tcp: add cwnd_event_tx_start to tcp_congestion_ops

(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.

No change in kernel size:

$ scripts/bloat-o-meter -t vmlinux.0 vmlinux
add/remove: 4/4 grow/shrink: 3/1 up/down: 564/-568 (-4)
Function                                     old     new   delta
bbr_cwnd_event_tx_start                        -     450    +450
cubictcp_cwnd_event_tx_start                   -      70     +70
__pfx_cubictcp_cwnd_event_tx_start             -      16     +16
__pfx_bbr_cwnd_event_tx_start                  -      16     +16
tcp_unregister_congestion_control             93      99      +6
tcp_update_congestion_control                518     521      +3
tcp_register_congestion_control              422     425      +3
__tcp_transmit_skb                          3308    3306      -2
__pfx_cubictcp_cwnd_event                     16       -     -16
__pfx_bbr_cwnd_event                          16       -     -16
cubictcp_cwnd_event                           80       -     -80
bbr_cwnd_event                               454       -    -454
Total: Before=25240512, After=25240508, chg -0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260323234920.1097858-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

8 files changed, 50 insertions, 29 deletions

diff --git a/net/ipv4/bpf_tcp_ca.c b/net/ipv4/bpf_tcp_ca.c
index e01492234b0b..008edc7f6688 100644
--- a/net/ipv4/bpf_tcp_ca.c
+++ b/net/ipv4/bpf_tcp_ca.c
@@ -272,6 +272,10 @@ static void bpf_tcp_ca_cwnd_event(struct sock *sk, enum tcp_ca_event ev)
 {
 }
 
+static void bpf_tcp_ca_cwnd_event_tx_start(struct sock *sk)
+{
+}
+
 static void bpf_tcp_ca_in_ack_event(struct sock *sk, u32 flags)
 {
 }
@@ -313,6 +317,7 @@ static struct tcp_congestion_ops __bpf_ops_tcp_congestion_ops = {
 	.cong_avoid = bpf_tcp_ca_cong_avoid,
 	.set_state = bpf_tcp_ca_set_state,
 	.cwnd_event = bpf_tcp_ca_cwnd_event,
+	.cwnd_event_tx_start = bpf_tcp_ca_cwnd_event_tx_start,
 	.in_ack_event = bpf_tcp_ca_in_ack_event,
 	.pkts_acked = bpf_tcp_ca_pkts_acked,
 	.min_tso_segs = bpf_tcp_ca_min_tso_segs,
diff --git a/net/ipv4/tcp_bbr.c b/net/ipv4/tcp_bbr.c
index 05d52372ca8f..1ddc20a399b0 100644
--- a/net/ipv4/tcp_bbr.c
+++ b/net/ipv4/tcp_bbr.c
@@ -330,12 +330,12 @@ static void bbr_save_cwnd(struct sock *sk)
 		bbr->prior_cwnd = max(bbr->prior_cwnd, tcp_snd_cwnd(tp));
 }
 
-__bpf_kfunc static void bbr_cwnd_event(struct sock *sk, enum tcp_ca_event event)
+__bpf_kfunc static void bbr_cwnd_event_tx_start(struct sock *sk)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct bbr *bbr = inet_csk_ca(sk);
 
-	if (event == CA_EVENT_TX_START && tp->app_limited) {
+	if (tp->app_limited) {
 		bbr->idle_restart = 1;
 		bbr->ack_epoch_mstamp = tp->tcp_mstamp;
 		bbr->ack_epoch_acked = 0;
@@ -1149,7 +1149,7 @@ static struct tcp_congestion_ops tcp_bbr_cong_ops __read_mostly = {
 	.cong_control	= bbr_main,
 	.sndbuf_expand	= bbr_sndbuf_expand,
 	.undo_cwnd	= bbr_undo_cwnd,
-	.cwnd_event	= bbr_cwnd_event,
+	.cwnd_event_tx_start	= bbr_cwnd_event_tx_start,
 	.ssthresh	= bbr_ssthresh,
 	.min_tso_segs	= bbr_min_tso_segs,
 	.get_info	= bbr_get_info,
@@ -1161,7 +1161,7 @@ BTF_ID_FLAGS(func, bbr_init)
 BTF_ID_FLAGS(func, bbr_main)
 BTF_ID_FLAGS(func, bbr_sndbuf_expand)
 BTF_ID_FLAGS(func, bbr_undo_cwnd)
-BTF_ID_FLAGS(func, bbr_cwnd_event)
+BTF_ID_FLAGS(func, bbr_cwnd_event_tx_start)
 BTF_ID_FLAGS(func, bbr_ssthresh)
 BTF_ID_FLAGS(func, bbr_min_tso_segs)
 BTF_ID_FLAGS(func, bbr_set_state)
diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index 76c23675ae50..ab78b5ae8d0e 100644
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -139,24 +139,21 @@ __bpf_kfunc static void cubictcp_init(struct sock *sk)
 		tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
 }
 
-__bpf_kfunc static void cubictcp_cwnd_event(struct sock *sk, enum tcp_ca_event event)
+__bpf_kfunc static void cubictcp_cwnd_event_tx_start(struct sock *sk)
 {
-	if (event == CA_EVENT_TX_START) {
-		struct bictcp *ca = inet_csk_ca(sk);
-		u32 now = tcp_jiffies32;
-		s32 delta;
-
-		delta = now - tcp_sk(sk)->lsndtime;
-
-		/* We were application limited (idle) for a while.
-		 * Shift epoch_start to keep cwnd growth to cubic curve.
-		 */
-		if (ca->epoch_start && delta > 0) {
-			ca->epoch_start += delta;
-			if (after(ca->epoch_start, now))
-				ca->epoch_start = now;
-		}
-		return;
+	struct bictcp *ca = inet_csk_ca(sk);
+	u32 now = tcp_jiffies32;
+	s32 delta;
+
+	delta = now - tcp_sk(sk)->lsndtime;
+
+	/* We were application limited (idle) for a while.
+	 * Shift epoch_start to keep cwnd growth to cubic curve.
+	 */
+	if (ca->epoch_start && delta > 0) {
+		ca->epoch_start += delta;
+		if (after(ca->epoch_start, now))
+			ca->epoch_start = now;
 	}
 }
 
@@ -481,7 +478,7 @@ static struct tcp_congestion_ops cubictcp __read_mostly = {
 	.cong_avoid	= cubictcp_cong_avoid,
 	.set_state	= cubictcp_state,
 	.undo_cwnd	= tcp_reno_undo_cwnd,
-	.cwnd_event	= cubictcp_cwnd_event,
+	.cwnd_event_tx_start = cubictcp_cwnd_event_tx_start,
 	.pkts_acked     = cubictcp_acked,
 	.owner		= THIS_MODULE,
 	.name		= "cubic",
@@ -492,7 +489,7 @@ BTF_ID_FLAGS(func, cubictcp_init)
 BTF_ID_FLAGS(func, cubictcp_recalc_ssthresh)
 BTF_ID_FLAGS(func, cubictcp_cong_avoid)
 BTF_ID_FLAGS(func, cubictcp_state)
-BTF_ID_FLAGS(func, cubictcp_cwnd_event)
+BTF_ID_FLAGS(func, cubictcp_cwnd_event_tx_start)
 BTF_ID_FLAGS(func, cubictcp_acked)
 BTF_KFUNCS_END(tcp_cubic_check_kfunc_ids)
 
diff --git a/net/ipv4/tcp_dctcp.c b/net/ipv4/tcp_dctcp.c
index 03abe0848420..96c99999e09d 100644
--- a/net/ipv4/tcp_dctcp.c
+++ b/net/ipv4/tcp_dctcp.c
@@ -203,15 +203,19 @@ __bpf_kfunc static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev)
 		tcp_plb_update_state_upon_rto(sk, &ca->plb);
 		dctcp_react_to_loss(sk);
 		break;
-	case CA_EVENT_TX_START:
-		tcp_plb_check_rehash(sk, &ca->plb); /* Maybe rehash when inflight is 0 */
-		break;
 	default:
 		/* Don't care for the rest. */
 		break;
 	}
 }
 
+__bpf_kfunc static void dctcp_cwnd_event_tx_start(struct sock *sk)
+{
+	struct dctcp *ca = inet_csk_ca(sk);
+
+	tcp_plb_check_rehash(sk, &ca->plb); /* Maybe rehash when inflight is 0 */
+}
+
 static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr,
 			     union tcp_cc_info *info)
 {
@@ -252,6 +256,7 @@ static struct tcp_congestion_ops dctcp __read_mostly = {
 	.init		= dctcp_init,
 	.in_ack_event   = dctcp_update_alpha,
 	.cwnd_event	= dctcp_cwnd_event,
+	.cwnd_event_tx_start = dctcp_cwnd_event_tx_start,
 	.ssthresh	= dctcp_ssthresh,
 	.cong_avoid	= tcp_reno_cong_avoid,
 	.undo_cwnd	= dctcp_cwnd_undo,
@@ -275,6 +280,7 @@ BTF_KFUNCS_START(tcp_dctcp_check_kfunc_ids)
 BTF_ID_FLAGS(func, dctcp_init)
 BTF_ID_FLAGS(func, dctcp_update_alpha)
 BTF_ID_FLAGS(func, dctcp_cwnd_event)
+BTF_ID_FLAGS(func, dctcp_cwnd_event_tx_start)
 BTF_ID_FLAGS(func, dctcp_ssthresh)
 BTF_ID_FLAGS(func, dctcp_cwnd_undo)
 BTF_ID_FLAGS(func, dctcp_state)
diff --git a/net/ipv4/tcp_vegas.c b/net/ipv4/tcp_vegas.c
index 786848ad37ea..cf12fb6be079 100644
--- a/net/ipv4/tcp_vegas.c
+++ b/net/ipv4/tcp_vegas.c
@@ -151,12 +151,17 @@ EXPORT_SYMBOL_GPL(tcp_vegas_state);
  */
 void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event)
 {
-	if (event == CA_EVENT_CWND_RESTART ||
-	    event == CA_EVENT_TX_START)
+	if (event == CA_EVENT_CWND_RESTART)
 		tcp_vegas_init(sk);
 }
 EXPORT_SYMBOL_GPL(tcp_vegas_cwnd_event);
 
+void tcp_vegas_cwnd_event_tx_start(struct sock *sk)
+{
+	tcp_vegas_init(sk);
+}
+EXPORT_SYMBOL_GPL(tcp_vegas_cwnd_event_tx_start);
+
 static inline u32 tcp_vegas_ssthresh(struct tcp_sock *tp)
 {
 	return  min(tp->snd_ssthresh, tcp_snd_cwnd(tp));
diff --git a/net/ipv4/tcp_vegas.h b/net/ipv4/tcp_vegas.h
index 4f24d0e37d9c..602af8e600c7 100644
--- a/net/ipv4/tcp_vegas.h
+++ b/net/ipv4/tcp_vegas.h
@@ -20,6 +20,7 @@ void tcp_vegas_init(struct sock *sk);
 void tcp_vegas_state(struct sock *sk, u8 ca_state);
 void tcp_vegas_pkts_acked(struct sock *sk, const struct ack_sample *sample);
 void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event);
+void tcp_vegas_cwnd_event_tx_start(struct sock *sk);
 size_t tcp_vegas_get_info(struct sock *sk, u32 ext, int *attr,
 			  union tcp_cc_info *info);
 
diff --git a/net/ipv4/tcp_veno.c b/net/ipv4/tcp_veno.c
index 366ff6f214b2..1b2e1b947901 100644
--- a/net/ipv4/tcp_veno.c
+++ b/net/ipv4/tcp_veno.c
@@ -112,10 +112,15 @@ static void tcp_veno_state(struct sock *sk, u8 ca_state)
  */
 static void tcp_veno_cwnd_event(struct sock *sk, enum tcp_ca_event event)
 {
-	if (event == CA_EVENT_CWND_RESTART || event == CA_EVENT_TX_START)
+	if (event == CA_EVENT_CWND_RESTART)
 		tcp_veno_init(sk);
 }
 
+static void tcp_veno_cwnd_event_tx_start(struct sock *sk)
+{
+	tcp_veno_init(sk);
+}
+
 static void tcp_veno_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
@@ -213,6 +218,7 @@ static struct tcp_congestion_ops tcp_veno __read_mostly = {
 	.pkts_acked	= tcp_veno_pkts_acked,
 	.set_state	= tcp_veno_state,
 	.cwnd_event	= tcp_veno_cwnd_event,
+	.cwnd_event_tx_start = tcp_veno_cwnd_event_tx_start,
 
 	.owner		= THIS_MODULE,
 	.name		= "veno",
diff --git a/net/ipv4/tcp_yeah.c b/net/ipv4/tcp_yeah.c
index 18b07ff5d20e..b22b3dccd05e 100644
--- a/net/ipv4/tcp_yeah.c
+++ b/net/ipv4/tcp_yeah.c
@@ -212,6 +212,7 @@ static struct tcp_congestion_ops tcp_yeah __read_mostly = {
 	.cong_avoid	= tcp_yeah_cong_avoid,
 	.set_state	= tcp_vegas_state,
 	.cwnd_event	= tcp_vegas_cwnd_event,
+	.cwnd_event_tx_start = tcp_vegas_cwnd_event_tx_start,
 	.get_info	= tcp_vegas_get_info,
 	.pkts_acked	= tcp_vegas_pkts_acked,