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

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Message-ID: <cd4f2982-00ff-4e7b-88e1-6f6697da2c2f@kernel.org>
Date: Thu, 19 Jun 2025 14:09:11 +0200
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Toke Høiland-Jørgensen <toke@...hat.com>,
 Lorenzo Bianconi <lorenzo@...nel.org>,
 Stanislav Fomichev <stfomichev@...il.com>
Cc: Daniel Borkmann <daniel@...earbox.net>, bpf@...r.kernel.org,
 netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann
 <borkmann@...earbox.net>, Eric Dumazet <eric.dumazet@...il.com>,
 "David S. Miller" <davem@...emloft.net>, Paolo Abeni <pabeni@...hat.com>,
 sdf@...ichev.me, kernel-team@...udflare.com, arthur@...hurfabre.com,
 jakub@...udflare.com, Magnus Karlsson <magnus.karlsson@...el.com>,
 Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
 Willem Ferguson <wferguson@...udflare.com>,
 Jesse Brandeburg <jbrandeburg@...udflare.com>
Subject: Re: Performance impact of disabling VLAN offload [was: Re: [PATCH
 bpf-next V1 7/7] net: xdp: update documentation for xdp-rx-metadata.rst]

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