Re: Performance impact of disabling VLAN offload [was: Re: [PATCH bpf-next V1 7/7] net: xdp: update documentation for xdp-rx-metadata.rst]

[Jesper Dangaard Brouer <hawk@kernel.org>]

On 17/06/2025 17.10, Toke Høiland-Jørgensen wrote:
>> Later we will look at using the vlan tag. Today we have disabled HW
>> vlan-offloading, because XDP originally didn't support accessing HW vlan
>> tags.
> 
> Side note (with changed subject to disambiguate): Do you have any data
> on the performance impact of disabling VLAN offload that you can share?
> I've been sort of wondering whether saving those couple of bytes has any
> measurable impact on real workloads (where you end up looking at the
> headers anyway, so saving the cache miss doesn't matter so much)?
> 

Our production setup have two different VLAN IDs, one for INTERNAL-ID
and one for EXTERNAL-ID (Internet) traffic.  On (many) servers this is
on the same physical net_device.

Our Unimog XDP load-balancer *only* handles EXTERNAL-ID.  Thus, the very
first thing Unimog does is checking the VLAN ID.  If this doesn't match
EXTERNAL-ID it returns XDP_PASS.  This is the first time packet data
area is read which (due to our AMD-CPUs) will be a cache-miss.

If this were INTERNAL-ID then we have caused a cache-miss earlier than
needed.  The NIC driver have already started a net_prefetch.  Thus, if
we can return XDP_PASS without touching packet data, then we can
(latency) hide part of the cache-miss (behind SKB-zero-ing). (We could
also CPUMAP redirect the INTERNAL-ID to a remote CPU for further gains).
  Using the kfunc (bpf_xdp_metadata_rx_vlan_tag[1]) for reading VLAN ID
doesn't touch/read packet data.

I hope this makes it clear why reading the HW offloaded VLAN tag from
the RX-descriptor is a performance benefit?

--Jesper

[1] https://docs.kernel.org/networking/xdp-rx-metadata.html