Re: [PATCH net-next,v2] tcp: Set pingpong threshold via sysctl

From: Kuniyuki Iwashima
Date: Tue Oct 10 2023 - 16:12:19 EST

From: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
Date: Tue, 10 Oct 2023 12:23:30 -0700
> 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@xxxxxxxxxxxxx>
> ---
> v2: Make it per-namesapce setting, and other updates suggested by Neal Cardwell,
> and Kuniyuki Iwashima.
>
> ---
> Documentation/networking/ip-sysctl.rst | 8 ++++++++
> include/net/inet_connection_sock.h | 16 ++++++++++++----
> include/net/netns/ipv4.h | 1 +
> net/ipv4/sysctl_net_ipv4.c | 8 ++++++++
> net/ipv4/tcp_ipv4.c | 2 ++
> net/ipv4/tcp_output.c | 4 ++--
> 6 files changed, 33 insertions(+), 6 deletions(-)
>
> diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
> index 5bfa1837968c..c0308b65dc2f 100644
> --- a/Documentation/networking/ip-sysctl.rst
> +++ b/Documentation/networking/ip-sysctl.rst
> @@ -1183,6 +1183,14 @@ tcp_plb_cong_thresh - INTEGER
>
> Default: 128
>
> +tcp_pingpong_thresh - INTEGER
> + TCP pingpong threshold is 1 by default, but some application 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 5d2fcc137b88..0182f27bce40 100644
> --- a/include/net/inet_connection_sock.h
> +++ b/include/net/inet_connection_sock.h
> @@ -325,11 +325,10 @@ 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)
> @@ -339,7 +338,16 @@ 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 d96d05b08819..9f1b3eb9473e 100644
> --- a/include/net/netns/ipv4.h
> +++ b/include/net/netns/ipv4.h
> @@ -191,6 +191,7 @@ struct netns_ipv4 {
> u8 sysctl_tcp_plb_rehash_rounds;
> u8 sysctl_tcp_plb_suspend_rto_sec;
> int sysctl_tcp_plb_cong_thresh;
> + u8 sysctl_tcp_pingpong_thresh;
>
> int sysctl_udp_wmem_min;
> int sysctl_udp_rmem_min;

Maybe a hole after sysctl_tcp_backlog_ack_defer is a good place
to put a new TCP knob.

After sysctl_tcp_plb_cong_thresh, we can fill 1-byte hole but the
cacheline seems cold for TCP.

$ pahole -C netns_ipv4 vmlinux
struct netns_ipv4 {
...
u8 sysctl_tcp_backlog_ack_defer; /* 402 1 */

/* XXX 1 byte hole, try to pack */

int sysctl_tcp_reordering; /* 404 4 */
...
int sysctl_tcp_plb_cong_thresh; /* 572 4 */
/* --- cacheline 9 boundary (576 bytes) --- */
int sysctl_udp_wmem_min; /* 576 4 */
int sysctl_udp_rmem_min; /* 580 4 */
u8 sysctl_fib_notify_on_flag_change; /* 584 1 */
u8 sysctl_tcp_syn_linear_timeouts; /* 585 1 */
u8 sysctl_igmp_llm_reports; /* 586 1 */

/* XXX 1 byte hole, try to pack */
...


> diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
> index e7f024d93572..f63a545a7374 100644
> --- 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 a441740616d7..f603ad9307af 100644
> --- 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 8885552dff8e..5736a736b59c 100644
> --- 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. */
> --
> 2.25.1