tcp: make undo_cwnd mandatory for congestion modules

The undo_cwnd fallback in the stack doubles cwnd based on ssthresh,
which un-does reno halving behaviour.

It seems more appropriate to let congctl algorithms pair .ssthresh
and .undo_cwnd properly. Add a 'tcp_reno_undo_cwnd' function and wire it
up for all congestion algorithms that used to rely on the fallback.

Cc: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

8 files changed, 19 insertions, 6 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 123979fe12bf..7de80739adab 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -958,6 +958,7 @@ u32 tcp_slow_start(struct tcp_sock *tp, u32 acked);
 void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w, u32 acked);
 
 u32 tcp_reno_ssthresh(struct sock *sk);
+u32 tcp_reno_undo_cwnd(struct sock *sk);
 void tcp_reno_cong_avoid(struct sock *sk, u32 ack, u32 acked);
 extern struct tcp_congestion_ops tcp_reno;
 
diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
index 1294af4e0127..38905ec5f508 100644
--- a/net/ipv4/tcp_cong.c
+++ b/net/ipv4/tcp_cong.c
@@ -68,8 +68,9 @@ int tcp_register_congestion_control(struct tcp_congestion_ops *ca)
 {
         int ret = 0;
 
-        /* all algorithms must implement ssthresh and cong_avoid ops */
-        if (!ca->ssthresh || !(ca->cong_avoid || ca->cong_control)) {
+        /* all algorithms must implement these */
+        if (!ca->ssthresh || !ca->undo_cwnd ||
+            !(ca->cong_avoid || ca->cong_control)) {
                 pr_err("%s does not implement required ops\n", ca->name);
                 return -EINVAL;
         }
@@ -441,10 +442,19 @@ u32 tcp_reno_ssthresh(struct sock *sk)
 }
 EXPORT_SYMBOL_GPL(tcp_reno_ssthresh);
 
+u32 tcp_reno_undo_cwnd(struct sock *sk)
+{
+        const struct tcp_sock *tp = tcp_sk(sk);
+
+        return max(tp->snd_cwnd, tp->snd_ssthresh << 1);
+}
+EXPORT_SYMBOL_GPL(tcp_reno_undo_cwnd);
+
 struct tcp_congestion_ops tcp_reno = {
         .flags          = TCP_CONG_NON_RESTRICTED,
         .name           = "reno",
         .owner          = THIS_MODULE,
         .ssthresh       = tcp_reno_ssthresh,
         .cong_avoid     = tcp_reno_cong_avoid,
+        .undo_cwnd      = tcp_reno_undo_cwnd,
 };
diff --git a/net/ipv4/tcp_dctcp.c b/net/ipv4/tcp_dctcp.c
index 51139175bf61..bde22ebb92a8 100644
--- a/net/ipv4/tcp_dctcp.c
+++ b/net/ipv4/tcp_dctcp.c
@@ -342,6 +342,7 @@ static struct tcp_congestion_ops dctcp __read_mostly = {
 static struct tcp_congestion_ops dctcp_reno __read_mostly = {
         .ssthresh       = tcp_reno_ssthresh,
         .cong_avoid     = tcp_reno_cong_avoid,
+        .undo_cwnd      = tcp_reno_undo_cwnd,
         .get_info       = dctcp_get_info,
         .owner          = THIS_MODULE,
         .name           = "dctcp-reno",
diff --git a/net/ipv4/tcp_hybla.c b/net/ipv4/tcp_hybla.c
index 083831e359df..0f7175c3338e 100644
--- a/net/ipv4/tcp_hybla.c
+++ b/net/ipv4/tcp_hybla.c
@@ -166,6 +166,7 @@ static void hybla_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 static struct tcp_congestion_ops tcp_hybla __read_mostly = {
         .init           = hybla_init,
         .ssthresh       = tcp_reno_ssthresh,
+        .undo_cwnd      = tcp_reno_undo_cwnd,
         .cong_avoid     = hybla_cong_avoid,
         .set_state      = hybla_state,
 
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index a70046fea0e8..22e6a2097ff6 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2394,10 +2394,7 @@ static void tcp_undo_cwnd_reduction(struct sock *sk, bool unmark_loss)
         if (tp->prior_ssthresh) {
                 const struct inet_connection_sock *icsk = inet_csk(sk);
 
-                if (icsk->icsk_ca_ops->undo_cwnd)
-                        tp->snd_cwnd = icsk->icsk_ca_ops->undo_cwnd(sk);
-                else
-                        tp->snd_cwnd = max(tp->snd_cwnd, tp->snd_ssthresh << 1);
+                tp->snd_cwnd = icsk->icsk_ca_ops->undo_cwnd(sk);
 
                 if (tp->prior_ssthresh > tp->snd_ssthresh) {
                         tp->snd_ssthresh = tp->prior_ssthresh;
diff --git a/net/ipv4/tcp_lp.c b/net/ipv4/tcp_lp.c
index c67ece1390c2..046fd3910873 100644
--- a/net/ipv4/tcp_lp.c
+++ b/net/ipv4/tcp_lp.c
@@ -316,6 +316,7 @@ static void tcp_lp_pkts_acked(struct sock *sk, const struct ack_sample *sample)
 static struct tcp_congestion_ops tcp_lp __read_mostly = {
         .init = tcp_lp_init,
         .ssthresh = tcp_reno_ssthresh,
+        .undo_cwnd = tcp_reno_undo_cwnd,
         .cong_avoid = tcp_lp_cong_avoid,
         .pkts_acked = tcp_lp_pkts_acked,
 
diff --git a/net/ipv4/tcp_vegas.c b/net/ipv4/tcp_vegas.c
index 4c4bac1b5eab..218cfcc77650 100644
--- a/net/ipv4/tcp_vegas.c
+++ b/net/ipv4/tcp_vegas.c
@@ -307,6 +307,7 @@ EXPORT_SYMBOL_GPL(tcp_vegas_get_info);
 static struct tcp_congestion_ops tcp_vegas __read_mostly = {
         .init           = tcp_vegas_init,
         .ssthresh       = tcp_reno_ssthresh,
+        .undo_cwnd      = tcp_reno_undo_cwnd,
         .cong_avoid     = tcp_vegas_cong_avoid,
         .pkts_acked     = tcp_vegas_pkts_acked,
         .set_state      = tcp_vegas_state,
diff --git a/net/ipv4/tcp_westwood.c b/net/ipv4/tcp_westwood.c
index 4b03a2e2a050..fed66dc0e0f5 100644
--- a/net/ipv4/tcp_westwood.c
+++ b/net/ipv4/tcp_westwood.c
@@ -278,6 +278,7 @@ static struct tcp_congestion_ops tcp_westwood __read_mostly = {
         .init           = tcp_westwood_init,
         .ssthresh       = tcp_reno_ssthresh,
         .cong_avoid     = tcp_reno_cong_avoid,
+        .undo_cwnd      = tcp_reno_undo_cwnd,
         .cwnd_event     = tcp_westwood_event,
         .in_ack_event   = tcp_westwood_ack,
         .get_info       = tcp_westwood_info,