netdev - Re: [PATCH bpf-next V2 0/7] xdp: Allow BPF to set RX hints for XDP

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Message-ID: <4eaf6d02-6b4e-4713-a8f8-6b00a031d255@linux.dev>
Date: Tue, 29 Jul 2025 12:47:23 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Jesper Dangaard Brouer <hawk@...nel.org>, Jakub Kicinski
 <kuba@...nel.org>, Lorenzo Bianconi <lorenzo@...nel.org>
Cc: Stanislav Fomichev <stfomichev@...il.com>, bpf@...r.kernel.org,
 netdev@...r.kernel.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,
 Jesse Brandeburg <jbrandeburg@...udflare.com>,
 Andrew Rzeznik <arzeznik@...udflare.com>
Subject: Re: [PATCH bpf-next V2 0/7] xdp: Allow BPF to set RX hints for
 XDP_REDIRECTed packets

On 7/29/25 4:15 AM, Jesper Dangaard Brouer wrote:
> That idea has been considered before, but it unfortunately doesn't work
> from a performance angle. The performance model of XDP_REDIRECT into
> CPUMAP relies on moving the expensive SKB allocation+init to a remote
> CPU. This keeps the ingress CPU free to process packets at near line
> rate (our DDoS use-case). If we allocate the SKB on the ingress-CPU
> before the redirect, we destroy this load-balancing model and create the
> exact bottleneck we designed CPUMAP to avoid.

iirc, a xdp prog can be attached to a cpumap. The skb can be created by that xdp 
prog running on the remote cpu. It should be like a xdp prog returning a 
XDP_PASS + an optional skb. The xdp prog can set some fields in the skb. Other 
than setting fields in the skb, something else may be also possible in the 
future, e.g. look up sk, earlier demux ...etc.