Re: [PATCH net-next v2] seg6: reallocate the skb head on L2 encapsulation only when needed

[Eric Dumazet]

On Thu, Sep 3, 2026 at 10:23 AM Yuya Kusakabe <yuya.kusakabe@xxxxxxxxx> wrote:
>
> 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
>
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Yuya Kusakabe <yuya.kusakabe@xxxxxxxxx>

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>

Thanks!