netdev - Re: [PATCH bpf-next v8 10/12] bpf: make TCP tx timestamp bpf extension work

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Message-ID: <CAL+tcoAXn7hct4HjU2QpTkPJohCdTH4eqH0AwAj54fqxXLrqsA@mail.gmail.com>
Date: Wed, 5 Feb 2025 10:15:11 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com, 
	dsahern@...nel.org, willemdebruijn.kernel@...il.com, willemb@...gle.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@...ichev.me, 
	haoluo@...gle.com, jolsa@...nel.org, horms@...nel.org, bpf@...r.kernel.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next v8 10/12] bpf: make TCP tx timestamp bpf
 extension work

On Wed, Feb 5, 2025 at 9:57 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Wed,  5 Feb 2025 02:30:22 +0800 Jason Xing wrote:
> > +     if (cgroup_bpf_enabled(CGROUP_SOCK_OPS) &&
> > +         SK_BPF_CB_FLAG_TEST(sk, SK_BPF_CB_TX_TIMESTAMPING) && skb) {
> > +             struct skb_shared_info *shinfo = skb_shinfo(skb);
> > +             struct tcp_skb_cb *tcb = TCP_SKB_CB(skb);
> > +
> > +             tcb->txstamp_ack_bpf = 1;
> > +             shinfo->tx_flags |= SKBTX_BPF;
> > +             shinfo->tskey = TCP_SKB_CB(skb)->seq + skb->len - 1;
> > +     }
>
> If BPF program is attached we'll timestamp all skbs? Am I reading this
> right?

For now, not really because tcp_tx_timestamp() gets called only when
dealing with the last part of this sendmsg(). So not all the skbs will
be traced.

>
> Wouldn't it be better to let BPF_SOCK_OPS_TS_SND_CB return whether it's
> interested in tracing current packet all the way thru the stack?

This flag is mainly used to correlate the sendmsg timestamp with
corresponding tskey, or else the skb travers the qdisc layer without
knowing how to match its sendmsg.

Thanks,
Jason