Re: [RFC PATCHv3 4/6] nvme: add sysfs attribute adp_weight_timeout

[Hannes Reinecke <hare@suse.de>]

On 10/27/25 10:29, Nilay Shroff wrote:
> By default, the adaptive 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 sysfs attribute, adp_weight_timeout,
> which allows users to configure the path weight calculation interval
> based on their workload requirements.
> 
> Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
> ---
>   drivers/nvme/host/core.c      |  6 ++++++
>   drivers/nvme/host/multipath.c | 38 +++++++++++++++++++++++++++++++++--
>   drivers/nvme/host/nvme.h      |  4 +++-
>   drivers/nvme/host/sysfs.c     |  1 +
>   4 files changed, 46 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index ab09b9724674..f48c6bc25055 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -3248,6 +3248,12 @@ static int nvme_init_subsystem(struct nvme_ctrl *ctrl, struct nvme_id_ctrl *id)
>   	 * used while adding latency sample for adaptive iopolicy.
>   	 */
>   	subsys->adp_ewma_shift = NVME_DEFAULT_ADP_EWMA_SHIFT;
> +	/*
> +	 * Path weight calculation timeout interval used for adaptive iopolicy.
> +	 * The default value of this paremeter is set to 15 seconds. However, it
> +	 * could be also changed through sysfs.
> +	 */
> +	subsys->adp_weight_timeout = NVME_DEFAULT_ADP_WEIGHT_TIMEOUT;
>   #endif
>   	subsys->dev.class = &nvme_subsys_class;
>   	subsys->dev.release = nvme_release_subsystem;
> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
> index 95407c0f2f4b..d4df01511ee9 100644
> --- a/drivers/nvme/host/multipath.c
> +++ b/drivers/nvme/host/multipath.c
> @@ -362,8 +362,11 @@ static void nvme_mpath_add_sample(struct request *rq, struct nvme_ns *ns)
>   	stat->batch_count++;
>   	stat->nr_samples++;
>   
> -	if (now > stat->last_weight_ts &&
> -	    (now - stat->last_weight_ts) >= NVME_DEFAULT_ADP_WEIGHT_TIMEOUT) {
> +	if (now > stat->last_weight_ts) {
> +		u64 timeout = READ_ONCE(head->subsys->adp_weight_timeout);
> +
> +		if ((now - stat->last_weight_ts) < timeout)
> +			return;
>   
>   		stat->last_weight_ts = now;
>   
> @@ -1495,6 +1498,37 @@ static ssize_t nvme_subsys_adp_ewma_shift_store(struct device *dev,
>   SUBSYS_ATTR_RW(adp_ewma_shift, 0644, nvme_subsys_adp_ewma_shift_show,
>   		nvme_subsys_adp_ewma_shift_store);
>   
> +static ssize_t nvme_subsys_adp_weight_timeout_show(struct device *dev,
> +		struct device_attribute *attr, char *buf)
> +{
> +	struct nvme_subsystem *subsys =
> +		container_of(dev, struct nvme_subsystem, dev);
> +
> +	return sysfs_emit(buf, "%llu\n",
> +		div_u64(READ_ONCE(subsys->adp_weight_timeout), NSEC_PER_SEC));
> +}
> +
> +static ssize_t nvme_subsys_adp_weight_timeout_store(struct device *dev,
> +		struct device_attribute *attr, const char *buf, size_t count)
> +{
> +	int timeout, err;
> +	struct nvme_subsystem *subsys =
> +		container_of(dev, struct nvme_subsystem, dev);
> +
> +	err = kstrtoint(buf, 0, &timeout);
> +	if (err)
> +		return -EINVAL;
> +
> +	if (timeout <= 0)
> +		return -EINVAL;
> +
> +	WRITE_ONCE(subsys->adp_weight_timeout, timeout * NSEC_PER_SEC);
> +	return count;
> +}
> +
> +SUBSYS_ATTR_RW(adp_weight_timeout, 0644, nvme_subsys_adp_weight_timeout_show,
> +		nvme_subsys_adp_weight_timeout_store);
> +
>   static ssize_t ana_grpid_show(struct device *dev, struct device_attribute *attr,
>   		char *buf)
>   {
> diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
> index 9f5b233c747a..2e58d4d6902a 100644
> --- a/drivers/nvme/host/nvme.h
> +++ b/drivers/nvme/host/nvme.h
> @@ -450,7 +450,8 @@ struct nvme_subsystem {
>   	struct ida		ns_ida;
>   #ifdef CONFIG_NVME_MULTIPATH
>   	enum nvme_iopolicy	iopolicy;
> -	int			adp_ewma_shift; /* used for adaptive iopolicy */
> +	int			adp_ewma_shift;     /* used for adaptive iopolicy */
> +	u64			adp_weight_timeout; /* used for adaptive iopolicy */
>   #endif
>   };
>   
> @@ -1045,6 +1046,7 @@ extern struct device_attribute dev_attr_numa_nodes;
>   extern struct device_attribute dev_attr_delayed_removal_secs;
>   extern struct device_attribute subsys_attr_iopolicy;
>   extern struct device_attribute subsys_attr_adp_ewma_shift;
> +extern struct device_attribute subsys_attr_adp_weight_timeout;
>   
>   static inline bool nvme_disk_is_ns_head(struct gendisk *disk)
>   {
> diff --git a/drivers/nvme/host/sysfs.c b/drivers/nvme/host/sysfs.c
> index cf9711961b00..18d7eddd477a 100644
> --- a/drivers/nvme/host/sysfs.c
> +++ b/drivers/nvme/host/sysfs.c
> @@ -918,6 +918,7 @@ static struct attribute *nvme_subsys_attrs[] = {
>   #ifdef CONFIG_NVME_MULTIPATH
>   	&subsys_attr_iopolicy.attr,
>   	&subsys_attr_adp_ewma_shift.attr,
> +	&subsys_attr_adp_weight_timeout.attr,
>   #endif
>   	NULL,
>   };

Similar comment: can we please move it to debugfs?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
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