tcp: add cwnd_undo functions to various tcp cc algorithms

congestion control algorithms that do not halve cwnd in their .ssthresh
should provide a .cwnd_undo rather than rely on current fallback which
assumes reno halving (and thus doubles the cwnd).

All of these do 'something else' in their .ssthresh implementation, thus
store the cwnd on loss and provide .undo_cwnd to restore it again.

A followup patch will remove the fallback and all algorithms will
need to provide a .cwnd_undo function.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 15 insertions, 0 deletions

diff --git a/net/ipv4/tcp_scalable.c b/net/ipv4/tcp_scalable.c
index bf5ea9e9bbc1..f2123075ce6e 100644
--- a/net/ipv4/tcp_scalable.c
+++ b/net/ipv4/tcp_scalable.c
@@ -15,6 +15,10 @@
 #define TCP_SCALABLE_AI_CNT     50U
 #define TCP_SCALABLE_MD_SCALE   3
 
+struct scalable {
+        u32 loss_cwnd;
+};
+
 static void tcp_scalable_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 {
         struct tcp_sock *tp = tcp_sk(sk);
@@ -32,12 +36,23 @@ static void tcp_scalable_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 static u32 tcp_scalable_ssthresh(struct sock *sk)
 {
         const struct tcp_sock *tp = tcp_sk(sk);
+        struct scalable *ca = inet_csk_ca(sk);
+
+        ca->loss_cwnd = tp->snd_cwnd;
 
         return max(tp->snd_cwnd - (tp->snd_cwnd>>TCP_SCALABLE_MD_SCALE), 2U);
 }
 
+static u32 tcp_scalable_cwnd_undo(struct sock *sk)
+{
+        const struct scalable *ca = inet_csk_ca(sk);
+
+        return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd);
+}
+
 static struct tcp_congestion_ops tcp_scalable __read_mostly = {
         .ssthresh       = tcp_scalable_ssthresh,
+        .undo_cwnd      = tcp_scalable_cwnd_undo,
         .cong_avoid     = tcp_scalable_cong_avoid,
 
         .owner          = THIS_MODULE,