Re: [PATCH v8 net-next 0/2] Explicit TSO segment count

On Mon Aug 24, 2026 at 10:21 PM CEST, Chia-Yu Chang (Nokia) wrote:

quoted

-----Original Message-----
From: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Sent: Monday, August 24, 2026 10:10 PM
To: Jakub Kicinski <kuba@kernel.org>; Chia-Yu Chang (Nokia) <redacted>
Cc: john.fastabend@gmail.com; jakub@cloudflare.com; jiayuan.chen@linux.dev; netdev@vger.kernel.org; bpf@vger.kernel.org; ast@kernel.org; daniel@iogearbox.net; andrii@kernel.org; eddyz87@gmail.com; martin.lau@linux.dev; song@kernel.org; yonghong.song@linux.dev; jolsa@kernel.org; emil@etsalapatis.com; linux-kselftest@vger.kernel.org; shuah@kernel.org; horms@kernel.org; dsahern@kernel.org; pabeni@redhat.com; jhs@mojatatu.com; stephen@networkplumber.org; davem@davemloft.net; edumazet@google.com; andrew+netdev@lunn.ch; donald.hunter@gmail.com; kuniyu@google.com; ij@kernel.org; ncardwell@google.com; Koen De Schepper (Nokia) <redacted>; g.white@cablelabs.com; ingemar.s.johansson@ericsson.com; mirja.kuehlewind@ericsson.com; cheshire@apple.com; rs.ietf@gmx.at; Jason_Livingood@comcast.com; vidhi_goel@apple.com; Mike_Rudolph@comcast.com; Jeff_Howe@comcast.com; srichard@netapp.com
Subject: Re: [PATCH v8 net-next 0/2] Explicit TSO segment count


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On Mon Aug 24, 2026 at 10:05 PM CEST, Jakub Kicinski wrote:

quoted

On Mon, 24 Aug 2026 18:31:37 +0200 chia-yu.chang@nokia-bell-labs.com
wrote:

quoted

From: Chia-Yu Chang <redacted>

Hello,

This series replaces the existing min_tso_segs() congestion control
callback with a new tso_segs() callback that allows congestion
control algorithms to provide an explicit TSO segment count for each data burst.

To support BPF congestion controls, the series also exposes
tcp_tso_autosize() as a BPF kfunc, allowing BPF implementations to
reuse the kernel TSO autosizing logic while implementing custom
tso_segs() callbacks.

AI CI says:

The bpf-ci build jobs are failing for this patch on all tested
architectures/toolchains (x86-64 gcc-15, x86-64 llvm-21, aarch64
gcc-15, s390x gcc-15), all with the same link-time error:

  net/ipv4/tcp_bbr.o: in function `__BTF_ID__set8__tcp_bbr_check_kfunc_ids':
  tcp_bbr.c:(.BTF_ids+0x...): undefined reference to `KF_DEPRECATED'

This comes from the change to net/ipv4/tcp_bbr.c that marks the
retained
bbr_min_tso_segs() kfunc entry with the KF_DEPRECATED flag:

  BTF_ID_FLAGS(func, bbr_min_tso_segs, KF_DEPRECATED)
  BTF_ID_FLAGS(func, bbr_tso_segs)

KF_DEPRECATED does not resolve to a usable symbol/flag in this tree,
so the BTF id set for tcp_bbr fails to link into vmlinux on every
architecture and toolchain -- it's a straight undefined-reference
error, not a per-arch/per-config issue.

Could you drop the KF_DEPRECATED flag (or use a flag that this kfunc
infrastructure actually defines) on bbr_min_tso_segs, and re-verify
that the kernel links cleanly? A local `make net/ipv4/tcp_bbr.o
vmlinux` build (or the bpf-ci selftest build job) should reproduce this quickly.

Full build log for reference:

  ld.lld-21: error: undefined symbol: KF_DEPRECATED
  >>> referenced by usercopy_64.c
  >>>               vmlinux.o:(__BTF_ID__set8__tcp_bbr_check_kfunc_ids)

I think Chia-Yu's AI is hallucinating, it was a proposed flag but not added yet...

Hi Kumar and Kuba,

I will drop it in the next version; however, this AI hallucination is from Sashiko's feedback I got:

https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260814173034.749151-1-chia-yu.chang%40nokia-bell-labs.com

BTW, any BPF-related flag is suggested to be added for graceful degradation?

We don't have anything yet, but I will add something this cycle and figure out
how this can be marked deprecated then.

Thanks.
Chia-Yu