[RFC PATCHv2 2/4] nvme-multipath: add support for adaptive I/O policy
Hannes Reinecke
hare at suse.de
Sun Oct 12 23:31:46 PDT 2025
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 at 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 at 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
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