seg6: reallocate the skb head on L2 encapsulation only when needed

The L2 encapsulation modes of the seg6 lwtunnel reallocate the skb head
on every packet, where the IPv6 encapsulation modes reallocate only when
they have to.  Ask for the whole encapsulation up front instead, so that
the reallocation happens at most once and only when the headroom really
is too small:

  skb->mac_len + sizeof(struct ipv6hdr) + ipv6_optlen(tinfo->srh)
  + dst_dev_overhead(cache_dst, skb)

__seg6_do_srh_encap() then finds the room it needs and its own
skb_cow_head() becomes a no-op.

Drivers reserve more than that on the forwarding path, so the
reallocation usually disappears altogether.  A single-segment policy
on ixgbe needs

  14 (mac_len) + 40 (ipv6hdr) + 24 (SRH) + 16 (LL_RESERVED_SPACE) = 94

against the 206 bytes the driver leaves.  Where the headroom is
smaller, as on a veth pair, pskb_expand_head() is called once per
forwarded packet instead of twice.  Asking only for skb->mac_len would
still take two whenever the skb is header-cloned, because the cow that
unclones it does not also make room for the outer header.

The cost is amplified by CONFIG_INIT_ON_ALLOC_DEFAULT_ON, which many
distributions enable: every new head is zeroed in full, and that memset
alone accounts for 16% of the datapath profile.

Throughput at 0.5% packet loss, 64-byte frames forwarded through one
2.30 GHz core (Xeon E5-2650 v3, ixgbe 82599ES), offered by TRex and
binary-searched over 10 runs of 10 s:

  Before: 654.6 kpps
  After:  965.7 kpps

Signed-off-by: Yuya Kusakabe <yuya.kusakabe@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>
Link: https://patch.msgid.link/20260925-seg6-l2cow-v3-1-fc83821542a7@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 8 insertions, 2 deletions

diff --git a/net/ipv6/seg6_iptunnel.c b/net/ipv6/seg6_iptunnel.c
index 61c6a27bf202..ecd8146089ee 100644
--- a/net/ipv6/seg6_iptunnel.c
+++ b/net/ipv6/seg6_iptunnel.c
@@ -400,6 +400,7 @@ static int seg6_do_srh(struct sk_buff *skb, struct dst_entry *cache_dst)
 	struct dst_entry *dst = skb_dst(skb);
 	struct seg6_iptunnel_encap *tinfo;
 	struct seg6_lwt *slwt;
+	unsigned int headroom;
 	int proto, err = 0;
 
 	slwt = seg6_lwt_lwtunnel(dst->lwtstate);
@@ -446,8 +447,13 @@ static int seg6_do_srh(struct sk_buff *skb, struct dst_entry *cache_dst)
 		if (!skb_mac_header_was_set(skb))
 			return -EINVAL;
 
-		if (pskb_expand_head(skb, skb->mac_len, 0, GFP_ATOMIC) < 0)
-			return -ENOMEM;
+		headroom = skb->mac_len + sizeof(struct ipv6hdr) +
+			   ipv6_optlen(tinfo->srh) +
+			   dst_dev_overhead(cache_dst, skb);
+
+		err = skb_cow_head(skb, headroom);
+		if (unlikely(err))
+			return err;
 
 		skb_mac_header_rebuild(skb);
 		skb_push(skb, skb->mac_len);