netdev - [PATCH RFC net-next] taprio: Handle short intervals and large packets

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Date:   Fri, 12 Mar 2021 10:28:23 +0100
From:   Kurt Kanzenbach <kurt@...utronix.de>
To:     Vinicius Costa Gomes <vinicius.gomes@...el.com>
Cc:     Vladimir Oltean <olteanv@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        Kurt Kanzenbach <kurt@...utronix.de>
Subject: [PATCH RFC net-next] taprio: Handle short intervals and large packets

When using short intervals e.g. below one millisecond, large packets won't be
transmitted at all. The software implementations checks whether the packet can
be fit into the remaining interval. Therefore, it takes the packet length and
the transmission speed into account. That is correct.

However, for large packets it may be that the transmission time will be larger
than the interval resulting in no packet transmission. The same situation works
fine with hardware offloading applied.

The problem has been observerd with the following schedule and iperf3:

|tc qdisc replace dev lan1 parent root handle 100 taprio \
|   num_tc 8 \
|   map 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 \
|   queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 \
|   base-time $base \
|   sched-entry S 0x40 500000 \
|   sched-entry S 0xbf 500000 \
|   clockid CLOCK_TAI \
|   flags 0x00

[...]

|root@tsn:~# iperf3 -c 192.168.2.105
|Connecting to host 192.168.2.105, port 5201
|[  5] local 192.168.2.121 port 52610 connected to 192.168.2.105 port 5201
|[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
|[  5]   0.00-1.00   sec  45.2 KBytes   370 Kbits/sec    0   1.41 KBytes
|[  5]   1.00-2.00   sec  0.00 Bytes  0.00 bits/sec    0   1.41 KBytes

After debugging, it seems that the packet length stored in the SKB is about
7000-8000 bytes. Using a 100 Mbit/s link the transmission time is about 600us
which larger than the interval of 500us.

Therefore, segment the SKB into smaller chunks if the packet is too big. This
yields similar results than the hardware offload:

|root@tsn:~# iperf3 -c 192.168.2.105
|Connecting to host 192.168.2.105, port 5201
|- - - - - - - - - - - - - - - - - - - - - - - - -
|[ ID] Interval           Transfer     Bitrate         Retr
|[  5]   0.00-10.00  sec  48.9 MBytes  41.0 Mbits/sec    0             sender
|[  5]   0.00-10.02  sec  48.7 MBytes  40.7 Mbits/sec                  receiver

Signed-off-by: Kurt Kanzenbach <kurt@...utronix.de>
---
 net/sched/sch_taprio.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index 8287894541e3..8434e87f79f7 100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c
@@ -411,6 +411,42 @@ static long get_packet_txtime(struct sk_buff *skb, struct Qdisc *sch)
 	return txtime;
 }
 
+/* Similar to net/sched/sch_tbf.c::tbf_segment */
+static int taprio_segment(struct sk_buff *skb, struct Qdisc *sch,
+			  struct Qdisc *child, struct sk_buff **to_free)
+{
+	netdev_features_t features = netif_skb_features(skb);
+	unsigned int len = 0, prev_len = qdisc_pkt_len(skb);
+	struct sk_buff *segs, *nskb;
+	int ret, nb;
+
+	segs = skb_gso_segment(skb, features & ~NETIF_F_GSO_MASK);
+
+	if (IS_ERR_OR_NULL(segs))
+		return qdisc_drop(skb, sch, to_free);
+
+	nb = 0;
+	skb_list_walk_safe(segs, segs, nskb) {
+		skb_mark_not_on_list(segs);
+		qdisc_skb_cb(segs)->pkt_len = segs->len;
+		len += segs->len;
+		ret = qdisc_enqueue(segs, child, to_free);
+		if (ret != NET_XMIT_SUCCESS) {
+			if (net_xmit_drop_count(ret))
+				qdisc_qstats_drop(sch);
+		} else {
+			nb++;
+		}
+	}
+
+	sch->q.qlen += nb;
+	if (nb > 1)
+		qdisc_tree_reduce_backlog(sch, 1 - nb, prev_len - len);
+	consume_skb(skb);
+
+	return nb > 0 ? NET_XMIT_SUCCESS : NET_XMIT_DROP;
+}
+
 static int taprio_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 			  struct sk_buff **to_free)
 {
@@ -433,6 +469,9 @@ static int taprio_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 			return qdisc_drop(skb, sch, to_free);
 	}
 
+	if (skb_is_gso(skb))
+		return taprio_segment(skb, sch, child, to_free);
+
 	qdisc_qstats_backlog_inc(sch, skb);
 	sch->q.qlen++;
 
-- 
2.20.1