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

[Eric Dumazet]

On Wed, Sep 2, 2026 at 10:34 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. This site only has to fit skb->mac_len before its
> skb_push(); the outer header that follows is already covered by
> __seg6_do_srh_encap()'s own skb_cow_head(). On a forwarding path neither
> reallocation is necessary: ixgbe leaves 206 bytes of headroom, against
>
>   14 (skb->mac_len) + 40 (ipv6hdr) + 24 (SRH with one segment)
>   + 16 (LL_RESERVED_SPACE, via dst_dev_overhead()) = 94
>
> for the whole encapsulation.
>
> 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.
>
> Use skb_cow_head() instead, matching the IPv6 encapsulation modes.
>
> 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: 660.7 kpps
>   After:  991.3 kpps
>
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Yuya Kusakabe <yuya.kusakabe@xxxxxxxxx>
> ---
>  net/ipv6/seg6_iptunnel.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv6/seg6_iptunnel.c b/net/ipv6/seg6_iptunnel.c
> index 61c6a27bf202..a6556efd8e0b 100644
> --- a/net/ipv6/seg6_iptunnel.c
> +++ b/net/ipv6/seg6_iptunnel.c
> @@ -446,8 +446,9 @@ 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;
> +               err = skb_cow_head(skb, skb->mac_len);
> +               if (unlikely(err))
> +                       return err;
>
>                 skb_mac_header_rebuild(skb);
>                 skb_push(skb, skb->mac_len);
>

This is a nice improvement for forwarded traffic, but locally
generated TCP traffic
is still hitting two expensive reallocations (second one in
__seg6_do_srh_encap())

Can we combine needed headrooms so that only a single re-alloc occurs?

 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;