Re: [RFC net-next 0/6] net: configurable extra TX headroom for local egress

[Daniel Borkmann <daniel@iogearbox.net>]

On 10/16/26 8:48 PM, Dragos Tatulea wrote:
> On 16.10.26 10:02, David Woodhouse wrote:
>> On Thu, 2026-10-15 at 23:53 +0000, David Keogh wrote:
>>>
>>> Measurements (net-next at commit 05b5019b0, veth, 500000 UDP datagrams, a
>>> 20-byte ip-in-ip push attached at TC egress, only tx_extra_headroom varied):
>>>
>>>    tx_extra_headroom   pskb_expand_head reallocs / 500k
>>>    0 (off)             500003   (~1 per packet)
>>>    64                  4        (background)
>>>    128                 2        (background)
>>
>> I remember frowning at this years ago, in the context of PPPoE.
>>
>> I have the same question I had back then: why, if *every* packet ends
>> up having to call pskb_expand_head() because it didn't have enough
>> headroom, does our kernel need to be *told* by userspace that it needs
>> to expand the allocations?
>>
>> We gained dev->needed_headroom and ip_tunnel_adj_headroom() since then,
>> and the kernel doesn't actually need to be told.
>>
>> You explained why you don't want to use dev->needed_headroom; I don't
>> have much of an opinion on that front. But if you do want to introduce
>> a new tx_extra_headroom, could we try really hard to have it auto-
>> tuning?
> For TC-BPF I suppose only the BPF program knows how much extra
> headroom is needed.
> 
> Maybe this tx_extra_headroom could be declared as a BTF decl tag
> in the program. If this is the only usecase there's no need for a
> rtnetlink uAPI.
Fwiw, there was some work in this area here in the past ...

   - b9ed315d3c4c ("netkit: Allow for configuring needed_{head,tail}room")
   - b73b8146d7ff ("rtnetlink: add needed_{head,tail}room attributes")

... which we've integrated into Cilium which orchestrates the K8s
Pods + BPF progs, so it can calc needed headroom for the Pod's netdev
based on the config/needs in the hostns (e.g. tunneling).

Cheers,
Daniel