Subject: Re: [PATCH net-next] tcp: add cwnd_event_tx_start to tcp_congestion_ops

From: Jakub Kicinski <[email-masked]>
Date: Fri, 13 Mar 2026 15:07:10 -0700

On Fri, 13 Mar 2026 20:28:02 +0000 Eric Dumazet wrote:
> (tcp_congestion_ops)->cwnd_event() is called very often, with
> @event oscillating between CA_EVENT_TX_START and other values.
> 
> This is not branch prediction friendly.
> 
> Provide a new cwnd_event_tx_start pointer dedicated for CA_EVENT_TX_START.
> 
> Both BBR and CUBIC benefit from this change, since they only care
> about CA_EVENT_TX_START.

BPF selftest are not happy:
857
  progs/tcp_ca_kfunc.c:121:33: error: use of undeclared identifier 'cwnd_event_tx_start'
    121 |         .cwnd_event_tx_start = (void *)cwnd_event_tx_start,
        |                                        ^~~~~~~~~~~~~~~~~~~
-- 
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