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Message-ID: <20251105093837.711053-1-edumazet@google.com>
Date: Wed, 5 Nov 2025 09:38:36 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, Neal Cardwell <ncardwell@...gle.com>,
Kuniyuki Iwashima <kuniyu@...gle.com>, netdev@...r.kernel.org, eric.dumazet@...il.com,
Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next] 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 100.
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
Documentation/networking/ip-sysctl.rst | 13 +++++++++++--
include/net/netns/ipv4.h | 1 +
net/ipv4/sysctl_net_ipv4.c | 9 +++++++++
net/ipv4/tcp_input.c | 26 ++++++++++++++++++--------
net/ipv4/tcp_ipv4.c | 1 +
5 files changed, 40 insertions(+), 10 deletions(-)
diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index 7cd35bfd39e68c5b2650eb9d0fbb76e34aed3f2b..ebc11f593305bf87e7d4ad4d50ef085b22aef7da 100644
--- 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 : 100 %
+
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
--- 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
--- 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 6db1d4c36a88bfa64b48388ee95e4e9218d9a9fd..d4ee74da018ee97209bed3402688f5e18759866b 100644
--- 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 @@ static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
}
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 @@ static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
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 b7526a7888cbe296c0f4ba6350772741cfe1765b..a4411cd0229cb7fc5903d206e549d0889d177937 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -3596,6 +3596,7 @@ 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 = 100;
net->ipv4.sysctl_tcp_backlog_ack_defer = 1;
net->ipv4.sysctl_tcp_fastopen = TFO_CLIENT_ENABLE;
net->ipv4.sysctl_tcp_fastopen_blackhole_timeout = 0;
--
2.51.2.1026.g39e6a42477-goog
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