Re: [PATCH bpf-next 2/2] bpf: Require only one of cong_avoid() and cong_control() from a TCP CC

On Thu, Jun 09, 2022 at 10:55:25AM +0200, Jörn-Thorben Hinz wrote:

Thanks for the feedback, Martin.

On Wed, 2022-06-08 at 11:33 -0700, Martin KaFai Lau wrote:

quoted

On Wed, Jun 08, 2022 at 07:48:43PM +0200, Jörn-Thorben Hinz wrote:

quoted

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 <redacted>
---
 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.

You mean completely remove this part of the validation from
bpf_tcp_ca.c and just rely on tcp_register_congestion_control()? True,

Yes.

that would be even easier to maintain at this point, make
tcp_register_congestion_control() the one-and-only place that has to
know about required and optional functions.

Will rework the second patch.

quoted

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 ?

I cannot do that just now, unfortunately. It’s still earlier work in
progress. Also, it will have an additional, external dependency which
might make it unfit to be included here/as a selftest. I will keep it
in mind for later this year, though.

What is the external dependency ?  Could you share some high level
of the CC you are developing ?
The reason for this question is to see if there is something
missing from the kernel side to write the tcp-cc in bpf that you
are developing.

In the meantime, I could look into adding a more naive/trivial test,
that implements cong_control() without cong_avoid() and relies on
sk_pacing_* being writable, if you would prefer that? Would that be
fine as a follow-up patch (might take me a moment) or better be
included in this series?

Yeah, it will do and the test should be submitted together in
this series.