netdev - Re: [PATCH bpf-next] bpf: add sacked flag in BPF_SOCK_OPS_RETRANS

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Message-ID: <CANn89iLWMhAOq0R7N3utrXdro_zTmp=9cs8a7_eviNcTK-_5+w@mail.gmail.com>
Date: Wed, 17 Apr 2024 15:11:26 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Philo Lu <lulie@...ux.alibaba.com>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org, davem@...emloft.net, 
	kuba@...nel.org, pabeni@...hat.com, ast@...nel.org, daniel@...earbox.net, 
	andrii@...nel.org, martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org, 
	yonghong.song@...ux.dev, john.fastabend@...il.com, kpsingh@...nel.org, 
	sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org, dsahern@...nel.org, 
	laoar.shao@...il.com, xuanzhuo@...ux.alibaba.com, fred.cc@...baba-inc.com
Subject: Re: [PATCH bpf-next] bpf: add sacked flag in BPF_SOCK_OPS_RETRANS_CB

On Wed, Apr 17, 2024 at 2:46 PM Philo Lu <lulie@...ux.alibaba.com> wrote:
>
> Add TCP_SKB_CB(skb)->sacked as the 4th arg of sockops passed to bpf
> program. Then we can get the retransmission efficiency by counting skbs
> w/ and w/o TCPCB_EVER_RETRANS mark. And for this purpose, sacked
> updating is moved after the BPF_SOCK_OPS_RETRANS_CB hook.
>
> Signed-off-by: Philo Lu <lulie@...ux.alibaba.com>

This might be a naive question, but how the bpf program know what is the meaning
of each bit ?

Are they exposed already, and how future changes in TCP stack could
break old bpf programs ?

#define TCPCB_SACKED_ACKED 0x01 /* SKB ACK'd by a SACK block */
#define TCPCB_SACKED_RETRANS 0x02 /* SKB retransmitted */
#define TCPCB_LOST 0x04 /* SKB is lost */
#define TCPCB_TAGBITS 0x07 /* All tag bits */
#define TCPCB_REPAIRED 0x10 /* SKB repaired (no skb_mstamp_ns) */
#define TCPCB_EVER_RETRANS 0x80 /* Ever retransmitted frame */
#define TCPCB_RETRANS (TCPCB_SACKED_RETRANS|TCPCB_EVER_RETRANS| \
TCPCB_REPAIRED)