-----Original Message-----
From: Alexei Starovoitov <redacted> 
Sent: Wednesday, July 1, 2026 1:21 AM
To: Chia-Yu Chang (Nokia) <redacted>; jolsa@kernel.org; yonghong.song@linux.dev; song@kernel.org; linux-kselftest@vger.kernel.org; memxor@gmail.com; shuah@kernel.org; martin.lau@linux.dev; ast@kernel.org; daniel@iogearbox.net; andrii@kernel.org; eddyz87@gmail.com; horms@kernel.org; dsahern@kernel.org; bpf@vger.kernel.org; netdev@vger.kernel.org; pabeni@redhat.com; jhs@mojatatu.com; kuba@kernel.org; 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
Subject: Re: [PATCH v3 net-next 1/1] tcp: Replace min_tso_segs() with tso_segs() CC callback

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On Tue Jun 30, 2026 at 5:01 AM PDT, chia-yu.chang wrote:

quoted

From: Chia-Yu Chang <redacted>

This patch replaces existing min_tso_segs() with tso_segs() CC callbak 
for CC algorithm to provides explicit tso segment number of each data 
burst and overrides tcp_tso_autosize().

This change provides below impacts on BPF struct_ops users:
- The callback is renamed from min_tso_segs to tso_segs
- The signature gains an extra u32 mss_now argument
- The return value semantics is changed from "floor value passed into
  tcp_tso_autosize()" to "final tso_segs value", bypassing autosizing

As a result, BPF programs shall be updated, beccause retuning a small 
constans will now directly limit tso_segs instead of the minimum.

Signed-off-by: Ilpo Järvinen <ij@kernel.org>
Signed-off-by: Chia-Yu Chang <redacted>
---
 include/net/tcp.h                                | 13 +++++++++++--
 net/ipv4/bpf_tcp_ca.c                            |  8 +++++---
 net/ipv4/tcp_bbr.c                               | 13 ++++++++++---
 net/ipv4/tcp_output.c                            | 13 +++++++------
 tools/testing/selftests/bpf/progs/tcp_ca_kfunc.c |  8 ++++----
 5 files changed, 37 insertions(+), 18 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h index 

6d376ea4d1c0..7fb42a0ce7da 100644

--- a/include/net/tcp.h
+++ b/include/net/tcp.h

@@ -824,6 +824,9 @@ unsigned int tcp_sync_mss(struct sock *sk, u32 

pmtu);  unsigned int tcp_current_mss(struct sock *sk);
 u32 tcp_clamp_probe0_to_user_timeout(const struct sock *sk, u32 
when);

+u32 tcp_tso_autosize(const struct sock *sk, unsigned int mss_now,
+                  int min_tso_segs);
+
 /* Bound MSS / TSO packet size with the half of the window */  static 
inline int tcp_bound_to_half_wnd(struct tcp_sock *tp, int pktsize)  { 

@@ -1361,8 +1364,14 @@ struct tcp_congestion_ops {
      /* hook for packet ack accounting (optional) */
      void (*pkts_acked)(struct sock *sk, const struct ack_sample 

*sample);

-     /* override sysctl_tcp_min_tso_segs (optional) */
-     u32 (*min_tso_segs)(struct sock *sk);
+     /*
+      * Override tcp_tso_autosize (optional)
+      *
+      * If provided, this callback returns the final TSO segment number
+      * and will bypass tcp_tso_autosize() entirely. The implementation
+      * must derive an appropriate value and ensure the result is valid.
+      */
+     u32 (*tso_segs)(struct sock *sk, u32 mss_now);

I don't like this interface change.
It introduces churn for no good reason.
At least I don't see why you cannot live with the existing api.

Hi Alexei,

This patch was part of TCP Prague preparation series: https://lore.kernel.org/all/20260611161504.228319-4-chia-yu.chang@nokia-bell-labs.com/ (local)
Our original patch is to add an extra tso_segs, and after discussion it's recommended to replace exisiting min_tso_segs.

This is needed because TCP Prague would set the exact TSO size rather than using autosizing from TCP.
The TCP Prague itself is planned to be submitted after all preparation commits are accepted.
You can find its current stauts: https://github.com/L4STeam/linux-net-next/blob/upstream_l4steam/net/ipv4/tcp_prague.c

Thanks.
Chia-Yu