tcp: add net.ipv4.tcp_comp_sack_rtt_percent

TCP SACK compression has been added in 2018 in commit
5d9f4262b7ea ("tcp: add SACK compression").

It is working great for WAN flows (with large RTT).
Wifi in particular gets a significant boost _when_ ACK are suppressed.

Add a new sysctl so that we can tune the very conservative 5 % value
that has been used so far in this formula, so that small RTT flows
can benefit from this feature.

delay = min ( 5 % of RTT, 1 ms)

This patch adds new tcp_comp_sack_rtt_percent sysctl
to ease experiments and tuning.

Given that we cap the delay to 1ms (tcp_comp_sack_delay_ns sysctl),
set the default value to 33 %.

Quoting Neal Cardwell ( https://lore.kernel.org/netdev/CADVnQymZ1tFnEA1Q=vtECs0=Db7zHQ8=+WCQtnhHFVbEOzjVnQ@mail.gmail.com/ )

The rationale for 33% is basically to try to facilitate pipelining,
where there are always at least 3 ACKs and 3 GSO/TSO skbs per SRTT, so
that the path can maintain a budget for 3 full-sized GSO/TSO skbs "in
flight" at all times:

+ 1 skb in the qdisc waiting to be sent by the NIC next
+ 1 skb being sent by the NIC (being serialized by the NIC out onto the wire)
+ 1 skb being received and aggregated by the receiver machine's
aggregation mechanism (some combination of LRO, GRO, and sack
compression)

Note that this is basically the same magic number (3) and the same
rationales as:

(a) tcp_tso_should_defer() ensuring that we defer sending data for no
longer than cwnd/tcp_tso_win_divisor (where tcp_tso_win_divisor = 3),
and
(b) bbr_quantization_budget() ensuring that cwnd is at least 3 GSO/TSO
skbs to maintain pipelining and full throughput at low RTTs

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Link: https://patch.msgid.link/20251106115236.3450026-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index 7cd35bfd39e68c5b2650eb9d0fbb76e34aed3f2b..2bae61be18593a8111a83d9f034517e4646eb653 100644 (file)
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -854,9 +854,18 @@ tcp_sack - BOOLEAN
 
        Default: 1 (enabled)
 
+tcp_comp_sack_rtt_percent - INTEGER
+       Percentage of SRTT used for the compressed SACK feature.
+       See tcp_comp_sack_nr, tcp_comp_sack_delay_ns, tcp_comp_sack_slack_ns.
+
+       Possible values : 1 - 1000
+
+       Default : 33 %
+
 tcp_comp_sack_delay_ns - LONG INTEGER
-       TCP tries to reduce number of SACK sent, using a timer
-       based on 5% of SRTT, capped by this sysctl, in nano seconds.
+       TCP tries to reduce number of SACK sent, using a timer based
+       on tcp_comp_sack_rtt_percent of SRTT, capped by this sysctl
+       in nano seconds.
        The default is 1ms, based on TSO autosizing period.
 
        Default : 1,000,000 ns (1 ms)

diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
index 0e96c90e56c6d987a16598ef885c403d5c3eae52..de9d36acc8e22d3203120d8015b3d172e85de121 100644 (file)
--- a/include/net/netns/ipv4.h
+++ b/include/net/netns/ipv4.h
@@ -221,6 +221,7 @@ struct netns_ipv4 {
        int sysctl_tcp_pacing_ss_ratio;
        int sysctl_tcp_pacing_ca_ratio;
        unsigned int sysctl_tcp_child_ehash_entries;
+       int sysctl_tcp_comp_sack_rtt_percent;
        unsigned long sysctl_tcp_comp_sack_delay_ns;
        unsigned long sysctl_tcp_comp_sack_slack_ns;
        int sysctl_max_syn_backlog;

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 0c7c8f9041cbf4aa4e51dcebd607aa5d8ac80dcd..35367f8e2da32f2c7de5a06164f5e47c8929c8f1 100644 (file)
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -1451,6 +1451,15 @@ static struct ctl_table ipv4_net_table[] = {
                .mode           = 0644,
                .proc_handler   = proc_doulongvec_minmax,
        },
+       {
+               .procname       = "tcp_comp_sack_rtt_percent",
+               .data           = &init_net.ipv4.sysctl_tcp_comp_sack_rtt_percent,
+               .maxlen         = sizeof(int),
+               .mode           = 0644,
+               .proc_handler   = proc_dointvec_minmax,
+               .extra1         = SYSCTL_ONE,
+               .extra2         = SYSCTL_ONE_THOUSAND,
+       },
        {
                .procname       = "tcp_comp_sack_slack_ns",
                .data           = &init_net.ipv4.sysctl_tcp_comp_sack_slack_ns,

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 804ec56bdd244143f78a7d7e29c04e637f88db28..9df5d75156057e6ba6b64ff7a0517809e8d1d49a 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5893,7 +5893,9 @@ static inline void tcp_data_snd_check(struct sock *sk)
 static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
 {
        struct tcp_sock *tp = tcp_sk(sk);
-       unsigned long rtt, delay;
+       struct net *net = sock_net(sk);
+       unsigned long rtt;
+       u64 delay;
 
            /* More than one full frame received... */
        if (((tp->rcv_nxt - tp->rcv_wup) > inet_csk(sk)->icsk_ack.rcv_mss &&
@@ -5912,7 +5914,7 @@ static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
                 * Defer the ack until tcp_release_cb().
                 */
                if (sock_owned_by_user_nocheck(sk) &&
-                   READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_backlog_ack_defer)) {
+                   READ_ONCE(net->ipv4.sysctl_tcp_backlog_ack_defer)) {
                        set_bit(TCP_ACK_DEFERRED, &sk->sk_tsq_flags);
                        return;
                }
@@ -5927,7 +5929,7 @@ send_now:
        }
 
        if (!tcp_is_sack(tp) ||
-           tp->compressed_ack >= READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_comp_sack_nr))
+           tp->compressed_ack >= READ_ONCE(net->ipv4.sysctl_tcp_comp_sack_nr))
                goto send_now;
 
        if (tp->compressed_ack_rcv_nxt != tp->rcv_nxt) {
@@ -5942,18 +5944,26 @@ send_now:
        if (hrtimer_is_queued(&tp->compressed_ack_timer))
                return;
 
-       /* compress ack timer : 5 % of rtt, but no more than tcp_comp_sack_delay_ns */
+       /* compress ack timer : comp_sack_rtt_percent of rtt,
+        * but no more than tcp_comp_sack_delay_ns.
+        */
 
        rtt = tp->rcv_rtt_est.rtt_us;
        if (tp->srtt_us && tp->srtt_us < rtt)
                rtt = tp->srtt_us;
 
-       delay = min_t(unsigned long,
-                     READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_comp_sack_delay_ns),
-                     rtt * (NSEC_PER_USEC >> 3)/20);
+       /* delay = (rtt >> 3) * NSEC_PER_USEC * comp_sack_rtt_percent / 100
+        * ->
+        * delay = rtt * 1.25 * comp_sack_rtt_percent
+        */
+       delay = (u64)(rtt + (rtt >> 2)) *
+               READ_ONCE(net->ipv4.sysctl_tcp_comp_sack_rtt_percent);
+
+       delay = min(delay, READ_ONCE(net->ipv4.sysctl_tcp_comp_sack_delay_ns));
+
        sock_hold(sk);
        hrtimer_start_range_ns(&tp->compressed_ack_timer, ns_to_ktime(delay),
-                              READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_comp_sack_slack_ns),
+                              READ_ONCE(net->ipv4.sysctl_tcp_comp_sack_slack_ns),
                               HRTIMER_MODE_REL_PINNED_SOFT);
 }
 

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 0cfebac33a910d7f304272df65c4aa6ffd94f07a..a7d9fec2950b915e24f0586b2cb964e0e68866ed 100644 (file)
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -3595,6 +3595,7 @@ static int __net_init tcp_sk_init(struct net *net)
        net->ipv4.sysctl_tcp_comp_sack_delay_ns = NSEC_PER_MSEC;
        net->ipv4.sysctl_tcp_comp_sack_slack_ns = 100 * NSEC_PER_USEC;
        net->ipv4.sysctl_tcp_comp_sack_nr = 44;
+       net->ipv4.sysctl_tcp_comp_sack_rtt_percent = 33;
        net->ipv4.sysctl_tcp_backlog_ack_defer = 1;
        net->ipv4.sysctl_tcp_fastopen = TFO_CLIENT_ENABLE;
        net->ipv4.sysctl_tcp_fastopen_blackhole_timeout = 0;