netdev - Re: [PATCH bpf-next 2/2] bpf: Require only one of cong_avoid() and cong

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Date:   Wed, 8 Jun 2022 11:33:56 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Jörn-Thorben Hinz <jthinz@...lbox.tu-berlin.de>
Cc:     bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next 2/2] bpf: Require only one of cong_avoid() and
 cong_control() from a TCP CC

On Wed, Jun 08, 2022 at 07:48:43PM +0200, Jörn-Thorben Hinz wrote:
> When a CC implements tcp_congestion_ops.cong_control(), the alternate
> cong_avoid() is not in use in the TCP stack. Do not force a BPF CC to
> implement cong_avoid() as a no-op by always requiring it.
> 
> An incomplete BPF CC implementing neither cong_avoid() nor
> cong_control() will still get rejected by
> tcp_register_congestion_control().
> 
> Signed-off-by: Jörn-Thorben Hinz <jthinz@...lbox.tu-berlin.de>
> ---
>  net/ipv4/bpf_tcp_ca.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/ipv4/bpf_tcp_ca.c b/net/ipv4/bpf_tcp_ca.c
> index 1f5c53ede4e5..37290d0bf134 100644
> --- a/net/ipv4/bpf_tcp_ca.c
> +++ b/net/ipv4/bpf_tcp_ca.c
> @@ -17,6 +17,7 @@ extern struct bpf_struct_ops bpf_tcp_congestion_ops;
>  static u32 optional_ops[] = {
>  	offsetof(struct tcp_congestion_ops, init),
>  	offsetof(struct tcp_congestion_ops, release),
> +	offsetof(struct tcp_congestion_ops, cong_avoid),
At least one of the cong_avoid() or cong_control() is needed.
It is better to remove is_optional(moff) check and its optional_ops[]
here.  Only depends on the tcp_register_congestion_control() which
does a similar check at the beginning.

Patch 1 looks good.  tcp_bbr.c also needs the sk_pacing fields.

A selftest is needed.  Can you share your bpf tcp-cc and
use it as a selftest to exercise the change in this patch
set ?


>  	offsetof(struct tcp_congestion_ops, set_state),
>  	offsetof(struct tcp_congestion_ops, cwnd_event),
>  	offsetof(struct tcp_congestion_ops, in_ack_event),
> -- 
> 2.30.2
>