tcp: Set pingpong threshold via sysctl

TCP pingpong threshold is 1 by default. But some applications, like SQL DB
may prefer a higher pingpong threshold to activate delayed acks in quick
ack mode for better performance.

The pingpong threshold and related code were changed to 3 in the year
2019 in:
  commit 4a41f453bedf ("tcp: change pingpong threshold to 3")
And reverted to 1 in the year 2022 in:
  commit 4d8f24eeedc5 ("Revert "tcp: change pingpong threshold to 3"")

There is no single value that fits all applications.
Add net.ipv4.tcp_pingpong_thresh sysctl tunable, so it can be tuned for
optimal performance based on the application needs.

Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/1697056244-21888-1-git-send-email-haiyangz@microsoft.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index f7dfde3..e7ec902 100644 (file)
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -1183,6 +1183,19 @@ tcp_plb_cong_thresh - INTEGER
 
        Default: 128
 
+tcp_pingpong_thresh - INTEGER
+       The number of estimated data replies sent for estimated incoming data
+       requests that must happen before TCP considers that a connection is a
+       "ping-pong" (request-response) connection for which delayed
+       acknowledgments can provide benefits.
+
+       This threshold is 1 by default, but some applications may need a higher
+       threshold for optimal performance.
+
+       Possible Values: 1 - 255
+
+       Default: 1
+
 UDP variables
 =============
 

diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
index d6d9d1c..086d119 100644 (file)
--- a/include/net/inet_connection_sock.h
+++ b/include/net/inet_connection_sock.h
@@ -328,11 +328,10 @@ void inet_csk_update_fastreuse(struct inet_bind_bucket *tb,
 
 struct dst_entry *inet_csk_update_pmtu(struct sock *sk, u32 mtu);
 
-#define TCP_PINGPONG_THRESH    1
-
 static inline void inet_csk_enter_pingpong_mode(struct sock *sk)
 {
-       inet_csk(sk)->icsk_ack.pingpong = TCP_PINGPONG_THRESH;
+       inet_csk(sk)->icsk_ack.pingpong =
+               READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_pingpong_thresh);
 }
 
 static inline void inet_csk_exit_pingpong_mode(struct sock *sk)
@@ -342,7 +341,16 @@ static inline void inet_csk_exit_pingpong_mode(struct sock *sk)
 
 static inline bool inet_csk_in_pingpong_mode(struct sock *sk)
 {
-       return inet_csk(sk)->icsk_ack.pingpong >= TCP_PINGPONG_THRESH;
+       return inet_csk(sk)->icsk_ack.pingpong >=
+              READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_pingpong_thresh);
+}
+
+static inline void inet_csk_inc_pingpong_cnt(struct sock *sk)
+{
+       struct inet_connection_sock *icsk = inet_csk(sk);
+
+       if (icsk->icsk_ack.pingpong < U8_MAX)
+               icsk->icsk_ack.pingpong++;
 }
 
 static inline bool inet_csk_has_ulp(const struct sock *sk)

diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
index d96d05b..73f43f6 100644 (file)
--- a/include/net/netns/ipv4.h
+++ b/include/net/netns/ipv4.h
@@ -133,6 +133,8 @@ struct netns_ipv4 {
        u8 sysctl_tcp_migrate_req;
        u8 sysctl_tcp_comp_sack_nr;
        u8 sysctl_tcp_backlog_ack_defer;
+       u8 sysctl_tcp_pingpong_thresh;
+
        int sysctl_tcp_reordering;
        u8 sysctl_tcp_retries1;
        u8 sysctl_tcp_retries2;

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index e7f024d..f63a545 100644 (file)
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -1498,6 +1498,14 @@ static struct ctl_table ipv4_net_table[] = {
                .extra1         = SYSCTL_ZERO,
                .extra2         = SYSCTL_ONE,
        },
+       {
+               .procname       = "tcp_pingpong_thresh",
+               .data           = &init_net.ipv4.sysctl_tcp_pingpong_thresh,
+               .maxlen         = sizeof(u8),
+               .mode           = 0644,
+               .proc_handler   = proc_dou8vec_minmax,
+               .extra1         = SYSCTL_ONE,
+       },
        { }
 };
 

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index a441740..f603ad9 100644 (file)
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -3288,6 +3288,8 @@ static int __net_init tcp_sk_init(struct net *net)
        net->ipv4.sysctl_tcp_syn_linear_timeouts = 4;
        net->ipv4.sysctl_tcp_shrink_window = 0;
 
+       net->ipv4.sysctl_tcp_pingpong_thresh = 1;
+
        return 0;
 }
 

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index d5961a8..8e6ebf3 100644 (file)
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -170,10 +170,10 @@ static void tcp_event_data_sent(struct tcp_sock *tp,
        tp->lsndtime = now;
 
        /* If it is a reply for ato after last received
-        * packet, enter pingpong mode.
+        * packet, increase pingpong count.
         */
        if ((u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
-               inet_csk_enter_pingpong_mode(sk);
+               inet_csk_inc_pingpong_cnt(sk);
 }
 
 /* Account for an ACK we sent. */