[PATCH net-next v1 5/7] taprio: Add support for txtime offload mode.

[Vedang Patel <vedang.patel@intel.com>]

Currently, we are seeing non-critical packets being transmitted outside
of their timeslice. We can confirm that the packets are being dequeued
at the right time. So, the delay is induced in the hardware side.  The
most likely reason is the hardware queues are starving the lower
priority queues.

In order to improve the performance of taprio, we will be making use of the
txtime feature provided by the ETF qdisc. For all the packets which do not have
the SO_TXTIME option set, taprio will set the transmit timestamp (set in
skb->tstamp) in this mode. TAPrio Qdisc will ensure that the transmit time for
the packet is set to when the gate is open. If SO_TXTIME is set, the TAPrio
qdisc will validate whether the timestamp (in skb->tstamp) occurs when the gate
corresponding to skb's traffic class is open.

Following is the example configuration for enabling txtime offload:

tc qdisc replace dev eth0 parent root handle 100 taprio \\
      num_tc 3 \\
      map 2 2 1 0 2 2 2 2 2 2 2 2 2 2 2 2 \\
      queues 1@0 1@0 1@0 \\
      base-time 1558653424279842568 \\
      sched-entry S 01 300000 \\
      sched-entry S 02 300000 \\
      sched-entry S 04 400000 \\
      offload 2 \\
      txtime-delay 40000 \\
      clockid CLOCK_TAI

tc qdisc replace dev $IFACE parent 100:1 etf skip_sock_check \\
      offload delta 200000 clockid CLOCK_TAI

Here, the "offload" parameter is indicating that the TXTIME_OFFLOAD mode is
enabled. Also, all the traffic classes are mapped to the same queue.  This is
only possible in taprio when txtime offload is enabled. Also note that the ETF
Qdisc is enabled with offload mode set.

In this mode, if the packet's traffic class is open and the complete packet can
be transmitted, taprio will try to transmit the packet immediately. This will
be done by setting skb->tstamp to current_time + the time delta indicated in
the txtime_delay parameter. This parameter indicates the time taken (in
software) for packet to reach the network adapter.

If the packet cannot be transmitted in the current interval or if the packet's
traffic is not currently transmitting, the skb->tstamp is set to the next
available timestamp value. This is tracked in the next_launchtime parameter in
the struct sched_entry.

The behaviour w.r.t admin and oper schedules is not changed from what is
present in software mode.

The transmit time is already known in advance. So, we do not need the HR timers
to advance the schedule and wakeup the dequeue side of taprio.  So, HR timer
won't be run when this mode is enabled.

Signed-off-by: Vedang Patel <vedang.patel@intel.com>
---
 include/uapi/linux/pkt_sched.h |   1 +
 net/sched/sch_taprio.c         | 326 ++++++++++++++++++++++++++++++---
 2 files changed, 306 insertions(+), 21 deletions(-)

diff --git a/include/uapi/linux/pkt_sched.h b/include/uapi/linux/pkt_sched.h
index 3319255ffa25..afffda24e055 100644
--- a/include/uapi/linux/pkt_sched.h
+++ b/include/uapi/linux/pkt_sched.h
@@ -1174,6 +1174,7 @@ enum {
 	TCA_TAPRIO_ATTR_SCHED_CYCLE_TIME, /* s64 */
 	TCA_TAPRIO_ATTR_SCHED_CYCLE_TIME_EXTENSION, /* s64 */
 	TCA_TAPRIO_ATTR_OFFLOAD_FLAGS, /* u32 */
+	TCA_TAPRIO_ATTR_TXTIME_DELAY, /* s32 */
 	__TCA_TAPRIO_ATTR_MAX,
 };
 
diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index 8d87ba099130..1cd19eabc53b 100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c
@@ -21,6 +21,7 @@
 #include <net/pkt_sched.h>
 #include <net/pkt_cls.h>
 #include <net/sch_generic.h>
+#include <net/sock.h>
 
 static LIST_HEAD(taprio_list);
 static DEFINE_SPINLOCK(taprio_list_lock);
@@ -40,6 +41,7 @@ struct sched_entry {
 	 * packet leaves after this time.
 	 */
 	ktime_t close_time;
+	ktime_t next_txtime;
 	atomic_t budget;
 	int index;
 	u32 gate_mask;
@@ -76,6 +78,7 @@ struct taprio_sched {
 	struct sk_buff *(*peek)(struct Qdisc *sch);
 	struct hrtimer advance_timer;
 	struct list_head taprio_list;
+	int txtime_delay;
 };
 
 static ktime_t sched_base_time(const struct sched_gate_list *sched)
@@ -116,6 +119,235 @@ static void switch_schedules(struct taprio_sched *q,
 	*admin = NULL;
 }
 
+/* Get how much time has been already elapsed in the current cycle. */
+static inline s32 get_cycle_time_elapsed(struct sched_gate_list *sched, ktime_t time)
+{
+	ktime_t time_since_sched_start;
+	s32 time_elapsed;
+
+	time_since_sched_start = ktime_sub(time, sched->base_time);
+	div_s64_rem(time_since_sched_start, sched->cycle_time, &time_elapsed);
+
+	return time_elapsed;
+}
+
+static ktime_t get_interval_end_time(struct sched_gate_list *sched,
+				     struct sched_gate_list *admin,
+				     struct sched_entry *entry,
+				     ktime_t intv_start)
+{
+	s32 cycle_elapsed = get_cycle_time_elapsed(sched, intv_start);
+	ktime_t intv_end, cycle_ext_end, cycle_end;
+
+	cycle_end = ktime_add_ns(intv_start, sched->cycle_time - cycle_elapsed);
+	intv_end = ktime_add_ns(intv_start, entry->interval);
+	cycle_ext_end = ktime_add(cycle_end, sched->cycle_time_extension);
+
+	if (ktime_before(intv_end, cycle_end))
+		return intv_end;
+	else if (admin && admin != sched &&
+		 ktime_after(admin->base_time, cycle_end) &&
+		 ktime_before(admin->base_time, cycle_ext_end))
+		return admin->base_time;
+	else
+		return cycle_end;
+}
+
+static inline int length_to_duration(struct taprio_sched *q, int len)
+{
+	return (len * atomic64_read(&q->picos_per_byte)) / 1000;
+}
+
+/* Returns the entry corresponding to next available interval. If
+ * validate_interval is set, it only validates whether the timestamp occurs
+ * when the gate corresponding to the skb's traffic class is open.
+ */
+static struct sched_entry *find_entry_to_transmit(struct sk_buff *skb,
+						  struct Qdisc *sch,
+						  struct sched_gate_list *sched,
+						  struct sched_gate_list *admin,
+						  ktime_t time,
+						  ktime_t *interval_start,
+						  ktime_t *interval_end,
+						  bool validate_interval)
+{
+	ktime_t curr_intv_start, curr_intv_end, cycle_end, packet_transmit_time;
+	ktime_t earliest_txtime = KTIME_MAX, txtime, cycle, transmit_end_time;
+	struct sched_entry *entry = NULL, *entry_found = NULL;
+	struct taprio_sched *q = qdisc_priv(sch);
+	struct net_device *dev = qdisc_dev(sch);
+	int tc, entry_available = 0, n;
+	s32 cycle_elapsed;
+
+	tc = netdev_get_prio_tc_map(dev, skb->priority);
+	packet_transmit_time = length_to_duration(q, qdisc_pkt_len(skb));
+
+	*interval_start = 0;
+	*interval_end = 0;
+
+	if (!sched)
+		return NULL;
+
+	cycle = sched->cycle_time;
+	cycle_elapsed = get_cycle_time_elapsed(sched, time);
+	curr_intv_end = ktime_sub_ns(time, cycle_elapsed);
+	cycle_end = ktime_add_ns(curr_intv_end, cycle);
+
+	list_for_each_entry(entry, &sched->entries, list) {
+		curr_intv_start = curr_intv_end;
+		curr_intv_end = get_interval_end_time(sched, admin, entry,
+						      curr_intv_start);
+
+		if (ktime_after(curr_intv_start, cycle_end))
+			break;
+
+		if (!(entry->gate_mask & BIT(tc)) ||
+		    packet_transmit_time > entry->interval)
+			continue;
+
+		txtime = entry->next_txtime;
+
+		if (ktime_before(txtime, time) || validate_interval) {
+			transmit_end_time = ktime_add_ns(time, packet_transmit_time);
+			if ((ktime_before(curr_intv_start, time) &&
+			     ktime_before(transmit_end_time, curr_intv_end)) ||
+			    (ktime_after(curr_intv_start, time) && !validate_interval)) {
+				entry_found = entry;
+				*interval_start = curr_intv_start;
+				*interval_end = curr_intv_end;
+				break;
+			} else if (!entry_available && !validate_interval) {
+				/* Here, we are just trying to find out the
+				 * first available interval in the next cycle.
+				 */
+				entry_available = 1;
+				entry_found = entry;
+				*interval_start = ktime_add_ns(curr_intv_start, cycle);
+				*interval_end = ktime_add_ns(curr_intv_end, cycle);
+			}
+		} else if (ktime_before(txtime, earliest_txtime) &&
+			   !entry_available) {
+			earliest_txtime = txtime;
+			entry_found = entry;
+			n = div_s64(ktime_sub(txtime, curr_intv_start), cycle);
+			*interval_start = ktime_add(curr_intv_start, n * cycle);
+			*interval_end = ktime_add(curr_intv_end, n * cycle);
+		}
+	}
+
+	return entry_found;
+}
+
+static bool is_valid_interval(struct sk_buff *skb, struct Qdisc *sch)
+{
+	struct taprio_sched *q = qdisc_priv(sch);
+	struct sched_gate_list *sched, *admin;
+	ktime_t interval_start, interval_end;
+	struct sched_entry *entry;
+
+	rcu_read_lock();
+	sched = rcu_dereference(q->oper_sched);
+	admin = rcu_dereference(q->admin_sched);
+
+	entry = find_entry_to_transmit(skb, sch, sched, admin, skb->tstamp,
+				       &interval_start, &interval_end, true);
+	rcu_read_unlock();
+
+	return entry;
+}
+
+static inline ktime_t get_cycle_start(struct sched_gate_list *sched,
+				      ktime_t time)
+{
+	ktime_t cycle_elapsed;
+
+	cycle_elapsed = get_cycle_time_elapsed(sched, time);
+
+	return ktime_sub(time, cycle_elapsed);
+}
+
+/* There are a few scenarios where we will have to modify the txtime from
+ * what is read from next_txtime in sched_entry. They are:
+ * 1. If txtime is in the past,
+ *    a. The gate for the traffic class is currently open and packet can be
+ *       transmitted before it closes, schedule the packet right away.
+ *    b. If the gate corresponding to the traffic class is going to open later
+ *       in the cycle, set the txtime of packet to the interval start.
+ * 2. If txtime is in the future, there are packets corresponding to the
+ *    current traffic class waiting to be transmitted. So, the following
+ *    possibilities exist:
+ *    a. We can transmit the packet before the window containing the txtime
+ *       closes.
+ *    b. The window might close before the transmission can be completed
+ *       successfully. So, schedule the packet in the next open window.
+ */
+static long get_packet_txtime(struct sk_buff *skb, struct Qdisc *sch)
+{
+	ktime_t transmit_end_time, interval_end, interval_start;
+	int len, packet_transmit_time, sched_changed;
+	struct taprio_sched *q = qdisc_priv(sch);
+	ktime_t minimum_time, now, txtime;
+	struct sched_gate_list *sched, *admin;
+	struct sched_entry *entry;
+
+	now = q->get_time();
+	minimum_time = ktime_add_ns(now, q->txtime_delay);
+
+	rcu_read_lock();
+	admin = rcu_dereference(q->admin_sched);
+	sched = rcu_dereference(q->oper_sched);
+	if (admin && ktime_after(minimum_time, admin->base_time))
+		switch_schedules(q, &admin, &sched);
+
+	/* Until the schedule starts, all the queues are open */
+	if (!sched || ktime_before(minimum_time, sched->base_time)) {
+		txtime = minimum_time;
+		goto done;
+	}
+
+	len = qdisc_pkt_len(skb);
+	packet_transmit_time = length_to_duration(q, len);
+
+	do {
+		sched_changed = 0;
+
+		entry = find_entry_to_transmit(skb, sch, sched, admin,
+					       minimum_time,
+					       &interval_start, &interval_end,
+					       false);
+		if (!entry) {
+			txtime = 0;
+			goto done;
+		}
+
+		txtime = entry->next_txtime;
+		txtime = max_t(ktime_t, txtime, minimum_time);
+		txtime = max_t(ktime_t, txtime, interval_start);
+
+		if (admin && admin != sched &&
+		    ktime_after(txtime, admin->base_time)) {
+			sched = admin;
+			sched_changed = 1;
+			continue;
+		}
+
+		transmit_end_time = ktime_add(txtime, packet_transmit_time);
+		minimum_time = transmit_end_time;
+
+		/* Update the txtime of current entry to the next time it's
+		 * interval starts.
+		 */
+		if (ktime_after(transmit_end_time, interval_end))
+			entry->next_txtime = ktime_add(interval_start, sched->cycle_time);
+	} while (sched_changed || ktime_after(transmit_end_time, interval_end));
+
+	entry->next_txtime = transmit_end_time;
+
+done:
+	rcu_read_unlock();
+	return txtime;
+}
+
 static int taprio_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 			  struct sk_buff **to_free)
 {
@@ -129,6 +361,15 @@ static int taprio_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	if (unlikely(!child))
 		return qdisc_drop(skb, sch, to_free);
 
+	if (skb->sk && sock_flag(skb->sk, SOCK_TXTIME)) {
+		if (!is_valid_interval(skb, sch))
+			return qdisc_drop(skb, sch, to_free);
+	} else if (TXTIME_OFFLOAD_IS_ON(q->offload_flags)) {
+		skb->tstamp = get_packet_txtime(skb, sch);
+		if (!skb->tstamp)
+			return qdisc_drop(skb, sch, to_free);
+	}
+
 	qdisc_qstats_backlog_inc(sch, skb);
 	sch->q.qlen++;
 
@@ -206,11 +447,6 @@ static struct sk_buff *taprio_peek(struct Qdisc *sch)
 	return q->peek(sch);
 }
 
-static inline int length_to_duration(struct taprio_sched *q, int len)
-{
-	return div_u64(len * atomic64_read(&q->picos_per_byte), 1000);
-}
-
 static void taprio_set_budget(struct taprio_sched *q, struct sched_entry *entry)
 {
 	atomic_set(&entry->budget,
@@ -594,7 +830,8 @@ static int parse_taprio_schedule(struct nlattr **tb,
 
 static int taprio_parse_mqprio_opt(struct net_device *dev,
 				   struct tc_mqprio_qopt *qopt,
-				   struct netlink_ext_ack *extack)
+				   struct netlink_ext_ack *extack,
+				   u32 offload_flags)
 {
 	int i, j;
 
@@ -642,6 +879,9 @@ static int taprio_parse_mqprio_opt(struct net_device *dev,
 			return -EINVAL;
 		}
 
+		if (TXTIME_OFFLOAD_IS_ON(offload_flags))
+			continue;
+
 		/* Verify that the offset and counts do not overlap */
 		for (j = i + 1; j < qopt->num_tc; j++) {
 			if (last > qopt->offset[j]) {
@@ -804,6 +1044,9 @@ static int taprio_enable_offload(struct net_device *dev,
 	const struct net_device_ops *ops = dev->netdev_ops;
 	int err = 0;
 
+	if (TXTIME_OFFLOAD_IS_ON(offload_flags))
+		goto done;
+
 	if (!FULL_OFFLOAD_IS_ON(offload_flags)) {
 		NL_SET_ERR_MSG(extack, "Offload mode is not supported");
 		return -EOPNOTSUPP;
@@ -816,15 +1059,28 @@ static int taprio_enable_offload(struct net_device *dev,
 
 	/* FIXME: enable offloading */
 
-	q->dequeue = taprio_dequeue_offload;
-	q->peek = taprio_peek_offload;
-
-	if (err == 0)
+done:
+	if (err == 0) {
+		q->dequeue = taprio_dequeue_offload;
+		q->peek = taprio_peek_offload;
 		q->offload_flags = offload_flags;
+	}
 
 	return err;
 }
 
+static void setup_txtime(struct taprio_sched *q,
+			 struct sched_gate_list *sched, ktime_t base)
+{
+	struct sched_entry *entry;
+	u32 interval = 0;
+
+	list_for_each_entry(entry, &sched->entries, list) {
+		entry->next_txtime = ktime_add_ns(base, interval);
+		interval += entry->interval;
+	}
+}
+
 static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
 			 struct netlink_ext_ack *extack)
 {
@@ -846,7 +1102,10 @@ static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
 	if (tb[TCA_TAPRIO_ATTR_PRIOMAP])
 		mqprio = nla_data(tb[TCA_TAPRIO_ATTR_PRIOMAP]);
 
-	err = taprio_parse_mqprio_opt(dev, mqprio, extack);
+	if (tb[TCA_TAPRIO_ATTR_OFFLOAD_FLAGS])
+		offload_flags = nla_get_u32(tb[TCA_TAPRIO_ATTR_OFFLOAD_FLAGS]);
+
+	err = taprio_parse_mqprio_opt(dev, mqprio, extack, offload_flags);
 	if (err < 0)
 		return err;
 
@@ -887,6 +1146,10 @@ static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
 		goto free_sched;
 	}
 
+	/* preserve offload flags when changing the schedule. */
+	if (q->offload_flags)
+		offload_flags = q->offload_flags;
+
 	if (tb[TCA_TAPRIO_ATTR_SCHED_CLOCKID]) {
 		clockid = nla_get_s32(tb[TCA_TAPRIO_ATTR_SCHED_CLOCKID]);
 
@@ -914,7 +1177,10 @@ static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
 	/* Protects against enqueue()/dequeue() */
 	spin_lock_bh(qdisc_lock(sch));
 
-	if (!hrtimer_active(&q->advance_timer)) {
+	if (tb[TCA_TAPRIO_ATTR_TXTIME_DELAY])
+		q->txtime_delay = nla_get_s32(tb[TCA_TAPRIO_ATTR_TXTIME_DELAY]);
+
+	if (!TXTIME_OFFLOAD_IS_ON(offload_flags) && !hrtimer_active(&q->advance_timer)) {
 		hrtimer_init(&q->advance_timer, q->clockid, HRTIMER_MODE_ABS);
 		q->advance_timer.function = advance_sched;
 	}
@@ -966,20 +1232,35 @@ static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
 		goto unlock;
 	}
 
-	setup_first_close_time(q, new_admin, start);
+	if (TXTIME_OFFLOAD_IS_ON(offload_flags)) {
+		setup_txtime(q, new_admin, start);
+
+		if (!oper) {
+			rcu_assign_pointer(q->oper_sched, new_admin);
+			err = 0;
+			new_admin = NULL;
+			goto unlock;
+		}
+
+		rcu_assign_pointer(q->admin_sched, new_admin);
+		if (admin)
+			call_rcu(&admin->rcu, taprio_free_sched_cb);
+	} else {
+		setup_first_close_time(q, new_admin, start);
 
-	/* Protects against advance_sched() */
-	spin_lock_irqsave(&q->current_entry_lock, flags);
+		/* Protects against advance_sched() */
+		spin_lock_irqsave(&q->current_entry_lock, flags);
 
-	taprio_start_sched(sch, start, new_admin);
+		taprio_start_sched(sch, start, new_admin);
 
-	rcu_assign_pointer(q->admin_sched, new_admin);
-	if (admin)
-		call_rcu(&admin->rcu, taprio_free_sched_cb);
-	new_admin = NULL;
+		rcu_assign_pointer(q->admin_sched, new_admin);
+		if (admin)
+			call_rcu(&admin->rcu, taprio_free_sched_cb);
 
-	spin_unlock_irqrestore(&q->current_entry_lock, flags);
+		spin_unlock_irqrestore(&q->current_entry_lock, flags);
+	}
 
+	new_admin = NULL;
 	err = 0;
 
 unlock:
@@ -1225,6 +1506,9 @@ static int taprio_dump(struct Qdisc *sch, struct sk_buff *skb)
 	if (nla_put_u32(skb, TCA_TAPRIO_ATTR_OFFLOAD_FLAGS, q->offload_flags))
 		goto options_error;
 
+	if (nla_put_s32(skb, TCA_TAPRIO_ATTR_TXTIME_DELAY, q->txtime_delay))
+		goto options_error;
+
 	if (oper && dump_schedule(skb, oper))
 		goto options_error;
 
-- 
2.17.0