RDMA/rtrs-clt: Add a minimum latency multipath policy

This patch adds new multipath policy: min-latency.  Client checks the
latency of each path when it sends the heart-beat.  And it sends IO to the
path with the minimum latency.

Link: https://lore.kernel.org/r/20210407113444.150961-2-gi-oh.kim@ionos.com
Signed-off-by: Gioh Kim <gi-oh.kim@ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

Author: Gioh Kim

drivers/infiniband/ulp/rtrs/rtrs-clt-sysfs.c

@@ -101,6 +101,9 @@ static ssize_t mpath_policy_show(struct device *dev,
 	case MP_POLICY_MIN_INFLIGHT:
 		return sysfs_emit(page, "min-inflight (MI: %d)\n",
 				  clt->mp_policy);
+	case MP_POLICY_MIN_LATENCY:
+		return sysfs_emit(page, "min-latency (ML: %d)\n",
+				  clt->mp_policy);
 	default:
 		return sysfs_emit(page, "Unknown (%d)\n", clt->mp_policy);
 	}
@@ -114,22 +117,32 @@ static ssize_t mpath_policy_store(struct device *dev,
 	struct rtrs_clt *clt;
 	int value;
 	int ret;
+	size_t len = 0;
 
 	clt = container_of(dev, struct rtrs_clt, dev);
 
 	ret = kstrtoint(buf, 10, &value);
 	if (!ret && (value == MP_POLICY_RR ||
-		     value == MP_POLICY_MIN_INFLIGHT)) {
+		     value == MP_POLICY_MIN_INFLIGHT ||
+		     value == MP_POLICY_MIN_LATENCY)) {
 		clt->mp_policy = value;
 		return count;
 	}
 
+	/* distinguish "mi" and "min-latency" with length */
+	len = strnlen(buf, NAME_MAX);
+	if (buf[len - 1] == '\n')
+		len--;
+
 	if (!strncasecmp(buf, "round-robin", 11) ||
-	    !strncasecmp(buf, "rr", 2))
+	    (len == 2 && !strncasecmp(buf, "rr", 2)))
 		clt->mp_policy = MP_POLICY_RR;
 	else if (!strncasecmp(buf, "min-inflight", 12) ||
-		 !strncasecmp(buf, "mi", 2))
+		 (len == 2 && !strncasecmp(buf, "mi", 2)))
 		clt->mp_policy = MP_POLICY_MIN_INFLIGHT;
+	else if (!strncasecmp(buf, "min-latency", 11) ||
+		 (len == 2 && !strncasecmp(buf, "ml", 2)))
+		clt->mp_policy = MP_POLICY_MIN_LATENCY;
 	else
 		return -EINVAL;
 

drivers/infiniband/ulp/rtrs/rtrs-clt.c

@@ -628,6 +628,8 @@ static void rtrs_clt_rdma_done(struct ib_cq *cq, struct ib_wc *wc)
 		} else if (imm_type == RTRS_HB_ACK_IMM) {
 			WARN_ON(con->c.cid);
 			sess->s.hb_missed_cnt = 0;
+			sess->s.hb_cur_latency =
+				ktime_sub(ktime_get(), sess->s.hb_last_sent);
 			if (sess->flags & RTRS_MSG_NEW_RKEY_F)
 				return  rtrs_clt_recv_done(con, wc);
 		} else {
@@ -826,6 +828,57 @@ static struct rtrs_clt_sess *get_next_path_min_inflight(struct path_it *it)
 	return min_path;
 }
 
+/**
+ * get_next_path_min_latency() - Returns path with minimal latency.
+ * @it:	the path pointer
+ *
+ * Return: a path with the lowest latency or NULL if all paths are tried
+ *
+ * Locks:
+ *    rcu_read_lock() must be hold.
+ *
+ * Related to @MP_POLICY_MIN_LATENCY
+ *
+ * This DOES skip an already-tried path.
+ * There is a skip-list to skip a path if the path has tried but failed.
+ * It will try the minimum latency path and then the second minimum latency
+ * path and so on. Finally it will return NULL if all paths are tried.
+ * Therefore the caller MUST check the returned
+ * path is NULL and trigger the IO error.
+ */
+static struct rtrs_clt_sess *get_next_path_min_latency(struct path_it *it)
+{
+	struct rtrs_clt_sess *min_path = NULL;
+	struct rtrs_clt *clt = it->clt;
+	struct rtrs_clt_sess *sess;
+	ktime_t min_latency = INT_MAX;
+	ktime_t latency;
+
+	list_for_each_entry_rcu(sess, &clt->paths_list, s.entry) {
+		if (unlikely(READ_ONCE(sess->state) != RTRS_CLT_CONNECTED))
+			continue;
+
+		if (unlikely(!list_empty(raw_cpu_ptr(sess->mp_skip_entry))))
+			continue;
+
+		latency = sess->s.hb_cur_latency;
+
+		if (latency < min_latency) {
+			min_latency = latency;
+			min_path = sess;
+		}
+	}
+
+	/*
+	 * add the path to the skip list, so that next time we can get
+	 * a different one
+	 */
+	if (min_path)
+		list_add(raw_cpu_ptr(min_path->mp_skip_entry), &it->skip_list);
+
+	return min_path;
+}
+
 static inline void path_it_init(struct path_it *it, struct rtrs_clt *clt)
 {
 	INIT_LIST_HEAD(&it->skip_list);
@@ -834,8 +887,10 @@ static inline void path_it_init(struct path_it *it, struct rtrs_clt *clt)
 
 	if (clt->mp_policy == MP_POLICY_RR)
 		it->next_path = get_next_path_rr;
-	else
+	else if (clt->mp_policy == MP_POLICY_MIN_INFLIGHT)
 		it->next_path = get_next_path_min_inflight;
+	else
+		it->next_path = get_next_path_min_latency;
 }
 
 static inline void path_it_deinit(struct path_it *it)

drivers/infiniband/ulp/rtrs/rtrs-clt.h

@@ -29,6 +29,7 @@ enum rtrs_clt_state {
 enum rtrs_mp_policy {
 	MP_POLICY_RR,
 	MP_POLICY_MIN_INFLIGHT,
+	MP_POLICY_MIN_LATENCY,
 };
 
 /* see Documentation/ABI/testing/sysfs-class-rtrs-client for details */

drivers/infiniband/ulp/rtrs/rtrs-pri.h

@@ -112,6 +112,8 @@ struct rtrs_sess {
 	unsigned int		hb_interval_ms;
 	unsigned int		hb_missed_cnt;
 	unsigned int		hb_missed_max;
+	ktime_t			hb_last_sent;
+	ktime_t			hb_cur_latency;
 };
 
 /* rtrs information unit */

drivers/infiniband/ulp/rtrs/rtrs.c

@@ -337,6 +337,9 @@ static void hb_work(struct work_struct *work)
 		schedule_hb(sess);
 		return;
 	}
+
+	sess->hb_last_sent = ktime_get();
+
 	imm = rtrs_to_imm(RTRS_HB_MSG_IMM, 0);
 	err = rtrs_post_rdma_write_imm_empty(usr_con, sess->hb_cqe, imm,
 					     0, NULL);