netdev - Re: [PATCH bpf-next v11 11/12] bpf: support selective sampling for bpf timestamping

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Message-ID: <CAL+tcoC=PROxQfPoa_LGJZ0JAPW1XuqSnTTHwJssjsC7-MPV_A@mail.gmail.com>
Date: Sun, 16 Feb 2025 00:17:56 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	pabeni@...hat.com, dsahern@...nel.org, 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 v11 11/12] bpf: support selective sampling for
 bpf timestamping

On Sat, Feb 15, 2025 at 11:10 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> Jason Xing wrote:
> > Add the bpf_sock_ops_enable_tx_tstamp kfunc to allow BPF programs to
> > selectively enable TX timestamping on a skb during tcp_sendmsg().
> >
> > For example, BPF program will limit tracking X numbers of packets
> > and then will stop there instead of tracing all the sendmsgs of
> > matched flow all along. It would be helpful for users who cannot
> > afford to calculate latencies from every sendmsg call probably
> > due to the performance or storage space consideration.
> >
> > Signed-off-by: Jason Xing <kerneljasonxing@...il.com>
> > ---
> >  kernel/bpf/btf.c  |  1 +
> >  net/core/filter.c | 33 ++++++++++++++++++++++++++++++++-
> >  2 files changed, 33 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 9433b6467bbe..740210f883dc 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -8522,6 +8522,7 @@ static int bpf_prog_type_to_kfunc_hook(enum bpf_prog_type prog_type)
> >       case BPF_PROG_TYPE_CGROUP_SOCK_ADDR:
> >       case BPF_PROG_TYPE_CGROUP_SOCKOPT:
> >       case BPF_PROG_TYPE_CGROUP_SYSCTL:
> > +     case BPF_PROG_TYPE_SOCK_OPS:
> >               return BTF_KFUNC_HOOK_CGROUP;
> >       case BPF_PROG_TYPE_SCHED_ACT:
> >               return BTF_KFUNC_HOOK_SCHED_ACT;
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 7f56d0bbeb00..3b4c1e7b1470 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -12102,6 +12102,27 @@ __bpf_kfunc int bpf_sk_assign_tcp_reqsk(struct __sk_buff *s, struct sock *sk,
> >  #endif
> >  }
> >
> > +__bpf_kfunc int bpf_sock_ops_enable_tx_tstamp(struct bpf_sock_ops_kern *skops,
> > +                                           u64 flags)
> > +{
> > +     struct sk_buff *skb;
> > +     struct sock *sk;
> > +
> > +     if (skops->op != BPF_SOCK_OPS_TS_SND_CB)
> > +             return -EOPNOTSUPP;
> > +
> > +     if (flags)
> > +             return -EINVAL;
> > +
> > +     skb = skops->skb;
> > +     sk = skops->sk;
>
> nit: not used

BPF programs can use this in the future if necessary whereas the
selftests don't reflect it.

>
> > +     skb_shinfo(skb)->tx_flags |= SKBTX_BPF;
> > +     TCP_SKB_CB(skb)->txstamp_ack |= TSTAMP_ACK_BPF;
> > +     skb_shinfo(skb)->tskey = TCP_SKB_CB(skb)->seq + skb->len - 1;
>
> Can this overwrite the seqno previously calculated by tcp_tx_timestamp?

seqno? If you are referring to seqno, I don't think the BPF program is
allowed to modify it because SOCK_OPS_GET_OR_SET_FIELD() only supports
overwriting sk_txhash only. Please see sock_ops_convert_ctx_access().

Thanks,
Jason




>
> I suppose that that is safe as long as both calculate the same value.
> But good to have explicit.
>
> > +
> > +     return 0;
> > +}
> > +