* [PATCHv6 1/8] block: expose blk_stat_{enable,disable}_accounting() to drivers
2026-05-20 18:20 [PATCHv6 0/8] nvme-multipath: introduce latency I/O policy Nilay Shroff
@ 2026-05-20 18:20 ` Nilay Shroff
2026-05-21 12:06 ` John Garry
2026-05-20 18:20 ` [PATCHv6 2/8] nvme-multipath: pass I/O type to nvme_find_path() Nilay Shroff
` (6 subsequent siblings)
7 siblings, 1 reply; 13+ messages in thread
From: Nilay Shroff @ 2026-05-20 18:20 UTC (permalink / raw)
To: linux-nvme
Cc: hare, kbusch, hch, sagi, dwagner, kanie, jmeneghi, randyj,
martin.petersen, john.g.garry, gjoyce
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.
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
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 cc5b66e7ee60..b1614a1ef4ad 100644
--- a/block/blk-stat.h
+++ b/block/blk-stat.h
@@ -70,10 +70,6 @@ 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 18a2388ba581..88aabc2dd738 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -753,6 +753,10 @@ 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),
--
2.53.0
^ permalink raw reply related [flat|nested] 13+ messages in thread* Re: [PATCHv6 1/8] block: expose blk_stat_{enable,disable}_accounting() to drivers
2026-05-20 18:20 ` [PATCHv6 1/8] block: expose blk_stat_{enable,disable}_accounting() to drivers Nilay Shroff
@ 2026-05-21 12:06 ` John Garry
2026-05-22 6:18 ` Nilay Shroff
0 siblings, 1 reply; 13+ messages in thread
From: John Garry @ 2026-05-21 12:06 UTC (permalink / raw)
To: Nilay Shroff, linux-nvme
Cc: hare, kbusch, hch, sagi, dwagner, kanie, jmeneghi, randyj,
martin.petersen, gjoyce
On 20/05/2026 19:20, 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.
>
> Reviewed-by: Hannes Reinecke<hare@suse.de>
> Reviewed-by: Sagi Grimberg<sagi@grimberg.me>
> Signed-off-by: Nilay Shroff<nilay@linux.ibm.com>
> ---
> block/blk-stat.h | 4 ----
> include/linux/blk-mq.h | 4 ++++
struct request_queue comes from blkdev.h, so maybe that would be a more
appropriate place to locate the prototype.
Thanks,
John
^ permalink raw reply [flat|nested] 13+ messages in thread* Re: [PATCHv6 1/8] block: expose blk_stat_{enable,disable}_accounting() to drivers
2026-05-21 12:06 ` John Garry
@ 2026-05-22 6:18 ` Nilay Shroff
2026-05-22 12:10 ` Christoph Hellwig
0 siblings, 1 reply; 13+ messages in thread
From: Nilay Shroff @ 2026-05-22 6:18 UTC (permalink / raw)
To: John Garry, linux-nvme
Cc: hare, kbusch, hch, sagi, dwagner, kanie, jmeneghi, randyj,
martin.petersen, gjoyce
On 5/21/26 5:36 PM, John Garry wrote:
> On 20/05/2026 19:20, 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.
>>
>> Reviewed-by: Hannes Reinecke<hare@suse.de>
>> Reviewed-by: Sagi Grimberg<sagi@grimberg.me>
>> Signed-off-by: Nilay Shroff<nilay@linux.ibm.com>
>> ---
>> block/blk-stat.h | 4 ----
>> include/linux/blk-mq.h | 4 ++++
>
> struct request_queue comes from blkdev.h, so maybe that would be a more appropriate place to locate the prototype.
>
yes both blk-mq.h and blkdev.h should work well. We do also have few
prototypes defined in linux/blk-mq.h which uses request_queue. Personally
I don't have anything against using blkdev.h. So I will move it under
blkdev.h.
Thanks,
--Nilay
^ permalink raw reply [flat|nested] 13+ messages in thread* Re: [PATCHv6 1/8] block: expose blk_stat_{enable,disable}_accounting() to drivers
2026-05-22 6:18 ` Nilay Shroff
@ 2026-05-22 12:10 ` Christoph Hellwig
0 siblings, 0 replies; 13+ messages in thread
From: Christoph Hellwig @ 2026-05-22 12:10 UTC (permalink / raw)
To: Nilay Shroff
Cc: John Garry, linux-nvme, hare, kbusch, hch, sagi, dwagner, kanie,
jmeneghi, randyj, martin.petersen, gjoyce
blk_stat_enable_accounting is currently only useful for blk-mq based
drivers, so please keep it there.
^ permalink raw reply [flat|nested] 13+ messages in thread
* [PATCHv6 2/8] nvme-multipath: pass I/O type to nvme_find_path()
2026-05-20 18:20 [PATCHv6 0/8] nvme-multipath: introduce latency I/O policy Nilay Shroff
2026-05-20 18:20 ` [PATCHv6 1/8] block: expose blk_stat_{enable,disable}_accounting() to drivers Nilay Shroff
@ 2026-05-20 18:20 ` Nilay Shroff
2026-05-27 7:16 ` Hannes Reinecke
2026-05-20 18:20 ` [PATCHv6 3/8] nvme-multipath: add support for latency I/O policy Nilay Shroff
` (5 subsequent siblings)
7 siblings, 1 reply; 13+ messages in thread
From: Nilay Shroff @ 2026-05-20 18:20 UTC (permalink / raw)
To: linux-nvme
Cc: hare, kbusch, hch, sagi, dwagner, kanie, jmeneghi, randyj,
martin.petersen, john.g.garry, gjoyce
Currently, nvme_find_path() only accepts an nvme_ns_head argument.
However, the upcoming latency-aware I/O policy also needs to know
the I/O type (read/write/other) associated with the request in order
to make path selection decisions.
Update nvme_find_path() to accept an additional argument describing
the I/O type. This patch does not introduce any functional change and
only prepares the interface for subsequent latency-policy changes.
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
drivers/nvme/host/ioctl.c | 38 ++++++++++++++++++++++++++++++++---
drivers/nvme/host/multipath.c | 9 +++++----
drivers/nvme/host/nvme.h | 19 +++++++++++++++++-
drivers/nvme/host/pr.c | 6 ++++--
drivers/nvme/host/sysfs.c | 2 +-
5 files changed, 63 insertions(+), 11 deletions(-)
diff --git a/drivers/nvme/host/ioctl.c b/drivers/nvme/host/ioctl.c
index 08889b20e5d8..7a9043350227 100644
--- a/drivers/nvme/host/ioctl.c
+++ b/drivers/nvme/host/ioctl.c
@@ -699,18 +699,29 @@ static int nvme_ns_head_ctrl_ioctl(struct nvme_ns *ns, unsigned int cmd,
int nvme_ns_head_ioctl(struct block_device *bdev, blk_mode_t mode,
unsigned int cmd, unsigned long arg)
{
+ u8 opcode;
struct nvme_ns_head *head = bdev->bd_disk->private_data;
bool open_for_write = mode & BLK_OPEN_WRITE;
void __user *argp = (void __user *)arg;
struct nvme_ns *ns;
int srcu_idx, ret = -EWOULDBLOCK;
unsigned int flags = 0;
+ unsigned int op_type = NVME_STAT_OTHER;
if (bdev_is_partition(bdev))
flags |= NVME_IOCTL_PARTITION;
+ if (cmd == NVME_IOCTL_SUBMIT_IO) {
+ if (get_user(opcode, (u8 *)argp))
+ return -EFAULT;
+ if (opcode == nvme_cmd_write)
+ op_type = NVME_STAT_WRITE;
+ else if (opcode == nvme_cmd_read)
+ op_type = NVME_STAT_READ;
+ }
+
srcu_idx = srcu_read_lock(&head->srcu);
- ns = nvme_find_path(head);
+ ns = nvme_find_path(head, op_type);
if (!ns)
goto out_unlock;
@@ -732,6 +743,7 @@ int nvme_ns_head_ioctl(struct block_device *bdev, blk_mode_t mode,
long nvme_ns_head_chr_ioctl(struct file *file, unsigned int cmd,
unsigned long arg)
{
+ u8 opcode;
bool open_for_write = file->f_mode & FMODE_WRITE;
struct cdev *cdev = file_inode(file)->i_cdev;
struct nvme_ns_head *head =
@@ -739,9 +751,19 @@ long nvme_ns_head_chr_ioctl(struct file *file, unsigned int cmd,
void __user *argp = (void __user *)arg;
struct nvme_ns *ns;
int srcu_idx, ret = -EWOULDBLOCK;
+ unsigned int op_type = NVME_STAT_OTHER;
+
+ if (cmd == NVME_IOCTL_SUBMIT_IO) {
+ if (get_user(opcode, (u8 *)argp))
+ return -EFAULT;
+ if (opcode == nvme_cmd_write)
+ op_type = NVME_STAT_WRITE;
+ else if (opcode == nvme_cmd_read)
+ op_type = NVME_STAT_READ;
+ }
srcu_idx = srcu_read_lock(&head->srcu);
- ns = nvme_find_path(head);
+ ns = nvme_find_path(head, op_type);
if (!ns)
goto out_unlock;
@@ -761,7 +783,17 @@ 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);
+ const struct nvme_uring_cmd *cmd = io_uring_sqe128_cmd(ioucmd->sqe,
+ struct nvme_uring_cmd);
+ __u8 opcode = READ_ONCE(cmd->opcode);
+ unsigned int op_type = NVME_STAT_OTHER;
+
+ if (opcode == nvme_cmd_write)
+ op_type = NVME_STAT_WRITE;
+ else if (opcode == nvme_cmd_read)
+ op_type = NVME_STAT_READ;
+
+ struct nvme_ns *ns = nvme_find_path(head, op_type);
int ret = -EINVAL;
if (ns)
diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 263161cb8ac0..90f449780e72 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -446,7 +446,8 @@ 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 op_type)
{
switch (READ_ONCE(head->subsys->iopolicy)) {
case NVME_IOPOLICY_QD:
@@ -508,7 +509,7 @@ 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, nvme_data_dir(bio_op(bio)));
if (likely(ns)) {
bio_set_dev(bio, ns->disk->part0);
bio->bi_opf |= REQ_NVME_MPATH;
@@ -550,7 +551,7 @@ 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, NVME_STAT_OTHER);
if (ns)
ret = nvme_ns_get_unique_id(ns, id, type);
srcu_read_unlock(&head->srcu, srcu_idx);
@@ -566,7 +567,7 @@ 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, NVME_STAT_OTHER);
if (ns)
ret = nvme_ns_report_zones(ns, sector, nr_zones, args);
srcu_read_unlock(&head->srcu, srcu_idx);
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index ccd5e05dac98..39e986e5f184 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -514,6 +514,13 @@ struct nvme_ns_ids {
u8 csi;
};
+enum nvme_stat_group {
+ NVME_STAT_READ,
+ NVME_STAT_WRITE,
+ NVME_STAT_OTHER,
+ NVME_NUM_STAT_GROUPS
+};
+
/*
* Anchor structure for namespaces. There is one for each namespace in a
* NVMe subsystem that any of our controllers can see, and the namespace
@@ -1017,7 +1024,17 @@ 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 op_type);
+static inline int nvme_data_dir(const enum req_op op)
+{
+ if (op == REQ_OP_READ)
+ return NVME_STAT_READ;
+ else if (op_is_write(op))
+ return NVME_STAT_WRITE;
+ else
+ return NVME_STAT_OTHER;
+}
+
#ifdef CONFIG_NVME_MULTIPATH
static inline bool nvme_ctrl_use_ana(struct nvme_ctrl *ctrl)
{
diff --git a/drivers/nvme/host/pr.c b/drivers/nvme/host/pr.c
index fe7dbe264815..80e106f151d2 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_STAT_OTHER);
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 e59758616f27..b6b718bd310d 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, NVME_STAT_OTHER);
if (!ns)
goto out_unlock;
--
2.53.0
^ permalink raw reply related [flat|nested] 13+ messages in thread* Re: [PATCHv6 2/8] nvme-multipath: pass I/O type to nvme_find_path()
2026-05-20 18:20 ` [PATCHv6 2/8] nvme-multipath: pass I/O type to nvme_find_path() Nilay Shroff
@ 2026-05-27 7:16 ` Hannes Reinecke
0 siblings, 0 replies; 13+ messages in thread
From: Hannes Reinecke @ 2026-05-27 7:16 UTC (permalink / raw)
To: Nilay Shroff, linux-nvme
Cc: kbusch, hch, sagi, dwagner, kanie, jmeneghi, randyj,
martin.petersen, john.g.garry, gjoyce
On 5/20/26 20:20, Nilay Shroff wrote:
> Currently, nvme_find_path() only accepts an nvme_ns_head argument.
> However, the upcoming latency-aware I/O policy also needs to know
> the I/O type (read/write/other) associated with the request in order
> to make path selection decisions.
>
> Update nvme_find_path() to accept an additional argument describing
> the I/O type. This patch does not introduce any functional change and
> only prepares the interface for subsequent latency-policy changes.
>
> Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
> ---
> drivers/nvme/host/ioctl.c | 38 ++++++++++++++++++++++++++++++++---
> drivers/nvme/host/multipath.c | 9 +++++----
> drivers/nvme/host/nvme.h | 19 +++++++++++++++++-
> drivers/nvme/host/pr.c | 6 ++++--
> drivers/nvme/host/sysfs.c | 2 +-
> 5 files changed, 63 insertions(+), 11 deletions(-)
>
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
^ permalink raw reply [flat|nested] 13+ messages in thread
* [PATCHv6 3/8] nvme-multipath: add support for latency I/O policy
2026-05-20 18:20 [PATCHv6 0/8] nvme-multipath: introduce latency I/O policy Nilay Shroff
2026-05-20 18:20 ` [PATCHv6 1/8] block: expose blk_stat_{enable,disable}_accounting() to drivers Nilay Shroff
2026-05-20 18:20 ` [PATCHv6 2/8] nvme-multipath: pass I/O type to nvme_find_path() Nilay Shroff
@ 2026-05-20 18:20 ` Nilay Shroff
2026-05-20 18:21 ` [PATCHv6 4/8] nvme: add generic debugfs support Nilay Shroff
` (4 subsequent siblings)
7 siblings, 0 replies; 13+ messages in thread
From: Nilay Shroff @ 2026-05-20 18:20 UTC (permalink / raw)
To: linux-nvme
Cc: hare, kbusch, hch, sagi, dwagner, kanie, jmeneghi, randyj,
martin.petersen, john.g.garry, gjoyce
This commit introduces a new I/O policy named "latency". Users can
configure it by writing "latency" to "/sys/class/nvme-subsystem/nvme-
subsystemX/iopolicy"
The "latency" policy dynamically distributes I/O based on measured I/O
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 * 64) / total_score
path_B_weight = (path_B_score * 64) / 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 0–64 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.
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
drivers/nvme/host/core.c | 18 +-
drivers/nvme/host/multipath.c | 430 +++++++++++++++++++++++++++++++++-
drivers/nvme/host/nvme.h | 53 ++++-
3 files changed, 487 insertions(+), 14 deletions(-)
diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index c3032d6ad6b1..82db59927556 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -677,6 +677,9 @@ static void nvme_free_ns_head(struct kref *ref)
cleanup_srcu_struct(&head->srcu);
nvme_put_subsystem(head->subsys);
kfree(head->plids);
+#ifdef CONFIG_NVME_MULTIPATH
+ free_percpu(head->latency_path);
+#endif
kfree(head);
}
@@ -694,6 +697,7 @@ 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);
@@ -4174,6 +4178,9 @@ 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
@@ -4198,7 +4205,7 @@ 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);
/*
@@ -4207,7 +4214,7 @@ 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);
@@ -4231,6 +4238,8 @@ 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);
@@ -4270,10 +4279,13 @@ static void nvme_ns_remove(struct nvme_ns *ns)
/*
* Ensure that !NVME_NS_READY is seen by other threads to prevent
- * this ns going back into current_path.
+ * this ns going back into current_path/latency_path.
*/
synchronize_srcu(&ns->head->srcu);
+ if (test_bit(NVME_NS_PATH_STAT, &ns->flags))
+ nvme_cancel_ns_latency_weight_work(ns);
+
/* wait for concurrent submissions */
if (nvme_mpath_clear_current_path(ns))
synchronize_srcu(&ns->head->srcu);
diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 90f449780e72..e1089a75a6fe 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -6,6 +6,9 @@
#include <linux/backing-dev.h>
#include <linux/moduleparam.h>
#include <linux/vmalloc.h>
+#include <linux/blk-mq.h>
+#include <linux/math64.h>
+#include <linux/rculist.h>
#include <trace/events/block.h>
#include "nvme.h"
@@ -66,9 +69,10 @@ 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_LATENCY] = "latency",
};
static int iopolicy = NVME_IOPOLICY_NUMA;
@@ -83,6 +87,8 @@ 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, "latency", 7))
+ iopolicy = NVME_IOPOLICY_LATENCY;
else
return -EINVAL;
@@ -185,6 +191,203 @@ 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_lat_stat *stat;
+ struct nvme_path_lat_work *work = container_of(weight_work,
+ struct nvme_path_lat_work, weight_work);
+ struct nvme_ns_head *head = work->ns->head;
+ int op_type = work->op_type;
+ 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->path_lat)[op_type].stat;
+ if (!READ_ONCE(stat->slat_ns)) {
+ stat->score = 0;
+ 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 0–64). 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_latency_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->path_lat)[op_type].stat;
+ weight = div_u64(stat->score * 64, 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);
+ }
+out:
+ srcu_read_unlock(&head->srcu, srcu_idx);
+ put_cpu();
+}
+
+/*
+ * Formula to calculate the EWMA (Exponentially Weighted Moving Average):
+ * ewma = (old_ewma * (EWMA_SHIFT - 1) + (EWMA_SHIFT)) / EWMA_SHIFT
+ * For instance, with EWMA_SHIFT = 3, this assigns 7/8 (~87.5 %) weight to
+ * the existing/old ewma and 1/8 (~12.5%) weight to the new sample.
+ */
+static inline u64 calc_ewma_update(u64 old, u64 new)
+{
+ return (old * ((1 << NVME_DEFAULT_LATENCY_EWMA_SHIFT) - 1)
+ + new) >> NVME_DEFAULT_LATENCY_EWMA_SHIFT;
+}
+
+static void nvme_mpath_add_sample(struct request *rq, struct nvme_ns *ns)
+{
+ int cpu;
+ unsigned int op_type;
+ struct nvme_path_lat *path_lat;
+ struct nvme_path_lat_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 64. Please
+ * also refer nvme_latency_path().
+ */
+ cpu = blk_mq_rq_cpu(rq);
+ op_type = nvme_data_dir(req_op(rq));
+ path_lat = &per_cpu_ptr(ns->path_lat, cpu)[op_type];
+ stat = &path_lat->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++;
+ dev_warn_ratelimited(ns->ctrl->device,
+ "ignoring sample with >1s latency (possible controller stall or timeout)\n");
+ 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_batch_ts && ((now - stat->last_batch_ts) >=
+ NVME_DEFAULT_LATENCY_BATCH_TIMEOUT)) {
+
+ /*
+ * Find simple average latency for the last epoch (~15 sec
+ * interval).
+ */
+ avg_lat_ns = div_u64(stat->batch, stat->batch_count);
+ stat->last_batch_ts = now;
+
+ /*
+ * 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 = calc_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, &path_lat->work.weight_work);
+ }
+}
+
void nvme_mpath_end_request(struct request *rq)
{
struct nvme_ns *ns = rq->q->queuedata;
@@ -192,6 +395,9 @@ 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),
@@ -225,6 +431,70 @@ static const char *nvme_ana_state_names[] = {
[NVME_ANA_CHANGE] = "change",
};
+static void nvme_reset_ns_latency_stat(struct nvme_ns *ns)
+{
+ int i, cpu;
+ struct nvme_path_lat_stat *stat;
+
+ for_each_possible_cpu(cpu) {
+ for (i = 0; i < NVME_NUM_STAT_GROUPS; i++) {
+ stat = &per_cpu_ptr(ns->path_lat, cpu)[i].stat;
+ memset(stat, 0, sizeof(struct nvme_path_lat_stat));
+ }
+ }
+}
+
+void nvme_cancel_ns_latency_weight_work(struct nvme_ns *ns)
+{
+ int i, cpu;
+ struct nvme_path_lat *path_lat;
+
+ for_each_online_cpu(cpu) {
+ for (i = 0; i < NVME_NUM_STAT_GROUPS; i++) {
+ path_lat = &per_cpu_ptr(ns->path_lat, cpu)[i];
+ cancel_work_sync(&path_lat->work.weight_work);
+ }
+ }
+}
+
+static bool nvme_enable_ns_latency_sampling(struct nvme_ns *ns)
+{
+ struct nvme_ns_head *head = ns->head;
+
+ if (!head->disk ||
+ READ_ONCE(head->subsys->iopolicy) != NVME_IOPOLICY_LATENCY)
+ 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_disable_ns_latency_sampling(struct nvme_ns *ns)
+{
+ int cpu;
+ struct nvme_ns_head *head = ns->head;
+ bool changed = false;
+
+ if (!test_and_clear_bit(NVME_NS_PATH_STAT, &ns->flags))
+ return false;
+
+ for_each_possible_cpu(cpu) {
+ if (ns == READ_ONCE(*per_cpu_ptr(head->latency_path, cpu))) {
+ WRITE_ONCE(*per_cpu_ptr(head->latency_path, cpu), NULL);
+ changed = true;
+ }
+ }
+
+ blk_stat_disable_accounting(ns->queue);
+ blk_queue_flag_clear(QUEUE_FLAG_SAME_FORCE, ns->queue);
+ nvme_reset_ns_latency_stat(ns);
+ return changed;
+}
+
bool nvme_mpath_clear_current_path(struct nvme_ns *ns)
{
struct nvme_ns_head *head = ns->head;
@@ -237,6 +507,10 @@ bool nvme_mpath_clear_current_path(struct nvme_ns *ns)
changed = true;
}
}
+
+ if (nvme_disable_ns_latency_sampling(ns))
+ changed = true;
+
return changed;
}
@@ -254,6 +528,45 @@ void nvme_mpath_clear_ctrl_paths(struct nvme_ctrl *ctrl)
srcu_read_unlock(&ctrl->srcu, srcu_idx);
}
+int nvme_alloc_ns_stat(struct nvme_ns *ns)
+{
+ int i, cpu;
+ struct nvme_path_lat_work *work;
+ gfp_t gfp = GFP_KERNEL | __GFP_ZERO;
+
+ if (!ns->head->disk)
+ return 0;
+
+ ns->path_lat = __alloc_percpu_gfp(NVME_NUM_STAT_GROUPS *
+ sizeof(struct nvme_path_lat),
+ __alignof__(struct nvme_path_lat), gfp);
+ if (!ns->path_lat)
+ return -ENOMEM;
+
+ for_each_possible_cpu(cpu) {
+ for (i = 0; i < NVME_NUM_STAT_GROUPS; i++) {
+ work = &per_cpu_ptr(ns->path_lat, cpu)[i].work;
+ work->ns = ns;
+ work->op_type = i;
+ INIT_WORK(&work->weight_work, nvme_mpath_weight_work);
+ }
+ }
+
+ return 0;
+}
+
+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_enable_ns_latency_sampling(ns);
+ srcu_read_unlock(&ctrl->srcu, srcu_idx);
+}
+
void nvme_mpath_revalidate_paths(struct nvme_ns *ns)
{
struct nvme_ns_head *head = ns->head;
@@ -266,6 +579,8 @@ 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_reset_ns_latency_stat(ns);
}
srcu_read_unlock(&head->srcu, srcu_idx);
@@ -390,6 +705,92 @@ 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_latency_path(struct nvme_ns_head *head,
+ unsigned int op_type)
+{
+ struct nvme_ns *ns, *start, *found = NULL;
+ struct nvme_path_lat_stat *stat;
+ u32 weight;
+ int cpu;
+
+ cpu = get_cpu();
+ ns = READ_ONCE(*this_cpu_ptr(head->latency_path));
+ if (unlikely(!ns)) {
+ ns = list_first_or_null_rcu(&head->list,
+ struct nvme_ns, siblings);
+ if (unlikely(!ns))
+ goto out;
+ }
+found_ns:
+ start = ns;
+ while (nvme_path_is_disabled(ns) ||
+ !nvme_state_is_live(ns->ana_state)) {
+ ns = list_next_entry_circular(ns, &head->list, siblings);
+
+ /*
+ * If we iterate through all paths in the list but find each
+ * path in list is either disabled or dead then bail out.
+ */
+ if (ns == start)
+ goto out;
+ }
+
+ stat = &this_cpu_ptr(ns->path_lat)[op_type].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 when the
+ * 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 (!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;
+ } else {
+ /*
+ * Refill credit from path weight and move to next path. The
+ * refilled credit of the current path will be used next when
+ * all remainng paths exhaust its credits.
+ */
+ weight = READ_ONCE(stat->weight);
+ stat->credit = weight;
+ ns = list_next_entry_circular(ns, &head->list, siblings);
+ if (likely(ns))
+ goto found_ns;
+ }
+out:
+ if (found) {
+ stat->sel++;
+ WRITE_ONCE(*this_cpu_ptr(head->latency_path), found);
+ }
+
+ 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;
@@ -450,6 +851,8 @@ inline struct nvme_ns *nvme_find_path(struct nvme_ns_head *head,
unsigned int op_type)
{
switch (READ_ONCE(head->subsys->iopolicy)) {
+ case NVME_IOPOLICY_LATENCY:
+ return nvme_latency_path(head, op_type);
case NVME_IOPOLICY_QD:
return nvme_queue_depth_path(head);
case NVME_IOPOLICY_RR:
@@ -728,6 +1131,10 @@ int nvme_mpath_alloc_disk(struct nvme_ctrl *ctrl, struct nvme_ns_head *head)
if (!nvme_is_unique_nsid(ctrl, head))
return 0;
+ head->latency_path = alloc_percpu_gfp(struct nvme_ns*, GFP_KERNEL);
+ if (!head->latency_path)
+ return -ENOMEM;
+
blk_set_stacking_limits(&lim);
lim.dma_alignment = 3;
lim.features |= BLK_FEAT_IO_STAT | BLK_FEAT_NOWAIT |
@@ -736,8 +1143,10 @@ int nvme_mpath_alloc_disk(struct nvme_ctrl *ctrl, struct nvme_ns_head *head)
lim.features |= BLK_FEAT_ZONED;
head->disk = blk_alloc_disk(&lim, ctrl->numa_node);
- if (IS_ERR(head->disk))
+ if (IS_ERR(head->disk)) {
+ free_percpu(head->latency_path);
return PTR_ERR(head->disk);
+ }
head->disk->fops = &nvme_ns_head_ops;
head->disk->private_data = head;
@@ -793,6 +1202,10 @@ static void nvme_mpath_set_live(struct nvme_ns *ns)
}
mutex_unlock(&head->lock);
+ mutex_lock(&nvme_subsystems_lock);
+ nvme_enable_ns_latency_sampling(ns);
+ mutex_unlock(&nvme_subsystems_lock);
+
synchronize_srcu(&head->srcu);
kblockd_schedule_work(&head->requeue_work);
}
@@ -841,11 +1254,6 @@ 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)
{
@@ -1023,10 +1431,12 @@ 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 39e986e5f184..22e54c74c1a6 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -28,7 +28,9 @@ extern unsigned int nvme_io_timeout;
extern unsigned int admin_timeout;
#define NVME_ADMIN_TIMEOUT (admin_timeout * HZ)
-#define NVME_DEFAULT_KATO 5
+#define NVME_DEFAULT_KATO 5
+#define NVME_DEFAULT_LATENCY_EWMA_SHIFT 3
+#define NVME_DEFAULT_LATENCY_BATCH_TIMEOUT (15 * NSEC_PER_SEC)
#ifdef CONFIG_ARCH_NO_SG_CHAIN
#define NVME_INLINE_SG_CNT 0
@@ -477,6 +479,7 @@ enum nvme_iopolicy {
NVME_IOPOLICY_NUMA,
NVME_IOPOLICY_RR,
NVME_IOPOLICY_QD,
+ NVME_IOPOLICY_LATENCY,
};
struct nvme_subsystem {
@@ -521,6 +524,30 @@ enum nvme_stat_group {
NVME_NUM_STAT_GROUPS
};
+struct nvme_path_lat_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_batch_ts; /* timestamp when last time avg. latency is calculated */
+ 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_lat_work {
+ struct nvme_ns *ns; /* owning namespace */
+ struct work_struct weight_work; /* deferred work for weight calculation */
+ int op_type; /* op type : READ/WRITE/OTHER */
+};
+
+struct nvme_path_lat {
+ struct nvme_path_lat_stat stat; /* path statistics */
+ struct nvme_path_lat_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
@@ -570,6 +597,9 @@ struct nvme_ns_head {
unsigned long flags;
struct delayed_work remove_work;
unsigned int delayed_removal_secs;
+
+ struct nvme_ns * __percpu *latency_path;
+
#define NVME_NSHEAD_DISK_LIVE 0
#define NVME_NSHEAD_QUEUE_IF_NO_PATH 1
struct nvme_ns __rcu *current_path[];
@@ -596,6 +626,7 @@ struct nvme_ns {
#ifdef CONFIG_NVME_MULTIPATH
enum nvme_ana_state ana_state;
u32 ana_grpid;
+ struct nvme_path_lat __percpu *path_lat;
#endif
struct list_head siblings;
struct kref kref;
@@ -607,6 +638,7 @@ 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;
@@ -1063,6 +1095,8 @@ 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);
+void nvme_cancel_ns_latency_weight_work(struct nvme_ns *ns);
static inline void nvme_trace_bio_complete(struct request *req)
{
@@ -1090,6 +1124,13 @@ 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->path_lat);
+}
#else
#define multipath false
static inline bool nvme_ctrl_use_ana(struct nvme_ctrl *ctrl)
@@ -1181,6 +1222,16 @@ static inline bool nvme_mpath_queue_if_no_path(struct nvme_ns_head *head)
{
return false;
}
+static inline void nvme_cancel_ns_latency_weight_work(struct nvme_ns *ns)
+{
+}
+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],
--
2.53.0
^ permalink raw reply related [flat|nested] 13+ messages in thread* [PATCHv6 4/8] nvme: add generic debugfs support
2026-05-20 18:20 [PATCHv6 0/8] nvme-multipath: introduce latency I/O policy Nilay Shroff
` (2 preceding siblings ...)
2026-05-20 18:20 ` [PATCHv6 3/8] nvme-multipath: add support for latency I/O policy Nilay Shroff
@ 2026-05-20 18:21 ` Nilay Shroff
2026-05-20 18:21 ` [PATCHv6 5/8] nvme-multipath: add debugfs attribute latency_ewma_shift Nilay Shroff
` (3 subsequent siblings)
7 siblings, 0 replies; 13+ messages in thread
From: Nilay Shroff @ 2026-05-20 18:21 UTC (permalink / raw)
To: linux-nvme
Cc: hare, kbusch, hch, sagi, dwagner, kanie, jmeneghi, randyj,
martin.petersen, john.g.garry, gjoyce
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.
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
drivers/nvme/host/Makefile | 2 +-
drivers/nvme/host/core.c | 3 +
drivers/nvme/host/debugfs.c | 138 ++++++++++++++++++++++++++++++++++
drivers/nvme/host/multipath.c | 2 +
drivers/nvme/host/nvme.h | 10 +++
5 files changed, 154 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/core.c b/drivers/nvme/host/core.c
index 82db59927556..ca2be0472e3f 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -4224,6 +4224,8 @@ 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);
@@ -4307,6 +4309,7 @@ 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
new file mode 100644
index 000000000000..26a50566e4a1
--- /dev/null
+++ b/drivers/nvme/host/debugfs.c
@@ -0,0 +1,138 @@
+// 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);
+ ssize_t (*write)(void *data, const char __user *buf, size_t count,
+ loff_t *ppos);
+ 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_obj(struct nvme_debugfs_ctx);
+ 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 ssize_t nvme_debugfs_write(struct file *file, const char __user *buf,
+ size_t count, loff_t *ppos)
+{
+ struct seq_file *m = file->private_data;
+ struct nvme_debugfs_ctx *ctx = m->private;
+ struct nvme_debugfs_attr *attr = ctx->attr;
+
+ if (!attr->write)
+ return -EPERM;
+
+ return attr->write(ctx->data, buf, count, ppos);
+}
+
+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,
+ .write = nvme_debugfs_write,
+ .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/multipath.c b/drivers/nvme/host/multipath.c
index e1089a75a6fe..541d12b73b74 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -1078,6 +1078,7 @@ 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);
@@ -1187,6 +1188,7 @@ static void nvme_mpath_set_live(struct nvme_ns *ns)
}
nvme_add_ns_head_cdev(head);
queue_work(nvme_wq, &head->partition_scan_work);
+ nvme_debugfs_register(head->disk);
}
nvme_mpath_add_sysfs_link(ns->head);
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index 22e54c74c1a6..7ee9689ce07e 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -1067,6 +1067,16 @@ static inline int nvme_data_dir(const enum req_op op)
return NVME_STAT_OTHER;
}
+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)
{
--
2.53.0
^ permalink raw reply related [flat|nested] 13+ messages in thread* [PATCHv6 5/8] nvme-multipath: add debugfs attribute latency_ewma_shift
2026-05-20 18:20 [PATCHv6 0/8] nvme-multipath: introduce latency I/O policy Nilay Shroff
` (3 preceding siblings ...)
2026-05-20 18:21 ` [PATCHv6 4/8] nvme: add generic debugfs support Nilay Shroff
@ 2026-05-20 18:21 ` Nilay Shroff
2026-05-20 18:21 ` [PATCHv6 6/8] nvme-multipath: add debugfs attribute latency_batch_timeout Nilay Shroff
` (2 subsequent siblings)
7 siblings, 0 replies; 13+ messages in thread
From: Nilay Shroff @ 2026-05-20 18:21 UTC (permalink / raw)
To: linux-nvme
Cc: hare, kbusch, hch, sagi, dwagner, kanie, jmeneghi, randyj,
martin.petersen, john.g.garry, gjoyce
By default, the EWMA (Exponentially Weighted Moving Average) shift
value, used for storing latency samples for latency iopolicy, is set
to 3. The EWMA is calculated using the following formula:
ewma = (old * ((1 << ewma_shift) - 1) + new) >> ewma_shift;
The default value of 3 assigns ~87.5% weight to the existing EWMA value
and ~12.5% weight to the new latency sample. This provides a stable
average that smooths out short-term variations.
However, different workloads may require faster or slower adaptation to
changing conditions. This commit introduces a new debugfs attribute,
latency_ewma_shift, allowing users to tune the weighting factor.
For example:
- latency_ewma_shift = 2 => 75% old, 25% new
- latency_ewma_shift = 1 => 50% old, 50% new
- latency_ewma_shift = 0 => 0% old, 100% new
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
drivers/nvme/host/debugfs.c | 46 +++++++++++++++++++++++++++++++++++
drivers/nvme/host/multipath.c | 9 ++++---
drivers/nvme/host/nvme.h | 1 +
3 files changed, 52 insertions(+), 4 deletions(-)
diff --git a/drivers/nvme/host/debugfs.c b/drivers/nvme/host/debugfs.c
index 26a50566e4a1..4371d7aafae8 100644
--- a/drivers/nvme/host/debugfs.c
+++ b/drivers/nvme/host/debugfs.c
@@ -105,8 +105,54 @@ static const struct file_operations nvme_debugfs_fops = {
.release = nvme_debugfs_release,
};
+#ifdef CONFIG_NVME_MULTIPATH
+static int nvme_latency_ewma_shift_show(void *data, struct seq_file *m)
+{
+ struct nvme_ns_head *head = data;
+
+ seq_printf(m, "%u\n", READ_ONCE(head->latency_ewma_shift));
+ return 0;
+}
+
+static ssize_t nvme_latency_ewma_shift_store(void *data,
+ const char __user *ubuf, size_t count, loff_t *ppos)
+{
+ struct nvme_ns_head *head = data;
+ char kbuf[8];
+ u32 res;
+ int ret;
+ size_t len;
+ char *arg;
+
+ len = min(sizeof(kbuf) - 1, count);
+
+ if (copy_from_user(kbuf, ubuf, len))
+ return -EFAULT;
+
+ kbuf[len] = '\0';
+ arg = strstrip(kbuf);
+
+ ret = kstrtou32(arg, 0, &res);
+ if (ret)
+ return ret;
+
+ /*
+ * Values greater than 8 are nonsensical, as they effectively assign
+ * zero weight to new samples.
+ */
+ if (res > 8)
+ return -EINVAL;
+
+ WRITE_ONCE(head->latency_ewma_shift, res);
+ return count;
+}
+#endif
static const struct nvme_debugfs_attr nvme_mpath_debugfs_attrs[] = {
+#ifdef CONFIG_NVME_MULTIPATH
+ {"latency_ewma_shift", 0600, nvme_latency_ewma_shift_show,
+ nvme_latency_ewma_shift_store},
+#endif
{},
};
diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 541d12b73b74..3e76e07a0376 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -281,10 +281,9 @@ static void nvme_mpath_weight_work(struct work_struct *weight_work)
* For instance, with EWMA_SHIFT = 3, this assigns 7/8 (~87.5 %) weight to
* the existing/old ewma and 1/8 (~12.5%) weight to the new sample.
*/
-static inline u64 calc_ewma_update(u64 old, u64 new)
+static inline u64 calc_ewma_update(u64 old, u64 new, u32 ewma_shift)
{
- return (old * ((1 << NVME_DEFAULT_LATENCY_EWMA_SHIFT) - 1)
- + new) >> NVME_DEFAULT_LATENCY_EWMA_SHIFT;
+ return (old * ((1 << ewma_shift) - 1) + new) >> ewma_shift;
}
static void nvme_mpath_add_sample(struct request *rq, struct nvme_ns *ns)
@@ -375,7 +374,8 @@ static void nvme_mpath_add_sample(struct request *rq, struct nvme_ns *ns)
if (unlikely(!stat->slat_ns))
WRITE_ONCE(stat->slat_ns, avg_lat_ns);
else {
- slat_ns = calc_ewma_update(stat->slat_ns, avg_lat_ns);
+ slat_ns = calc_ewma_update(stat->slat_ns, avg_lat_ns,
+ READ_ONCE(head->latency_ewma_shift));
WRITE_ONCE(stat->slat_ns, slat_ns);
}
@@ -1113,6 +1113,7 @@ int nvme_mpath_alloc_disk(struct nvme_ctrl *ctrl, struct nvme_ns_head *head)
INIT_WORK(&head->partition_scan_work, nvme_partition_scan_work);
INIT_DELAYED_WORK(&head->remove_work, nvme_remove_head_work);
head->delayed_removal_secs = 0;
+ head->latency_ewma_shift = NVME_DEFAULT_LATENCY_EWMA_SHIFT;
/*
* If "multipath_always_on" is enabled, a multipath node is added
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index 7ee9689ce07e..40009c024ab8 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -599,6 +599,7 @@ struct nvme_ns_head {
unsigned int delayed_removal_secs;
struct nvme_ns * __percpu *latency_path;
+ u32 latency_ewma_shift;
#define NVME_NSHEAD_DISK_LIVE 0
#define NVME_NSHEAD_QUEUE_IF_NO_PATH 1
--
2.53.0
^ permalink raw reply related [flat|nested] 13+ messages in thread* [PATCHv6 6/8] nvme-multipath: add debugfs attribute latency_batch_timeout
2026-05-20 18:20 [PATCHv6 0/8] nvme-multipath: introduce latency I/O policy Nilay Shroff
` (4 preceding siblings ...)
2026-05-20 18:21 ` [PATCHv6 5/8] nvme-multipath: add debugfs attribute latency_ewma_shift Nilay Shroff
@ 2026-05-20 18:21 ` Nilay Shroff
2026-05-20 18:21 ` [PATCHv6 7/8] nvme-multipath: add debugfs attribute latency_stat Nilay Shroff
2026-05-20 18:21 ` [PATCHv6 8/8] nvme-multipath: add documentation for latency I/O policy Nilay Shroff
7 siblings, 0 replies; 13+ messages in thread
From: Nilay Shroff @ 2026-05-20 18:21 UTC (permalink / raw)
To: linux-nvme
Cc: hare, kbusch, hch, sagi, dwagner, kanie, jmeneghi, randyj,
martin.petersen, john.g.garry, gjoyce
By default, the latency I/O policy accumulates latency samples over a
15-second window. When this window expires, the driver computes the
average latency and updates the smoothed (EWMA) latency value. The
path weight is then recalculated based on this data.
A 15-second window provides a good balance for most workloads, as it
helps smooth out transient latency spikes and produces a more stable
path weight profile. However, some workloads may benefit from faster
or slower adaptation to changing latency conditions.
This commit introduces a new debugfs attribute, latency_batch_timeout,
which allows users to configure the latency batch window and thus path
weight calculation interval based on their workload requirements.
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
drivers/nvme/host/debugfs.c | 37 +++++++++++++++++++++++++++++++++++
drivers/nvme/host/multipath.c | 8 ++++++--
drivers/nvme/host/nvme.h | 1 +
3 files changed, 44 insertions(+), 2 deletions(-)
diff --git a/drivers/nvme/host/debugfs.c b/drivers/nvme/host/debugfs.c
index 4371d7aafae8..63b0ad5d105b 100644
--- a/drivers/nvme/host/debugfs.c
+++ b/drivers/nvme/host/debugfs.c
@@ -146,12 +146,49 @@ static ssize_t nvme_latency_ewma_shift_store(void *data,
WRITE_ONCE(head->latency_ewma_shift, res);
return count;
}
+
+static int nvme_latency_batch_timeout_show(void *data, struct seq_file *m)
+{
+ struct nvme_ns_head *head = data;
+
+ seq_printf(m, "%llu\n",
+ div_u64(READ_ONCE(head->latency_batch_timeout), NSEC_PER_SEC));
+ return 0;
+}
+
+static ssize_t nvme_latency_batch_timeout_store(void *data,
+ const char __user *ubuf, size_t count, loff_t *ppos)
+{
+ struct nvme_ns_head *head = data;
+ char kbuf[8];
+ u32 res;
+ int ret;
+ size_t len;
+ char *arg;
+
+ len = min(sizeof(kbuf) - 1, count);
+
+ if (copy_from_user(kbuf, ubuf, len))
+ return -EFAULT;
+
+ kbuf[len] = '\0';
+ arg = strstrip(kbuf);
+
+ ret = kstrtou32(arg, 0, &res);
+ if (ret)
+ return ret;
+
+ WRITE_ONCE(head->latency_batch_timeout, res * NSEC_PER_SEC);
+ return count;
+}
#endif
static const struct nvme_debugfs_attr nvme_mpath_debugfs_attrs[] = {
#ifdef CONFIG_NVME_MULTIPATH
{"latency_ewma_shift", 0600, nvme_latency_ewma_shift_show,
nvme_latency_ewma_shift_store},
+ {"latency_batch_timeout", 0600, nvme_latency_batch_timeout_show,
+ nvme_latency_batch_timeout_store},
#endif
{},
};
diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 3e76e07a0376..aa817bfa4b81 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -349,8 +349,11 @@ static void nvme_mpath_add_sample(struct request *rq, struct nvme_ns *ns)
stat->batch_count++;
stat->nr_samples++;
- if (now > stat->last_batch_ts && ((now - stat->last_batch_ts) >=
- NVME_DEFAULT_LATENCY_BATCH_TIMEOUT)) {
+ if (now > stat->last_batch_ts) {
+ u64 timeout = READ_ONCE(head->latency_batch_timeout);
+
+ if ((now - stat->last_batch_ts) < timeout)
+ return;
/*
* Find simple average latency for the last epoch (~15 sec
@@ -1114,6 +1117,7 @@ int nvme_mpath_alloc_disk(struct nvme_ctrl *ctrl, struct nvme_ns_head *head)
INIT_DELAYED_WORK(&head->remove_work, nvme_remove_head_work);
head->delayed_removal_secs = 0;
head->latency_ewma_shift = NVME_DEFAULT_LATENCY_EWMA_SHIFT;
+ head->latency_batch_timeout = NVME_DEFAULT_LATENCY_BATCH_TIMEOUT;
/*
* If "multipath_always_on" is enabled, a multipath node is added
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index 40009c024ab8..a694dd091a16 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -600,6 +600,7 @@ struct nvme_ns_head {
struct nvme_ns * __percpu *latency_path;
u32 latency_ewma_shift;
+ u64 latency_batch_timeout;
#define NVME_NSHEAD_DISK_LIVE 0
#define NVME_NSHEAD_QUEUE_IF_NO_PATH 1
--
2.53.0
^ permalink raw reply related [flat|nested] 13+ messages in thread* [PATCHv6 7/8] nvme-multipath: add debugfs attribute latency_stat
2026-05-20 18:20 [PATCHv6 0/8] nvme-multipath: introduce latency I/O policy Nilay Shroff
` (5 preceding siblings ...)
2026-05-20 18:21 ` [PATCHv6 6/8] nvme-multipath: add debugfs attribute latency_batch_timeout Nilay Shroff
@ 2026-05-20 18:21 ` Nilay Shroff
2026-05-20 18:21 ` [PATCHv6 8/8] nvme-multipath: add documentation for latency I/O policy Nilay Shroff
7 siblings, 0 replies; 13+ messages in thread
From: Nilay Shroff @ 2026-05-20 18:21 UTC (permalink / raw)
To: linux-nvme
Cc: hare, kbusch, hch, sagi, dwagner, kanie, jmeneghi, randyj,
martin.petersen, john.g.garry, gjoyce
This commit introduces a new debugfs attribute, "latency_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 latency I/O policy to aid in debugging and performance
analysis.
For per-path entries, "latency_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 I/O path selection behavior
and help diagnose imbalance or instability in multipath performance.
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
drivers/nvme/host/debugfs.c | 124 ++++++++++++++++++++++++++++++++++++
1 file changed, 124 insertions(+)
diff --git a/drivers/nvme/host/debugfs.c b/drivers/nvme/host/debugfs.c
index 63b0ad5d105b..b4bc7c710050 100644
--- a/drivers/nvme/host/debugfs.c
+++ b/drivers/nvme/host/debugfs.c
@@ -181,6 +181,126 @@ static ssize_t nvme_latency_batch_timeout_store(void *data,
WRITE_ONCE(head->latency_batch_timeout, res * NSEC_PER_SEC);
return count;
}
+
+static void *nvme_mpath_latency_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;
+
+ if (!head->disk)
+ return NULL;
+
+ /* 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_latency_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_latency_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;
+
+ if (!head->disk)
+ return;
+
+ srcu_read_unlock(&head->srcu, srcu_idx);
+}
+
+static int nvme_mpath_latency_stat_show(struct seq_file *m, void *v)
+{
+ int i, cpu;
+ struct nvme_path_lat_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 (i = 0; i < NVME_NUM_STAT_GROUPS; i++) {
+ stat = &per_cpu_ptr(ns->path_lat, cpu)[i].stat;
+ seq_printf(m, "%u %u %llu %llu %llu %llu %llu ",
+ stat->weight, stat->credit, stat->score,
+ stat->slat_ns, stat->sel,
+ stat->nr_samples, stat->nr_ignored);
+ }
+ seq_putc(m, '\n');
+ }
+ return 0;
+}
+
+static const struct seq_operations nvme_mpath_latency_stat_seq_ops = {
+ .start = nvme_mpath_latency_stat_start,
+ .next = nvme_mpath_latency_stat_next,
+ .stop = nvme_mpath_latency_stat_stop,
+ .show = nvme_mpath_latency_stat_show
+};
+
+static void nvme_latency_stat_read_all(struct nvme_ns *ns,
+ struct nvme_path_lat_stat *batch)
+{
+ int i, cpu;
+ u32 ncpu[NVME_NUM_STAT_GROUPS] = {0};
+ struct nvme_path_lat_stat *stat;
+
+ for_each_online_cpu(cpu) {
+ for (i = 0; i < NVME_NUM_STAT_GROUPS; i++) {
+ stat = &per_cpu_ptr(ns->path_lat, cpu)[i].stat;
+ batch[i].sel += stat->sel;
+ batch[i].nr_samples += stat->nr_samples;
+ batch[i].nr_ignored += stat->nr_ignored;
+ batch[i].weight += stat->weight;
+ if (stat->weight)
+ ncpu[i]++;
+ }
+ }
+
+ for (i = 0; i < NVME_NUM_STAT_GROUPS; i++) {
+ if (!ncpu[i])
+ continue;
+ batch[i].weight = DIV_U64_ROUND_CLOSEST(batch[i].weight,
+ ncpu[i]);
+ }
+}
+
+static int nvme_ns_latency_stat_show(void *data, struct seq_file *m)
+{
+ int i;
+ struct nvme_path_lat_stat stat[NVME_NUM_STAT_GROUPS] = {0};
+ struct nvme_ns *ns = (struct nvme_ns *)data;
+
+ if (!ns->head->disk)
+ return 0;
+
+ nvme_latency_stat_read_all(ns, stat);
+ for (i = 0; i < NVME_NUM_STAT_GROUPS; i++) {
+ seq_printf(m, "%u %llu %llu %llu ",
+ stat[i].weight, stat[i].sel,
+ stat[i].nr_samples, stat[i].nr_ignored);
+ }
+ return 0;
+}
#endif
static const struct nvme_debugfs_attr nvme_mpath_debugfs_attrs[] = {
@@ -189,11 +309,15 @@ static const struct nvme_debugfs_attr nvme_mpath_debugfs_attrs[] = {
nvme_latency_ewma_shift_store},
{"latency_batch_timeout", 0600, nvme_latency_batch_timeout_show,
nvme_latency_batch_timeout_store},
+ {"latency_stat", 0400, .seq_ops = &nvme_mpath_latency_stat_seq_ops},
#endif
{},
};
static const struct nvme_debugfs_attr nvme_ns_debugfs_attrs[] = {
+#ifdef CONFIG_NVME_MULTIPATH
+ {"latency_stat", 0400, nvme_ns_latency_stat_show},
+#endif
{},
};
--
2.53.0
^ permalink raw reply related [flat|nested] 13+ messages in thread* [PATCHv6 8/8] nvme-multipath: add documentation for latency I/O policy
2026-05-20 18:20 [PATCHv6 0/8] nvme-multipath: introduce latency I/O policy Nilay Shroff
` (6 preceding siblings ...)
2026-05-20 18:21 ` [PATCHv6 7/8] nvme-multipath: add debugfs attribute latency_stat Nilay Shroff
@ 2026-05-20 18:21 ` Nilay Shroff
7 siblings, 0 replies; 13+ messages in thread
From: Nilay Shroff @ 2026-05-20 18:21 UTC (permalink / raw)
To: linux-nvme
Cc: hare, kbusch, hch, sagi, dwagner, kanie, jmeneghi, randyj,
martin.petersen, john.g.garry, gjoyce
Update the nvme-multipath documentation to describe the latency I/O
policy, its behavior, and when it is suitable for use.
Suggested-by: Guixin Liu <kanie@linux.alibaba.com>
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
Documentation/admin-guide/nvme-multipath.rst | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)
diff --git a/Documentation/admin-guide/nvme-multipath.rst b/Documentation/admin-guide/nvme-multipath.rst
index 97ca1ccef459..41a8054638ff 100644
--- a/Documentation/admin-guide/nvme-multipath.rst
+++ b/Documentation/admin-guide/nvme-multipath.rst
@@ -70,3 +70,22 @@ When to use the queue-depth policy:
1. High load with small I/Os: Effectively balances load across paths when
the load is high, and I/O operations consist of small, relatively
fixed-sized requests.
+
+Latency
+--------
+
+The latency policy manages I/O requests based on path latency. It periodically
+calculates a weight for each path and distributes I/O accordingly. Paths with
+higher latency receive lower weights, resulting in fewer I/O requests being sent
+to them, while paths with lower latency handle a proportionally larger share of
+the I/O load.
+
+When to use the latency policy
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+1. Homogeneous Path Performance: Utilizes all available paths efficiently when
+ their performance characteristics (e.g., latency, bandwidth) are similar.
+
+2. Heterogeneous Path Performance: Dynamically distributes I/O based on per-path
+ performance characteristics. Paths with lower latency receive a higher share
+ of I/O compared to those with higher latency.
--
2.53.0
^ permalink raw reply related [flat|nested] 13+ messages in thread