netdev - TCP get SND_CWND change on loss event

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Date:   Sat, 27 May 2017 14:12:22 +0200
From:   Lars Erik Storbukås <storbukas.dev@...il.com>
To:     Netdev <netdev@...r.kernel.org>
Subject: TCP get SND_CWND change on loss event

I want to store the value of snd_cwnd when a congestion event occurs
(value before snd_cwnd is reduced), and the new value of snd_cwnd (the
value it has been reduced to). In other words: the congestion window
before and after a congestion event occurs.

I'm uncertain where (and how) it would be logical to implement this. I
have found two possible locations in the tcp_input.c where (I think)
it could be implemented:

static void tcp_cong_control(...) {
 ...
  if (tcp_in_cwnd_reduction(sk)) {
    struct tcp_sock *tp = tcp_sk(sk);
    prior_congestion_window = tp->snd_cwnd;

    /* Reduce cwnd if state mandates */
    tcp_cwnd_reduction(sk, acked_sacked, flag);

    reduced_congestion_window = tp->snd_cwnd;
  }
 ...
}

or

static void tcp_fastretrans_alert(...) {
  ...
  default:
    ...
    struct tcp_sock *tp = tcp_sk(sk);
    prior_congestion_window = tp->snd_cwnd;

    /* Otherwise enter Recovery state */
    tcp_enter_recovery(sk, (flag & FLAG_ECE));
    fast_rexmit = 1;

    reduced_congestion_window = tp->snd_cwnd;
  ...
}

Does anyone have advice on where (and how) to implement this? Does any
of the proposed solutions above seem logical?

/ Lars Erik Storbukås