Merge branch 'tcp-allow-to-reduce-max-rto'

Eric Dumazet says:

====================
tcp: allow to reduce max RTO

This is a followup of a discussion started 6 months ago
by Jason Xing.

Some applications want to lower the time between each
retransmit attempts.

TCP_KEEPINTVL and TCP_KEEPCNT socket options don't
work around the issue.

This series adds:

- a new TCP level socket option (TCP_RTO_MAX_MS)
- a new sysctl (/proc/sys/net/ipv4/tcp_rto_max_ms)

Admins and/or applications can now change the max rto value
at their own risk.

Link: https://lore.kernel.org/netdev/20240715033118.32322-1-kerneljasonxing@gmail.com/T/
====================

Link: https://patch.msgid.link/20250207152830.2527578-1-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

4 files changed, 18 insertions, 8 deletions

diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
index 055aa80b05c6..d9978ffacc97 100644
--- a/include/net/inet_connection_sock.h
+++ b/include/net/inet_connection_sock.h
@@ -90,6 +90,7 @@ struct inet_connection_sock {
  	struct timer_list	  icsk_delack_timer;
 	__u32			  icsk_rto;
 	__u32                     icsk_rto_min;
+	u32			  icsk_rto_max;
 	__u32                     icsk_delack_max;
 	__u32			  icsk_pmtu_cookie;
 	const struct tcp_congestion_ops *icsk_ca_ops;
diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
index 46452da35206..45ac125e8aeb 100644
--- a/include/net/netns/ipv4.h
+++ b/include/net/netns/ipv4.h
@@ -181,6 +181,7 @@ struct netns_ipv4 {
 	u8 sysctl_tcp_window_scaling;
 	u8 sysctl_tcp_timestamps;
 	int sysctl_tcp_rto_min_us;
+	int sysctl_tcp_rto_max_ms;
 	u8 sysctl_tcp_recovery;
 	u8 sysctl_tcp_thin_linear_timeouts;
 	u8 sysctl_tcp_slow_start_after_idle;
diff --git a/include/net/tcp.h b/include/net/tcp.h
index bb7edf0e72aa..7fd2d7fa4532 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -143,8 +143,9 @@ static_assert((1 << ATO_BITS) > TCP_DELACK_MAX);
 #define TCP_DELACK_MIN	4U
 #define TCP_ATO_MIN	4U
 #endif
-#define TCP_RTO_MAX	((unsigned)(120*HZ))
-#define TCP_RTO_MIN	((unsigned)(HZ/5))
+#define TCP_RTO_MAX_SEC 120
+#define TCP_RTO_MAX	((unsigned)(TCP_RTO_MAX_SEC * HZ))
+#define TCP_RTO_MIN	((unsigned)(HZ / 5))
 #define TCP_TIMEOUT_MIN	(2U) /* Min timeout for TCP timers in jiffies */
 
 #define TCP_TIMEOUT_MIN_US (2*USEC_PER_MSEC) /* Min TCP timeout in microsecs */
@@ -740,10 +741,14 @@ int tcp_mtu_to_mss(struct sock *sk, int pmtu);
 int tcp_mss_to_mtu(struct sock *sk, int mss);
 void tcp_mtup_init(struct sock *sk);
 
+static inline unsigned int tcp_rto_max(const struct sock *sk)
+{
+	return READ_ONCE(inet_csk(sk)->icsk_rto_max);
+}
+
 static inline void tcp_bound_rto(struct sock *sk)
 {
-	if (inet_csk(sk)->icsk_rto > TCP_RTO_MAX)
-		inet_csk(sk)->icsk_rto = TCP_RTO_MAX;
+	inet_csk(sk)->icsk_rto = min(inet_csk(sk)->icsk_rto, tcp_rto_max(sk));
 }
 
 static inline u32 __tcp_set_rto(const struct tcp_sock *tp)
@@ -1424,10 +1429,12 @@ static inline unsigned long tcp_pacing_delay(const struct sock *sk)
 static inline void tcp_reset_xmit_timer(struct sock *sk,
 					const int what,
 					unsigned long when,
-					const unsigned long max_when)
+					bool pace_delay)
 {
-	inet_csk_reset_xmit_timer(sk, what, when + tcp_pacing_delay(sk),
-				  max_when);
+	if (pace_delay)
+		when += tcp_pacing_delay(sk);
+	inet_csk_reset_xmit_timer(sk, what, when,
+				  tcp_rto_max(sk));
 }
 
 /* Something is really bad, we could not queue an additional packet,
@@ -1456,7 +1463,7 @@ static inline void tcp_check_probe_timer(struct sock *sk)
 {
 	if (!tcp_sk(sk)->packets_out && !inet_csk(sk)->icsk_pending)
 		tcp_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
-				     tcp_probe0_base(sk), TCP_RTO_MAX);
+				     tcp_probe0_base(sk), true);
 }
 
 static inline void tcp_init_wl(struct tcp_sock *tp, u32 seq)
diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
index dbf896f3146c..32a27b4a5020 100644
--- a/include/uapi/linux/tcp.h
+++ b/include/uapi/linux/tcp.h
@@ -136,6 +136,7 @@ enum {
 #define TCP_AO_REPAIR		42	/* Get/Set SNEs and ISNs */
 
 #define TCP_IS_MPTCP		43	/* Is MPTCP being used? */
+#define TCP_RTO_MAX_MS		44	/* max rto time in ms */
 
 #define TCP_REPAIR_ON		1
 #define TCP_REPAIR_OFF		0