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Subject[PATCH] nvme-multipath: implement active-active round-robin path selector
Date
Some storage environments (e.g., dual-port NVMe SSD) provide higher
performance when using multiple paths simultaneously. Choosing a
path from multiple paths in a round-robin fashion is a simple and
efficient way to meet these requirements.

We implement the active-active round-robin path selector that
chooses the path that is NVME_CTRL_LIVE and next to the previous
path. By maintaining the structure of the active-standby path
selector, we can easily switch between the active-standby path
selector and the active-active round-robin path selector.

Example usage)
# cat /sys/block/nvme0n1/mpath_policy
[active-standby] round-robin
# echo round-robin > /sys/block/nvme0n1/mpath_policy
# cat /sys/block/nvme0n1/mpath_policy
active-standby [round-robin]

Below are the results from a physical dual-port NVMe SSD using fio.

(MB/s) active-standby round-robin
Random Read (4k) 1,672 2,640
Sequential Read (128k) 1,707 3,414
Random Write (4k) 1,450 1,728
Sequential Write (128k) 1,481 2,708

A single thread was used for sequential workloads and 16 threads
were used for random workloads. The queue depth for each thread
was 64.

Signed-off-by: Baegjae Sung <baegjae@gmail.com>
---
drivers/nvme/host/core.c | 49 +++++++++++++++++++++++++++++++++++++++++++
drivers/nvme/host/multipath.c | 45 ++++++++++++++++++++++++++++++++++++++-
drivers/nvme/host/nvme.h | 8 +++++++
3 files changed, 101 insertions(+), 1 deletion(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 7aeca5db7916..cc91e8b247d0 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -68,6 +68,13 @@ static bool streams;
module_param(streams, bool, 0644);
MODULE_PARM_DESC(streams, "turn on support for Streams write directives");

+#ifdef CONFIG_NVME_MULTIPATH
+static const char *const mpath_policy_name[] = {
+ [NVME_MPATH_ACTIVE_STANDBY] = "active-standby",
+ [NVME_MPATH_ROUND_ROBIN] = "round-robin",
+};
+#endif
+
/*
* nvme_wq - hosts nvme related works that are not reset or delete
* nvme_reset_wq - hosts nvme reset works
@@ -2603,12 +2610,51 @@ static ssize_t nsid_show(struct device *dev, struct device_attribute *attr,
}
static DEVICE_ATTR_RO(nsid);

+#ifdef CONFIG_NVME_MULTIPATH
+static ssize_t mpath_policy_show(struct device *dev,
+ struct device_attribute *attr,
+ char *buf)
+{
+ int i, len = 0;
+ struct nvme_ns_head *head = dev_to_ns_head(dev);
+
+ for (i = 0;i < ARRAY_SIZE(mpath_policy_name);i++) {
+ if (i == head->mpath_policy)
+ len += sprintf(buf + len, "[%s] ", mpath_policy_name[i]);
+ else
+ len += sprintf(buf + len, "%s ", mpath_policy_name[i]);
+ }
+ len += sprintf(buf + len, "\n");
+ return len;
+}
+static ssize_t mpath_policy_store(struct device *dev,
+ struct device_attribute *attr, const char *buf,
+ size_t count)
+{
+ int i;
+ struct nvme_ns_head *head = dev_to_ns_head(dev);
+
+ for (i = 0;i < ARRAY_SIZE(mpath_policy_name);i++) {
+ if (strncmp(buf, mpath_policy_name[i], count - 1) == 0) {
+ head->mpath_policy = i;
+ dev_info(dev, "change mpath policy to %s\n", mpath_policy_name[i]);
+ }
+ }
+ return count;
+}
+static DEVICE_ATTR(mpath_policy, S_IRUGO | S_IWUSR, mpath_policy_show, \
+ mpath_policy_store);
+#endif
+
static struct attribute *nvme_ns_id_attrs[] = {
&dev_attr_wwid.attr,
&dev_attr_uuid.attr,
&dev_attr_nguid.attr,
&dev_attr_eui.attr,
&dev_attr_nsid.attr,
+#ifdef CONFIG_NVME_MULTIPATH
+ &dev_attr_mpath_policy.attr,
+#endif
NULL,
};

@@ -2818,6 +2864,9 @@ static struct nvme_ns_head *nvme_alloc_ns_head(struct nvme_ctrl *ctrl,
head->subsys = ctrl->subsys;
head->ns_id = nsid;
kref_init(&head->ref);
+#ifdef CONFIG_NVME_MULTIPATH
+ head->mpath_policy = NVME_MPATH_ACTIVE_STANDBY;
+#endif

nvme_report_ns_ids(ctrl, nsid, id, &head->ids);

diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 060f69e03427..6b6a15ccb542 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -75,6 +75,42 @@ inline struct nvme_ns *nvme_find_path(struct nvme_ns_head *head)
return ns;
}

+inline struct nvme_ns *nvme_find_path_rr(struct nvme_ns_head *head)
+{
+ struct nvme_ns *prev_ns = srcu_dereference(head->current_path, &head->srcu);
+ struct nvme_ns *ns, *cand_ns = NULL;
+ bool after_prev_ns = false;
+
+ /*
+ * Active-active round-robin path selector
+ * Choose the path that is NVME_CTRL_LIVE and next to the previous path
+ */
+
+ /* Case 1. If there is no previous path, choose the first LIVE path */
+ if (!prev_ns) {
+ ns = __nvme_find_path(head);
+ return ns;
+ }
+
+ list_for_each_entry_rcu(ns, &head->list, siblings) {
+ /*
+ * Case 2-1. Choose the first LIVE path from the next path of
+ * previous path to end
+ */
+ if (after_prev_ns && ns->ctrl->state == NVME_CTRL_LIVE) {
+ rcu_assign_pointer(head->current_path, ns);
+ return ns;
+ }
+ /* Case 2-2. Mark the first LIVE path from start to previous path */
+ if (!cand_ns && ns->ctrl->state == NVME_CTRL_LIVE)
+ cand_ns = ns;
+ if (ns == prev_ns)
+ after_prev_ns = true;
+ }
+ rcu_assign_pointer(head->current_path, cand_ns);
+ return cand_ns;
+}
+
static blk_qc_t nvme_ns_head_make_request(struct request_queue *q,
struct bio *bio)
{
@@ -85,7 +121,14 @@ static blk_qc_t nvme_ns_head_make_request(struct request_queue *q,
int srcu_idx;

srcu_idx = srcu_read_lock(&head->srcu);
- ns = nvme_find_path(head);
+ switch (head->mpath_policy) {
+ case NVME_MPATH_ROUND_ROBIN:
+ ns = nvme_find_path_rr(head);
+ break;
+ case NVME_MPATH_ACTIVE_STANDBY:
+ default:
+ ns = nvme_find_path(head);
+ }
if (likely(ns)) {
bio->bi_disk = ns->disk;
bio->bi_opf |= REQ_NVME_MPATH;
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index d733b14ede9d..15e1163bbf2b 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -128,6 +128,13 @@ enum nvme_ctrl_state {
NVME_CTRL_DEAD,
};

+#ifdef CONFIG_NVME_MULTIPATH
+enum nvme_mpath_policy {
+ NVME_MPATH_ACTIVE_STANDBY,
+ NVME_MPATH_ROUND_ROBIN, /* active-active round-robin */
+};
+#endif
+
struct nvme_ctrl {
enum nvme_ctrl_state state;
bool identified;
@@ -250,6 +257,7 @@ struct nvme_ns_head {
struct bio_list requeue_list;
spinlock_t requeue_lock;
struct work_struct requeue_work;
+ enum nvme_mpath_policy mpath_policy;
#endif
struct list_head list;
struct srcu_struct srcu;
--
2.16.2
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