netdev - [RFC PATCH net-next 09/15] net/sched: taprio: calculate guard band against actual TC gate close time

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Date:   Sat, 28 Jan 2023 03:07:13 +0200
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     netdev@...r.kernel.org
Cc:     Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        Kurt Kanzenbach <kurt@...utronix.de>
Subject: [RFC PATCH net-next 09/15] net/sched: taprio: calculate guard band against actual TC gate close time

taprio_dequeue_from_txq() looks at the entry->end_time to determine
whether the skb will overrun its traffic class gate, as if at the end of
the schedule entry there surely is a "gate close" event for it. Hint:
maybe there isn't.

For each schedule entry, introduce an array of kernel times which
actually tracks when in the future will there be an *actual* gate close
event for that traffic class, and use that in the guard band overrun
calculation.

Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
---
 net/sched/sch_taprio.c | 40 ++++++++++++++++++++++++++++++++++------
 1 file changed, 34 insertions(+), 6 deletions(-)

diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index b3c25ab6a559..8ec3c0e1f741 100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c
@@ -40,12 +40,12 @@ struct sched_entry {
 	 */
 	u64 tc_gate_duration[TC_MAX_QUEUE];
 	atomic_t budget[TC_MAX_QUEUE];
-	struct list_head list;
-
-	/* The instant that this entry ends and the next one
-	 * should open, the qdisc will make some effort so that no
-	 * packet leaves after this time.
+	/* The qdisc makes some effort so that no packet leaves
+	 * after this time
 	 */
+	ktime_t tc_gate_close_time[TC_MAX_QUEUE];
+	struct list_head list;
+	/* Used to calculate when to advance the schedule */
 	ktime_t end_time;
 	ktime_t next_txtime;
 	int index;
@@ -142,6 +142,12 @@ static void taprio_calculate_tc_gate_durations(struct taprio_sched *q,
 	}
 }
 
+static bool taprio_entry_allows_tx(ktime_t skb_end_time,
+				   struct sched_entry *entry, int tc)
+{
+	return ktime_before(skb_end_time, entry->tc_gate_close_time[tc]);
+}
+
 static ktime_t sched_base_time(const struct sched_gate_list *sched)
 {
 	if (!sched)
@@ -629,7 +635,7 @@ static struct sk_buff *taprio_dequeue_from_txq(struct Qdisc *sch, int txq,
 	 * guard band ...
 	 */
 	if (gate_mask != TAPRIO_ALL_GATES_OPEN &&
-	    ktime_after(guard, entry->end_time))
+	    !taprio_entry_allows_tx(guard, entry, tc))
 		return NULL;
 
 	/* ... and no budget. */
@@ -761,10 +767,13 @@ static enum hrtimer_restart advance_sched(struct hrtimer *timer)
 {
 	struct taprio_sched *q = container_of(timer, struct taprio_sched,
 					      advance_timer);
+	struct net_device *dev = qdisc_dev(q->root);
 	struct sched_gate_list *oper, *admin;
+	int num_tc = netdev_get_num_tc(dev);
 	struct sched_entry *entry, *next;
 	struct Qdisc *sch = q->root;
 	ktime_t end_time;
+	int tc;
 
 	spin_lock(&q->current_entry_lock);
 	entry = rcu_dereference_protected(q->current_entry,
@@ -802,6 +811,14 @@ static enum hrtimer_restart advance_sched(struct hrtimer *timer)
 	end_time = ktime_add_ns(entry->end_time, next->interval);
 	end_time = min_t(ktime_t, end_time, oper->cycle_end_time);
 
+	for (tc = 0; tc < num_tc; tc++) {
+		if (next->tc_gate_duration[tc] == oper->cycle_time)
+			next->tc_gate_close_time[tc] = KTIME_MAX;
+		else
+			next->tc_gate_close_time[tc] = ktime_add_ns(entry->end_time,
+								    next->tc_gate_duration[tc]);
+	}
+
 	if (should_change_schedules(admin, oper, end_time)) {
 		/* Set things so the next time this runs, the new
 		 * schedule runs.
@@ -1107,8 +1124,11 @@ static int taprio_get_start_time(struct Qdisc *sch,
 static void setup_first_end_time(struct taprio_sched *q,
 				 struct sched_gate_list *sched, ktime_t base)
 {
+	struct net_device *dev = qdisc_dev(q->root);
+	int num_tc = netdev_get_num_tc(dev);
 	struct sched_entry *first;
 	ktime_t cycle;
+	int tc;
 
 	first = list_first_entry(&sched->entries,
 				 struct sched_entry, list);
@@ -1120,6 +1140,14 @@ static void setup_first_end_time(struct taprio_sched *q,
 
 	first->end_time = ktime_add_ns(base, first->interval);
 	taprio_set_budgets(q, sched, first);
+
+	for (tc = 0; tc < num_tc; tc++) {
+		if (first->tc_gate_duration[tc] == sched->cycle_time)
+			first->tc_gate_close_time[tc] = KTIME_MAX;
+		else
+			first->tc_gate_close_time[tc] = ktime_add_ns(base, first->tc_gate_duration[tc]);
+	}
+
 	rcu_assign_pointer(q->current_entry, NULL);
 }
 
-- 
2.34.1