Commit 1def9238 authored Jan 10, 2013 by Eric Dumazet Committed by David S. Miller Jan 10, 2013

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 fda55eca
(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>

parent 3d55b323

net/core/dev.c

+21 −1

Original line number	Diff line number	Diff line
		@@ -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

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