net_sched: more precise pkt_len computation

One long standing problem with TSO/GSO/GRO packets is that skb->len
doesn't represent a precise amount of bytes on wire.

Headers are only accounted for the first segment.
For TCP, thats typically 66 bytes per 1448 bytes segment missing,
an error of 4.5 % for normal MSS value.

As consequences :

1) TBF/CBQ/HTB/NETEM/... can send more bytes than the assigned limits.
2) Device stats are slightly under estimated as well.

Fix this by taking account of headers in qdisc_skb_cb(skb)->pkt_len
computation.

Packet schedulers should use qdisc pkt_len instead of skb->len for their
bandwidth limitations, and TSO enabled devices drivers could use pkt_len
if their statistics are not hardware assisted, and if they don't scratch
skb->cb[] first word.

Both egress and ingress paths work, thanks to commit fda55eca5a
(net: introduce skb_transport_header_was_set()) : If GRO built
a GSO packet, it also set the transport header for us.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: Paolo Valente <paolo.valente@unimore.it>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/core/dev.c b/net/core/dev.c
index 594830e83589b16cf07ae2a23d97d40757c91b84..4794cae84939d6a92bf7357e418fa346634190ff 100644 (file)
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2532,6 +2532,26 @@ struct netdev_queue *netdev_pick_tx(struct net_device *dev,
        return netdev_get_tx_queue(dev, queue_index);
 }
 
+static void qdisc_pkt_len_init(struct sk_buff *skb)
+{
+       const struct skb_shared_info *shinfo = skb_shinfo(skb);
+
+       qdisc_skb_cb(skb)->pkt_len = skb->len;
+
+       /* To get more precise estimation of bytes sent on wire,
+        * we add to pkt_len the headers size of all segments
+        */
+       if (shinfo->gso_size)  {
+               unsigned int hdr_len = skb_transport_offset(skb);
+
+               if (likely(shinfo->gso_type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6)))
+                       hdr_len += tcp_hdrlen(skb);
+               else
+                       hdr_len += sizeof(struct udphdr);
+               qdisc_skb_cb(skb)->pkt_len += (shinfo->gso_segs - 1) * hdr_len;
+       }
+}
+
 static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
                                 struct net_device *dev,
                                 struct netdev_queue *txq)
@@ -2540,7 +2560,7 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
        bool contended;
        int rc;
 
-       qdisc_skb_cb(skb)->pkt_len = skb->len;
+       qdisc_pkt_len_init(skb);
        qdisc_calculate_pkt_len(skb, q);
        /*
         * Heuristic to force contended enqueues to serialize on a