GFP_ZERO;
> +
> +	if (!ns->head->disk)
> +		return 0;
> +
> +	ns->info = __alloc_percpu_gfp(2 * sizeof(struct nvme_path_info),
> +			__alignof__(struct nvme_path_info), gfp);
> +	if (!ns->info)
> +		return -ENOMEM;
> +
> +	for_each_possible_cpu(cpu) {
> +		for (rw = 0; rw < 2; rw++) {
> +			work = &per_cpu_ptr(ns->info, cpu)[rw].work;
> +			work->ns = ns;
> +			work->rw = rw;
> +			INIT_WORK(&work->weight_work, nvme_mpath_weight_work);
> +		}
> +	}
> +
> +	return 0;
> +}
> +

The more I think about it the more I like the idea of having a third
direction (and not just READ and WRITE).
The latency of I/O commands _will_ be different from the latency of
admin commands, so the adaptive policy might come to different decisions
for admin commands.

>   void nvme_mpath_end_request(struct request *rq)
>   {
>   	struct nvme_ns *ns = rq->q->queuedata;
> @@ -203,6 +423,9 @@ void nvme_mpath_end_request(struct request *rq)
>   	if (nvme_req(rq)->flags & NVME_MPATH_CNT_ACTIVE)
>   		atomic_dec_if_positive(&ns->ctrl->nr_active);
>   
> +	if (test_bit(NVME_NS_PATH_STAT, &ns->flags))
> +		nvme_mpath_add_sample(rq, ns);
> +
>   	if (!(nvme_req(rq)->flags & NVME_MPATH_IO_STATS))
>   		return;
>   	bdev_end_io_acct(ns->head->disk->part0, req_op(rq),
> @@ -236,6 +459,54 @@ static const char *nvme_ana_state_names[] = {
>   	[NVME_ANA_CHANGE]		= "change",
>   };
>   
> +static void nvme_mpath_reset_current_stat(struct nvme_ns *ns)
> +{
> +	int rw, cpu;
> +	struct nvme_path_stat *stat;
> +
> +	for_each_possible_cpu(cpu) {
> +		for (rw = 0; rw < 2; rw++) {
> +			stat = &per_cpu_ptr(ns->info, cpu)[rw].stat;
> +			memset(stat, 0, sizeof(struct nvme_path_stat));
> +		}
> +	}
> +}
> +
> +static bool nvme_mpath_set_current_adaptive_path(struct nvme_ns *ns)
> +{
> +	struct nvme_ns_head *head = ns->head;
> +
> +	if (!head->disk || head->subsys->iopolicy != NVME_IOPOLICY_ADAPTIVE)
> +		return false;
> +
> +	if (test_and_set_bit(NVME_NS_PATH_STAT, &ns->flags))
> +		return false;
> +
> +	blk_queue_flag_set(QUEUE_FLAG_SAME_FORCE, ns->queue);
> +	blk_stat_enable_accounting(ns->queue);
> +	return true;
> +}
> +
> +static bool nvme_mpath_clear_current_adaptive_path(struct nvme_ns *ns)
> +{
> +	int rw, cpu;
> +	struct nvme_path_info *info;
> +
> +	if (!test_and_clear_bit(NVME_NS_PATH_STAT, &ns->flags))
> +		return false;
> +
> +	blk_stat_disable_accounting(ns->queue);
> +	blk_queue_flag_clear(QUEUE_FLAG_SAME_FORCE, ns->queue);
> +	for_each_online_cpu(cpu) {
> +		for (rw = 0; rw < 2; rw++) {
> +			info = &per_cpu_ptr(ns->info, cpu)[rw];
> +			cancel_work_sync(&info->work.weight_work);
> +		}
> +	}
> +	nvme_mpath_reset_current_stat(ns);
> +	return true;
> +}
> +
>   bool nvme_mpath_clear_current_path(struct nvme_ns *ns)
>   {
>   	struct nvme_ns_head *head = ns->head;
> @@ -251,6 +522,8 @@ bool nvme_mpath_clear_current_path(struct nvme_ns *ns)
>   			changed = true;
>   		}
>   	}
> +	if (nvme_mpath_clear_current_adaptive_path(ns))
> +		changed = true;
>   out:
>   	return changed;
>   }
> @@ -269,6 +542,18 @@ void nvme_mpath_clear_ctrl_paths(struct nvme_ctrl *ctrl)
>   	srcu_read_unlock(&ctrl->srcu, srcu_idx);
>   }
>   
> +static void nvme_mpath_set_ctrl_paths(struct nvme_ctrl *ctrl)
> +{
> +	struct nvme_ns *ns;
> +	int srcu_idx;
> +
> +	srcu_idx = srcu_read_lock(&ctrl->srcu);
> +	list_for_each_entry_srcu(ns, &ctrl->namespaces, list,
> +				srcu_read_lock_held(&ctrl->srcu))
> +		nvme_mpath_set_current_adaptive_path(ns);
> +	srcu_read_unlock(&ctrl->srcu, srcu_idx);
> +}
> +
>   void nvme_mpath_revalidate_paths(struct nvme_ns *ns)
>   {
>   	struct nvme_ns_head *head = ns->head;
> @@ -281,6 +566,8 @@ void nvme_mpath_revalidate_paths(struct nvme_ns *ns)
>   				 srcu_read_lock_held(&head->srcu)) {
>   		if (capacity != get_capacity(ns->disk))
>   			clear_bit(NVME_NS_READY, &ns->flags);
> +
> +		nvme_mpath_reset_current_stat(ns);
>   	}
>   	srcu_read_unlock(&head->srcu, srcu_idx);
>   
> @@ -405,6 +692,86 @@ static struct nvme_ns *nvme_round_robin_path(struct nvme_ns_head *head)
>   	return found;
>   }
>   
> +static inline bool nvme_state_is_live(enum nvme_ana_state state)
> +{
> +	return state == NVME_ANA_OPTIMIZED || state == NVME_ANA_NONOPTIMIZED;
> +}
> +
> +static struct nvme_ns *nvme_adaptive_path(struct nvme_ns_head *head,
> +		unsigned int rw)
> +{
> +	struct nvme_ns *ns, *found = NULL;
> +	struct nvme_path_stat *stat;
> +	u32 weight;
> +	int refill = 0;
> +
> +	get_cpu();
> +retry:
> +	list_for_each_entry_srcu(ns, &head->list, siblings,
> +			srcu_read_lock_held(&head->srcu)) {
> +
> +		if (nvme_path_is_disabled(ns) ||
> +				!nvme_state_is_live(ns->ana_state))
> +			continue;
> +
> +		stat = &this_cpu_ptr(ns->info)[rw].stat;
> +
> +		/*
> +		 * When the head path-list is singular we don't calculate the
> +		 * only path weight for optimization as we don't need to forward
> +		 * I/O to more than one path. The another possibility is whenthe
> +		 * path is newly added, we don't know its weight. So we go round
> +		 * -robin for each such path and forward I/O to it.Once we start
> +		 * getting response for such I/Os, the path weight calculation
> +		 * would kick in and then we start using path credit for
> +		 * forwarding I/O.
> +		 */
> +		weight = READ_ONCE(stat->weight);
> +		if (unlikely(!weight)) {
> +			found = ns;
> +			goto out;
> +		}
> +
> +		/*
> +		 * To keep path selection logic simple, we don't distinguish
> +		 * between ANA optimized and non-optimized states. The non-
> +		 * optimized path is expected to have a lower weight, and
> +		 * therefore fewer credits. As a result, only a small number of
> +		 * I/Os will be forwarded to paths in the non-optimized state.
> +		 */
> +		if (stat->credit > 0) {
> +			--stat->credit;
> +			found = ns;
> +			goto out;
> +		}
> +	}
> +
 > +	if (!found && !list_empty(&head->list)) {> +		/*
> +		 * Refill credits and retry.
> +		 */
> +		list_for_each_entry_srcu(ns, &head->list, siblings,
> +				srcu_read_lock_held(&head->srcu)) {
> +			if (nvme_path_is_disabled(ns) ||
> +					!nvme_state_is_live(ns->ana_state))
> +				continue;
> +
> +			stat = &this_cpu_ptr(ns->info)[rw].stat;
> +			weight = READ_ONCE(stat->weight);
> +			stat->credit = weight;
> +			refill = 1;
> +		}
> +		if (refill)
> +			goto retry;
> +	}

Hmm. Loop within a loop. Not pretty.
Can't we make do with just one loop?
After all, if we end up here the credits for this path are use up, and 
need to be refilled.
What would happen if we were _just_ refill and retry?
IE drop the second iteration and just execute the branch
under the loop?

> +out:
> +	if (found)
> +		stat->sel++;
> +
> +	put_cpu();
> +	return found;
> +}
> +
>   static struct nvme_ns *nvme_queue_depth_path(struct nvme_ns_head *head)
>   {
>   	struct nvme_ns *best_opt = NULL, *best_nonopt = NULL, *ns;
> @@ -461,9 +828,12 @@ static struct nvme_ns *nvme_numa_path(struct nvme_ns_head *head)
>   	return ns;
>   }
>   
> -inline struct nvme_ns *nvme_find_path(struct nvme_ns_head *head)
> +inline struct nvme_ns *nvme_find_path(struct nvme_ns_head *head,
> +		unsigned int rw)
>   {
>   	switch (READ_ONCE(head->subsys->iopolicy)) {
> +	case NVME_IOPOLICY_ADAPTIVE:
> +		return nvme_adaptive_path(head, rw);
>   	case NVME_IOPOLICY_QD:
>   		return nvme_queue_depth_path(head);
>   	case NVME_IOPOLICY_RR:
> @@ -523,7 +893,7 @@ static void nvme_ns_head_submit_bio(struct bio *bio)
>   		return;
>   
>   	srcu_idx = srcu_read_lock(&head->srcu);
> -	ns = nvme_find_path(head);
> +	ns = nvme_find_path(head, bio_data_dir(bio));
>   	if (likely(ns)) {
>   		bio_set_dev(bio, ns->disk->part0);
>   		bio->bi_opf |= REQ_NVME_MPATH;
> @@ -565,7 +935,7 @@ static int nvme_ns_head_get_unique_id(struct gendisk *disk, u8 id[16],
>   	int srcu_idx, ret = -EWOULDBLOCK;
>   
>   	srcu_idx = srcu_read_lock(&head->srcu);
> -	ns = nvme_find_path(head);
> +	ns = nvme_find_path(head, READ);
>   	if (ns)
>   		ret = nvme_ns_get_unique_id(ns, id, type);
>   	srcu_read_unlock(&head->srcu, srcu_idx);
> @@ -581,7 +951,7 @@ static int nvme_ns_head_report_zones(struct gendisk *disk, sector_t sector,
>   	int srcu_idx, ret = -EWOULDBLOCK;
>   
>   	srcu_idx = srcu_read_lock(&head->srcu);
> -	ns = nvme_find_path(head);
> +	ns = nvme_find_path(head, READ);
>   	if (ns)
>   		ret = nvme_ns_report_zones(ns, sector, nr_zones, cb, data);
>   	srcu_read_unlock(&head->srcu, srcu_idx);

See the discussion about the third option and not just 'READ'/'WRITE'.

> @@ -807,6 +1177,10 @@ static void nvme_mpath_set_live(struct nvme_ns *ns)
>   	}
>   	mutex_unlock(&head->lock);
>   
> +	mutex_lock(&nvme_subsystems_lock);
> +	nvme_mpath_set_current_adaptive_path(ns);
> +	mutex_unlock(&nvme_subsystems_lock);
> +
>   	synchronize_srcu(&head->srcu);
>   	kblockd_schedule_work(&head->requeue_work);
>   }
> @@ -855,11 +1229,6 @@ static int nvme_parse_ana_log(struct nvme_ctrl *ctrl, void *data,
>   	return 0;
>   }
>   
> -static inline bool nvme_state_is_live(enum nvme_ana_state state)
> -{
> -	return state == NVME_ANA_OPTIMIZED || state == NVME_ANA_NONOPTIMIZED;
> -}
> -
>   static void nvme_update_ns_ana_state(struct nvme_ana_group_desc *desc,
>   		struct nvme_ns *ns)
>   {
> @@ -1037,10 +1406,12 @@ static void nvme_subsys_iopolicy_update(struct nvme_subsystem *subsys,
>   
>   	WRITE_ONCE(subsys->iopolicy, iopolicy);
>   
> -	/* iopolicy changes clear the mpath by design */
> +	/* iopolicy changes clear/reset the mpath by design */
>   	mutex_lock(&nvme_subsystems_lock);
>   	list_for_each_entry(ctrl, &subsys->ctrls, subsys_entry)
>   		nvme_mpath_clear_ctrl_paths(ctrl);
> +	list_for_each_entry(ctrl, &subsys->ctrls, subsys_entry)
> +		nvme_mpath_set_ctrl_paths(ctrl);
>   	mutex_unlock(&nvme_subsystems_lock);
>   
>   	pr_notice("subsysnqn %s iopolicy changed from %s to %s\n",
> diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
> index 102fae6a231c..bc87dd80de62 100644
> --- a/drivers/nvme/host/nvme.h
> +++ b/drivers/nvme/host/nvme.h
> @@ -421,6 +421,7 @@ enum nvme_iopolicy {
>   	NVME_IOPOLICY_NUMA,
>   	NVME_IOPOLICY_RR,
>   	NVME_IOPOLICY_QD,
> +	NVME_IOPOLICY_ADAPTIVE,
>   };
>   
>   struct nvme_subsystem {
> @@ -459,6 +460,30 @@ struct nvme_ns_ids {
>   	u8	csi;
>   };
>   
> +struct nvme_path_stat {
> +	u64 nr_samples;		/* total num of samples processed */
> +	u64 nr_ignored;		/* num. of samples ignored */
> +	u64 slat_ns;		/* smoothed (ewma) latency in nanoseconds */
> +	u64 score;		/* score used for weight calculation */
> +	u64 last_weight_ts;	/* timestamp of the last weight calculation */
> +	u64 sel;		/* num of times this path is selcted for I/O */
> +	u64 batch;		/* accumulated latency sum for current window */
> +	u32 batch_count;	/* num of samples accumulated in current window */
> +	u32 weight;		/* path weight */
> +	u32 credit;		/* path credit for I/O forwarding */
> +};
> +
> +struct nvme_path_work {
> +	struct nvme_ns *ns;		/* owning namespace */
> +	struct work_struct weight_work;	/* deferred work for weight calculation */
> +	int rw;				/* op type : READ or WRITE */
> +};
> +
> +struct nvme_path_info {
> +	struct nvme_path_stat stat;	/* path statistics */
> +	struct nvme_path_work work;	/* background worker context */
> +};
> +
>   /*
>    * Anchor structure for namespaces.  There is one for each namespace in a
>    * NVMe subsystem that any of our controllers can see, and the namespace
> @@ -534,6 +559,7 @@ struct nvme_ns {
>   #ifdef CONFIG_NVME_MULTIPATH
>   	enum nvme_ana_state ana_state;
>   	u32 ana_grpid;
> +	struct nvme_path_info __percpu *info;
>   #endif
>   	struct list_head siblings;
>   	struct kref kref;
> @@ -545,6 +571,7 @@ struct nvme_ns {
>   #define NVME_NS_FORCE_RO		3
>   #define NVME_NS_READY			4
>   #define NVME_NS_SYSFS_ATTR_LINK	5
> +#define NVME_NS_PATH_STAT		6
>   
>   	struct cdev		cdev;
>   	struct device		cdev_device;
> @@ -949,7 +976,7 @@ extern const struct attribute_group *nvme_dev_attr_groups[];
>   extern const struct block_device_operations nvme_bdev_ops;
>   
>   void nvme_delete_ctrl_sync(struct nvme_ctrl *ctrl);
> -struct nvme_ns *nvme_find_path(struct nvme_ns_head *head);
> +struct nvme_ns *nvme_find_path(struct nvme_ns_head *head, unsigned int rw);
>   #ifdef CONFIG_NVME_MULTIPATH
>   static inline bool nvme_ctrl_use_ana(struct nvme_ctrl *ctrl)
>   {
> @@ -978,6 +1005,7 @@ void nvme_mpath_clear_ctrl_paths(struct nvme_ctrl *ctrl);
>   void nvme_mpath_remove_disk(struct nvme_ns_head *head);
>   void nvme_mpath_start_request(struct request *rq);
>   void nvme_mpath_end_request(struct request *rq);
> +int nvme_alloc_ns_stat(struct nvme_ns *ns);
>   
>   static inline void nvme_trace_bio_complete(struct request *req)
>   {
> @@ -1005,6 +1033,13 @@ static inline bool nvme_mpath_queue_if_no_path(struct nvme_ns_head *head)
>   		return true;
>   	return false;
>   }
> +static inline void nvme_free_ns_stat(struct nvme_ns *ns)
> +{
> +	if (!ns->head->disk)
> +		return;
> +
> +	free_percpu(ns->info);
> +}
>   #else
>   #define multipath false
>   static inline bool nvme_ctrl_use_ana(struct nvme_ctrl *ctrl)
> @@ -1096,6 +1131,13 @@ static inline bool nvme_mpath_queue_if_no_path(struct nvme_ns_head *head)
>   {
>   	return false;
>   }
> +static inline int nvme_alloc_ns_stat(struct nvme_ns *ns)
> +{
> +	return 0;
> +}
> +static inline void nvme_free_ns_stat(struct nvme_ns *ns)
> +{
> +}
>   #endif /* CONFIG_NVME_MULTIPATH */
>   
>   int nvme_ns_get_unique_id(struct nvme_ns *ns, u8 id[16],
> diff --git a/drivers/nvme/host/pr.c b/drivers/nvme/host/pr.c
> index ca6a74607b13..9f23793dc12f 100644
> --- a/drivers/nvme/host/pr.c
> +++ b/drivers/nvme/host/pr.c
> @@ -53,10 +53,12 @@ static int nvme_send_ns_head_pr_command(struct block_device *bdev,
>   		struct nvme_command *c, void *data, unsigned int data_len)
>   {
>   	struct nvme_ns_head *head = bdev->bd_disk->private_data;
> -	int srcu_idx = srcu_read_lock(&head->srcu);
> -	struct nvme_ns *ns = nvme_find_path(head);
> +	int srcu_idx;
> +	struct nvme_ns *ns;
>   	int ret = -EWOULDBLOCK;
>   
> +	srcu_idx = srcu_read_lock(&head->srcu);
> +	ns = nvme_find_path(head, nvme_is_write(c) ? WRITE : READ);
>   	if (ns) {
>   		c->common.nsid = cpu_to_le32(ns->head->ns_id);
>   		ret = nvme_submit_sync_cmd(ns->queue, c, data, data_len);
> diff --git a/drivers/nvme/host/sysfs.c b/drivers/nvme/host/sysfs.c
> index 29430949ce2f..4f9607e9698a 100644
> --- a/drivers/nvme/host/sysfs.c
> +++ b/drivers/nvme/host/sysfs.c
> @@ -194,7 +194,7 @@ static int ns_head_update_nuse(struct nvme_ns_head *head)
>   		return 0;
>   
>   	srcu_idx = srcu_read_lock(&head->srcu);
> -	ns = nvme_find_path(head);
> +	ns = nvme_find_path(head, READ);
>   	if (!ns)
>   		goto out_unlock;
>  
Other than that it really looks good.
Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich

Re: [RFC PATCHv2 3/4] nvme: add generic debugfs support

[Hannes Reinecke]

On 10/9/25 12:05, Nilay Shroff wrote:
> Add generic infrastructure for creating and managing debugfs files in
> the NVMe module. This introduces helper APIs that allow NVMe drivers to
> register and unregister debugfs entries, along with a reusable attribute
> structure for defining new debugfs files.
> 
> The implementation uses seq_file interfaces to safely expose per-NS and
> per-NS-head statistics, while supporting both simple show callbacks and
> full seq_operations.
> 
> Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
> ---
>   drivers/nvme/host/Makefile  |   2 +-
>   drivers/nvme/host/debugfs.c | 122 ++++++++++++++++++++++++++++++++++++
>   drivers/nvme/host/nvme.h    |  10 +++
>   3 files changed, 133 insertions(+), 1 deletion(-)
>   create mode 100644 drivers/nvme/host/debugfs.c
> 
Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich

Re: [RFC PATCHv2 4/4] nvme-multipath: add debugfs attribute for adaptive I/O policy stat

[Hannes Reinecke]

On 10/9/25 12:05, Nilay Shroff wrote:
> This commit introduces a new debugfs attribute, "adaptive_stat", under
> both per-path and head debugfs directories (defined under /sys/kernel/
> debug/block/). This attribute provides visibility into the internal
> state of the adaptive I/O policy to aid in debugging and performance
> analysis.
> 
> For per-path entries, "adaptive_stat" reports the corresponding path
> statistics such as I/O weight, selection count, processed samples, and
> ignored samples.
> 
> For head entries, it reports per-CPU statistics for each reachable path,
> including I/O weight, path score, smoothed (EWMA) latency, selection
> count, processed samples, and ignored samples.
> 
> These additions enhance observability of the adaptive I/O path selection
> behavior and help diagnose imbalance or instability in multipath
> performance.
> 
> Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
> ---
>   drivers/nvme/host/core.c      |   3 +
>   drivers/nvme/host/debugfs.c   | 117 ++++++++++++++++++++++++++++++++++
>   drivers/nvme/host/multipath.c |   2 +
>   3 files changed, 122 insertions(+)
> 
Reviewed-by: Hannes Reinecke <hare@suse.de>

Might be an idea to have support for queue-depth in here, too.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich

Re: [RFC PATCHv2 2/4] nvme-multipath: add support for adaptive I/O policy

[Nilay Shroff]

>> diff --git a/drivers/nvme/host/ioctl.c b/drivers/nvme/host/ioctl.c
>> index c212fa952c0f..69d2f72d0e86 100644
>> --- a/drivers/nvme/host/ioctl.c
>> +++ b/drivers/nvme/host/ioctl.c
>> @@ -711,7 +711,7 @@ int nvme_ns_head_ioctl(struct block_device *bdev, blk_mode_t mode,
>>           flags |= NVME_IOCTL_PARTITION;
>>         srcu_idx = srcu_read_lock(&head->srcu);
>> -    ns = nvme_find_path(head);
>> +    ns = nvme_find_path(head, open_for_write ? WRITE : READ);
>>       if (!ns)
>>           goto out_unlock;
>>   @@ -742,7 +742,7 @@ long nvme_ns_head_chr_ioctl(struct file *file, unsigned int cmd,
>>       int srcu_idx, ret = -EWOULDBLOCK;
>>         srcu_idx = srcu_read_lock(&head->srcu);
>> -    ns = nvme_find_path(head);
>> +    ns = nvme_find_path(head, open_for_write ? WRITE : READ);
>>       if (!ns)
>>           goto out_unlock;
>>   
> Are we sure that we should account for non-IO commands here, too?
> Thing is, the I/O policy should be just that, directing I/O to the
> various paths.
> But what about commands on the admin queue? Should they be influenced by
> the same policy? And can we even define what a 'read' command is?
> Wouldn't it be better to pass in a third option here (NONE?) to make it
> clear that this is a non-I/O command?
> 
Yeah makes sense to differentiate between read/write and others here.
I think we should be able to differentiate between read/write vs others.
I'd implement this in next patchset.

>> @@ -762,7 +762,8 @@ int nvme_ns_head_chr_uring_cmd(struct io_uring_cmd *ioucmd,
>>       struct cdev *cdev = file_inode(ioucmd->file)->i_cdev;
>>       struct nvme_ns_head *head = container_of(cdev, struct nvme_ns_head, cdev);
>>       int srcu_idx = srcu_read_lock(&head->srcu);
>> -    struct nvme_ns *ns = nvme_find_path(head);
>> +    struct nvme_ns *ns = nvme_find_path(head,
>> +            ioucmd->file->f_mode & FMODE_WRITE ? WRITE : READ);
>>       int ret = -EINVAL;
>>         if (ns)
> 
> See above. I really think that we should be able to pass in a third
> option for admin commands.
Yep, as mentioned will do this in the next patchset.

> 
>> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
>> index 3da980dc60d9..9ecdaca5e9a0 100644
>> --- a/drivers/nvme/host/multipath.c
>> +++ b/drivers/nvme/host/multipath.c
>> @@ -6,6 +6,8 @@
>>   #include <linux/backing-dev.h>
>>   #include <linux/moduleparam.h>
>>   #include <linux/vmalloc.h>
>> +#include <linux/blk-mq.h>
>> +#include <linux/math64.h>
>>   #include <trace/events/block.h>
>>   #include "nvme.h"
>>  
> Ouch.
> #include <math64.h>
> always spells disaster :-)

This is included for div_u64(). Well we may replace it with usual division
operator (e.g. dividend / divisor) but that may not work well on 32-bit
architecture. 

>>   +static void nvme_mpath_weight_work(struct work_struct *weight_work)
>> +{
>> +    int cpu, srcu_idx;
>> +    u32 weight;
>> +    struct nvme_ns *ns;
>> +    struct nvme_path_stat *stat;
>> +    struct nvme_path_work *work = container_of(weight_work,
>> +            struct nvme_path_work, weight_work);
>> +    struct nvme_ns_head *head = work->ns->head;
>> +    int rw = work->rw;
>> +    u64 total_score = 0;
>> +
>> +    cpu = get_cpu();
>> +
>> +    srcu_idx = srcu_read_lock(&head->srcu);
>> +    list_for_each_entry_srcu(ns, &head->list, siblings,
>> +            srcu_read_lock_held(&head->srcu)) {
>> +
>> +        stat = &this_cpu_ptr(ns->info)[rw].stat;
>> +        if (!READ_ONCE(stat->slat_ns))
>> +            continue;
>> +        /*
>> +         * Compute the path score as the inverse of smoothed
>> +         * latency, scaled by NSEC_PER_SEC. Floating point
>> +         * math is unavailable in the kernel, so fixed-point
>> +         * scaling is used instead. NSEC_PER_SEC is chosen
>> +         * because valid latencies are always < 1 second; longer
>> +         * latencies are ignored.
>> +         */
>> +        stat->score = div_u64(NSEC_PER_SEC, READ_ONCE(stat->slat_ns));
>> +
>> +        /* Compute total score. */
>> +        total_score += stat->score;
>> +    }
>> +
>> +    if (!total_score)
>> +        goto out;
>> +
>> +    /*
>> +     * After computing the total slatency, we derive per-path weight
>> +     * (normalized to the range 0–100). The weight represents the
>> +     * relative share of I/O the path should receive.
> 
> Why do we use the range 0-100?
> While this is nice for human consumption, it doesn't lend itself to a
> nice computation. Maybe we should select a different scale to allow us
> for more efficient computation (0-128? Maybe?)
Yes we can normalize this to any range. Lets scale it from 0-128 as
you suggested, and it also aligns with power of 2.

>> +
>> +#define NVME_EWMA_SHIFT    3
> 
> And we use this value why?
Oh yeah the reason being in the proposed code we want to assign  ~87.5%
weightage to the existing (smoothed) latency and ~12.5% weightage to the 
new latency. If you see the ewma_update() then you'd realize this 
fact. But yes I'd add a comment above this macro to make the intent
clear.

> 
>> +static inline u64 ewma_update(u64 old, u64 new)
>> +{
>> +    return (old * ((1 << NVME_EWMA_SHIFT) - 1) + new) >> NVME_EWMA_SHIFT;
>> +}
>> +
>> +static void nvme_mpath_add_sample(struct request *rq, struct nvme_ns *ns)
>> +{
>> +    int cpu;
>> +    unsigned int rw;
>> +    struct nvme_path_info *info;
>> +    struct nvme_path_stat *stat;
>> +    u64 now, latency, slat_ns, avg_lat_ns;
>> +    struct nvme_ns_head *head = ns->head;
>> +
>> +    if (list_is_singular(&head->list))
>> +        return;
>> +
>> +    now = ktime_get_ns();
>> +    latency = now >= rq->io_start_time_ns ? now - rq->io_start_time_ns : 0;
>> +    if (!latency)
>> +        return;
>> +
>> +    /*
>> +     * As completion code path is serialized(i.e. no same completion queue
>> +     * update code could run simultaneously on multiple cpu) we can safely
>> +     * access per cpu nvme path stat here from another cpu (in case the
>> +     * completion cpu is different from submission cpu).
>> +     * The only field which could be accessed simultaneously here is the
>> +     * path ->weight which may be accessed by this function as well as I/O
>> +     * submission path during path selection logic and we protect ->weight
>> +     * using READ_ONCE/WRITE_ONCE. Yes this may not be 100% accurate but
>> +     * we also don't need to be so accurate here as the path credit would
>> +     * be anyways refilled, based on path weight, once path consumes all
>> +     * its credits. And we limit path weight/credit max up to 100. Please
>> +     * also refer nvme_adaptive_path().
>> +     */
>> +    cpu = blk_mq_rq_cpu(rq);
>> +    rw = rq_data_dir(rq);
>> +    info = &per_cpu_ptr(ns->info, cpu)[rw];
>> +    stat = &info->stat;
>> +
> Hmm. While the 'SAME_CONP' attribute should help here, I remain
> sceptical.
> Wouldn't it be possible to look at the hwq map (eg by using something
> like blk_mq_map_queue_type()) to figure out the hw context, and then
> use the first cpu from that mask?
> That way we are guaranteed to always using the same per-cpu ptr.
> Might be overkill, though; but it would be good to have some
> checks in here in case we do run on the wrong CPU.
> 
Okay so I think you propose ensuring that submitting cpu should 
match the cpu on which we're processing completion I/O sample.
Well, that makes sense but here we use blk_mq_rq_cpu() which
yields the submitting cpu for the request and then use submitting cpu 
number to get per-cpu stat using per_cpu_ptr(). As we don't rely
on the current cpu on which completion I/O sample is being processed, I 
think we're all good. Agreed?

>> +    /*
>> +     * If latency > ~1s then ignore this sample to prevent EWMA from being
>> +     * skewed by pathological outliers (multi-second waits, controller
>> +     * timeouts etc.). This keeps path scores representative of normal
>> +     * performance and avoids instability from rare spikes. If such high
>> +     * latency is real, ANA state reporting or keep-alive error counters
>> +     * will mark the path unhealthy and remove it from the head node list,
>> +     * so we safely skip such sample here.
>> +     */
>> +    if (unlikely(latency > NSEC_PER_SEC)) {
>> +        stat->nr_ignored++;
>> +        return;
>> +    }
> 
> I would even go so far as to indicate that EWMA is unusable here.
> Can we issue a warning / debug message here to indicate that the path
> selector is running suboptimal?
> 
We can certainly do that, but as you could see above we're incrementing 
->nr_ignored here, and user could then access this value through 
debugfs, so is that sufficient? Or do you want an explicit WARN or debug
message to be also printed?

>> +
>> +    /*
>> +     * Accumulate latency samples and increment the batch count for each
>> +     * ~15 second interval. When the interval expires, compute the simple
>> +     * average latency over that window, then update the smoothed (EWMA)
>> +     * latency. The path weight is recalculated based on this smoothed
>> +     * latency.
>> +     */
> 
> Why 15 seconds? Can we make this changeable eg via debugfs?
Yes certainly we can make this configurable, BTW should it be
done through a new sysfs attribute instead of debugfs? 

> 
>> +    stat->batch += latency;
>> +    stat->batch_count++;
>> +    stat->nr_samples++;
>> +
>> +    if (now > stat->last_weight_ts &&
>> +            (now - stat->last_weight_ts) >= 15 * NSEC_PER_SEC) {
>> +
> 
> At the very least make this a #define. But ideally one should be able
> to modify that.
> 
Yes I'd define a macro for this 15 seconds interval, and also we'll make
it configurable. I'd implement this in next patchset.

>> +        stat->last_weight_ts = now;
>> +
>> +        /*
>> +         * Find simple average latency for the last epoch (~15 sec
>> +         * interval).
>> +         */
>> +        avg_lat_ns = div_u64(stat->batch, stat->batch_count);
>> +
>> +        /*
>> +         * Calculate smooth/EWMA (Exponentially Weighted Moving Average)
>> +         * latency. EWMA is preferred over simple average latency
>> +         * because it smooths naturally, reduces jitter from sudden
>> +         * spikes, and adapts faster to changing conditions. It also
>> +         * avoids storing historical samples, and works well for both
>> +         * slow and fast I/O rates.
>> +         * Formula:
>> +         * slat_ns = (prev_slat_ns * (WEIGHT - 1) + (latency)) / WEIGHT
>> +         * With WEIGHT = 8, this assigns 7/8 (~87.5 %) weight to the
>> +         * existing latency and 1/8 (~12.5%) weight to the new latency.
>> +         */
> 
> Similar comment to the WEIGHT. While 8 might be a nice choice, there is nothing indicating it's always the right choice.
> Please make it a #define and see if we cannot modify it via debugfs.
> 
Yeah sure, as discussed earlier this value is derived from NVME_EWMA_SHIFT
and I'd make it configurable in the next patchset.

>> +int nvme_alloc_ns_stat(struct nvme_ns *ns)
>> +{
>> +    int rw, cpu;
>> +    struct nvme_path_work *work;
>> +    gfp_t gfp = GFP_KERNEL | __GFP_ZERO;
>> +
>> +    if (!ns->head->disk)
>> +        return 0;
>> +
>> +    ns->info = __alloc_percpu_gfp(2 * sizeof(struct nvme_path_info),
>> +            __alignof__(struct nvme_path_info), gfp);
>> +    if (!ns->info)
>> +        return -ENOMEM;
>> +
>> +    for_each_possible_cpu(cpu) {
>> +        for (rw = 0; rw < 2; rw++) {
>> +            work = &per_cpu_ptr(ns->info, cpu)[rw].work;
>> +            work->ns = ns;
>> +            work->rw = rw;
>> +            INIT_WORK(&work->weight_work, nvme_mpath_weight_work);
>> +        }
>> +    }
>> +
>> +    return 0;
>> +}
>> +
> 
> The more I think about it the more I like the idea of having a third
> direction (and not just READ and WRITE).
> The latency of I/O commands _will_ be different from the latency of
> admin commands, so the adaptive policy might come to different decisions
> for admin commands.
Yes as I said above, I'd address this in the next patchset. 

>> +static struct nvme_ns *nvme_adaptive_path(struct nvme_ns_head *head,
>> +        unsigned int rw)
>> +{
>> +    struct nvme_ns *ns, *found = NULL;
>> +    struct nvme_path_stat *stat;
>> +    u32 weight;
>> +    int refill = 0;
>> +
>> +    get_cpu();
>> +retry:
>> +    list_for_each_entry_srcu(ns, &head->list, siblings,
>> +            srcu_read_lock_held(&head->srcu)) {
>> +
>> +        if (nvme_path_is_disabled(ns) ||
>> +                !nvme_state_is_live(ns->ana_state))
>> +            continue;
>> +
>> +        stat = &this_cpu_ptr(ns->info)[rw].stat;
>> +
>> +        /*
>> +         * When the head path-list is singular we don't calculate the
>> +         * only path weight for optimization as we don't need to forward
>> +         * I/O to more than one path. The another possibility is whenthe
>> +         * path is newly added, we don't know its weight. So we go round
>> +         * -robin for each such path and forward I/O to it.Once we start
>> +         * getting response for such I/Os, the path weight calculation
>> +         * would kick in and then we start using path credit for
>> +         * forwarding I/O.
>> +         */
>> +        weight = READ_ONCE(stat->weight);
>> +        if (unlikely(!weight)) {
>> +            found = ns;
>> +            goto out;
>> +        }
>> +
>> +        /*
>> +         * To keep path selection logic simple, we don't distinguish
>> +         * between ANA optimized and non-optimized states. The non-
>> +         * optimized path is expected to have a lower weight, and
>> +         * therefore fewer credits. As a result, only a small number of
>> +         * I/Os will be forwarded to paths in the non-optimized state.
>> +         */
>> +        if (stat->credit > 0) {
>> +            --stat->credit;
>> +            found = ns;
>> +            goto out;
>> +        }
>> +    }
>> +
>> +    if (!found && !list_empty(&head->list)) {> +        /*
>> +         * Refill credits and retry.
>> +         */
>> +        list_for_each_entry_srcu(ns, &head->list, siblings,
>> +                srcu_read_lock_held(&head->srcu)) {
>> +            if (nvme_path_is_disabled(ns) ||
>> +                    !nvme_state_is_live(ns->ana_state))
>> +                continue;
>> +
>> +            stat = &this_cpu_ptr(ns->info)[rw].stat;
>> +            weight = READ_ONCE(stat->weight);
>> +            stat->credit = weight;
>> +            refill = 1;
>> +        }
>> +        if (refill)
>> +            goto retry;
>> +    }
> 
> Hmm. Loop within a loop. Not pretty.
> Can't we make do with just one loop?
> After all, if we end up here the credits for this path are use up, and need to be refilled.
> What would happen if we were _just_ refill and retry?
> IE drop the second iteration and just execute the branch
> under the loop?
> 
Yes good pint. I think we should be able to address this
with one loop. I'd fix this in the next patchset. 

>> @@ -581,7 +951,7 @@ static int nvme_ns_head_report_zones(struct gendisk *disk, sector_t sector,
>>       int srcu_idx, ret = -EWOULDBLOCK;
>>         srcu_idx = srcu_read_lock(&head->srcu);
>> -    ns = nvme_find_path(head);
>> +    ns = nvme_find_path(head, READ);
>>       if (ns)
>>           ret = nvme_ns_report_zones(ns, sector, nr_zones, cb, data);
>>       srcu_read_unlock(&head->srcu, srcu_idx);
> 
> See the discussion about the third option and not just 'READ'/'WRITE'.
Yeah, noted!

>> diff --git a/drivers/nvme/host/sysfs.c b/drivers/nvme/host/sysfs.c
>> index 29430949ce2f..4f9607e9698a 100644
>> --- a/drivers/nvme/host/sysfs.c
>> +++ b/drivers/nvme/host/sysfs.c
>> @@ -194,7 +194,7 @@ static int ns_head_update_nuse(struct nvme_ns_head *head)
>>           return 0;
>>         srcu_idx = srcu_read_lock(&head->srcu);
>> -    ns = nvme_find_path(head);
>> +    ns = nvme_find_path(head, READ);
>>       if (!ns)
>>           goto out_unlock;
>>  
> Other than that it really looks good.
Thank you:)

--Nilay

Re: [RFC PATCHv2 4/4] nvme-multipath: add debugfs attribute for adaptive I/O policy stat

[Nilay Shroff]

On 10/13/25 12:03 PM, Hannes Reinecke wrote:
> On 10/9/25 12:05, Nilay Shroff wrote:
>> This commit introduces a new debugfs attribute, "adaptive_stat", under
>> both per-path and head debugfs directories (defined under /sys/kernel/
>> debug/block/). This attribute provides visibility into the internal
>> state of the adaptive I/O policy to aid in debugging and performance
>> analysis.
>>
>> For per-path entries, "adaptive_stat" reports the corresponding path
>> statistics such as I/O weight, selection count, processed samples, and
>> ignored samples.
>>
>> For head entries, it reports per-CPU statistics for each reachable path,
>> including I/O weight, path score, smoothed (EWMA) latency, selection
>> count, processed samples, and ignored samples.
>>
>> These additions enhance observability of the adaptive I/O path selection
>> behavior and help diagnose imbalance or instability in multipath
>> performance.
>>
>> Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
>> ---
>>   drivers/nvme/host/core.c      |   3 +
>>   drivers/nvme/host/debugfs.c   | 117 ++++++++++++++++++++++++++++++++++
>>   drivers/nvme/host/multipath.c |   2 +
>>   3 files changed, 122 insertions(+)
>>
> Reviewed-by: Hannes Reinecke <hare@suse.de>
> 
> Might be an idea to have support for queue-depth in here, too.
> 
Currently queue_depth and numa_nodes attributes are created under 
sysfs, so do you meant to move those attributes from sysfs to 
debugfs? 

Thanks,
--Nilay

Re: [RFC PATCHv2 4/4] nvme-multipath: add debugfs attribute for adaptive I/O policy stat

[Hannes Reinecke]

On 10/15/25 08:19, Nilay Shroff wrote:
> 
> 
> On 10/13/25 12:03 PM, Hannes Reinecke wrote:
>> On 10/9/25 12:05, Nilay Shroff wrote:
>>> This commit introduces a new debugfs attribute, "adaptive_stat", under
>>> both per-path and head debugfs directories (defined under /sys/kernel/
>>> debug/block/). This attribute provides visibility into the internal
>>> state of the adaptive I/O policy to aid in debugging and performance
>>> analysis.
>>>
>>> For per-path entries, "adaptive_stat" reports the corresponding path
>>> statistics such as I/O weight, selection count, processed samples, and
>>> ignored samples.
>>>
>>> For head entries, it reports per-CPU statistics for each reachable path,
>>> including I/O weight, path score, smoothed (EWMA) latency, selection
>>> count, processed samples, and ignored samples.
>>>
>>> These additions enhance observability of the adaptive I/O path selection
>>> behavior and help diagnose imbalance or instability in multipath
>>> performance.
>>>
>>> Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
>>> ---
>>>    drivers/nvme/host/core.c      |   3 +
>>>    drivers/nvme/host/debugfs.c   | 117 ++++++++++++++++++++++++++++++++++
>>>    drivers/nvme/host/multipath.c |   2 +
>>>    3 files changed, 122 insertions(+)
>>>
>> Reviewed-by: Hannes Reinecke <hare@suse.de>
>>
>> Might be an idea to have support for queue-depth in here, too.
>>
> Currently queue_depth and numa_nodes attributes are created under
> sysfs, so do you meant to move those attributes from sysfs to
> debugfs?
> 
Yes. sysfs information can be perused by udev rules, and that assumes
the information in there will not change without a 'CHANGE' uevent.
So it's questionable whether volatile information like the queue depth
or the round-robin path selection should be presented there.
I'd rather have it moved into debugfs.

But that might be personal preference.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich

Re: [RFC PATCHv2 4/4] nvme-multipath: add debugfs attribute for adaptive I/O policy stat

[Daniel Wagner]

On Thu, Oct 16, 2025 at 08:14:19AM +0200, Hannes Reinecke wrote:
> > Currently queue_depth and numa_nodes attributes are created under
> > sysfs, so do you meant to move those attributes from sysfs to
> > debugfs?
> > 
> Yes. sysfs information can be perused by udev rules, and that assumes
> the information in there will not change without a 'CHANGE' uevent.
> So it's questionable whether volatile information like the queue depth
> or the round-robin path selection should be presented there.
> I'd rather have it moved into debugfs.
> 
> But that might be personal preference.

If this moves to debugfs, I think nvme-cli should not show it per
default. Don't want to get lots of questions why the value is not
showed or updated.

Re: [RFC PATCHv2 4/4] nvme-multipath: add debugfs attribute for adaptive I/O policy stat

[Nilay Shroff]

On 10/16/25 1:57 PM, Daniel Wagner wrote:
> On Thu, Oct 16, 2025 at 08:14:19AM +0200, Hannes Reinecke wrote:
>>> Currently queue_depth and numa_nodes attributes are created under
>>> sysfs, so do you meant to move those attributes from sysfs to
>>> debugfs?
>>>
>> Yes. sysfs information can be perused by udev rules, and that assumes
>> the information in there will not change without a 'CHANGE' uevent.
>> So it's questionable whether volatile information like the queue depth
>> or the round-robin path selection should be presented there.
>> I'd rather have it moved into debugfs.
>>
>> But that might be personal preference.
> 
> If this moves to debugfs, I think nvme-cli should not show it per
> default. Don't want to get lots of questions why the value is not
> showed or updated.

That’s a fair concern — debugfs isn’t guaranteed to be available in
production systems (it may be disabled or not mounted), so depending
on it from nvme-cli would be unreliable. Given that, I think it’s
safer to keep queue_depth and numa_node in sysfs for now since they’re
already part of the sysfs and used by nvme-cli.

Thanks,
--Nilay