Re: [PATCH v3 net-next 1/1] tcp: Replace min_tso_segs() with tso_segs() CC callback

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Tue Jun 30, 2026 at 5:01 AM PDT, chia-yu.chang wrote:
> From: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
>
> 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 <chia-yu.chang@nokia-bell-labs.com>
> ---
>  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.