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torvalds-linux/include/net/netns/ipv4.h
Eric Dumazet 416dd649f3 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>
2025-11-07 18:41:44 -08:00

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/* SPDX-License-Identifier: GPL-2.0 */
/*
* ipv4 in net namespaces
*/
#ifndef __NETNS_IPV4_H__
#define __NETNS_IPV4_H__
#include <linux/uidgid.h>
#include <net/inet_frag.h>
#include <linux/rcupdate.h>
#include <linux/seqlock.h>
#include <linux/siphash.h>
struct ctl_table_header;
struct ipv4_devconf;
struct fib_rules_ops;
struct hlist_head;
struct fib_table;
struct sock;
struct local_ports {
u32 range; /* high << 16 | low */
bool warned;
};
struct ping_group_range {
seqlock_t lock;
kgid_t range[2];
};
struct inet_hashinfo;
struct inet_timewait_death_row {
refcount_t tw_refcount;
/* Padding to avoid false sharing, tw_refcount can be often written */
struct inet_hashinfo *hashinfo ____cacheline_aligned_in_smp;
int sysctl_max_tw_buckets;
};
struct tcp_fastopen_context;
#ifdef CONFIG_IP_ROUTE_MULTIPATH
struct sysctl_fib_multipath_hash_seed {
u32 user_seed;
u32 mp_seed;
};
#endif
struct udp_tunnel_gro {
struct sock __rcu *sk;
struct hlist_head list;
};
struct netns_ipv4 {
/* Cacheline organization can be found documented in
* Documentation/networking/net_cachelines/netns_ipv4_sysctl.rst.
* Please update the document when adding new fields.
*/
/* TX readonly hotpath cache lines */
__cacheline_group_begin(netns_ipv4_read_tx);
u8 sysctl_tcp_early_retrans;
u8 sysctl_tcp_tso_win_divisor;
u8 sysctl_tcp_tso_rtt_log;
u8 sysctl_tcp_autocorking;
int sysctl_tcp_min_snd_mss;
unsigned int sysctl_tcp_notsent_lowat;
int sysctl_tcp_limit_output_bytes;
int sysctl_tcp_min_rtt_wlen;
int sysctl_tcp_wmem[3];
u8 sysctl_ip_fwd_use_pmtu;
__cacheline_group_end(netns_ipv4_read_tx);
/* TXRX readonly hotpath cache lines */
__cacheline_group_begin(netns_ipv4_read_txrx);
u8 sysctl_tcp_moderate_rcvbuf;
__cacheline_group_end(netns_ipv4_read_txrx);
/* RX readonly hotpath cache line */
__cacheline_group_begin(netns_ipv4_read_rx);
u8 sysctl_ip_early_demux;
u8 sysctl_tcp_early_demux;
u8 sysctl_tcp_l3mdev_accept;
/* 3 bytes hole, try to pack */
int sysctl_tcp_reordering;
int sysctl_tcp_rmem[3];
__cacheline_group_end(netns_ipv4_read_rx);
struct inet_timewait_death_row tcp_death_row;
struct udp_table *udp_table;
#if IS_ENABLED(CONFIG_NET_UDP_TUNNEL)
/* Not in a pernet subsys because need to be available at GRO stage */
struct udp_tunnel_gro udp_tunnel_gro[2];
#endif
#ifdef CONFIG_SYSCTL
struct ctl_table_header *forw_hdr;
struct ctl_table_header *frags_hdr;
struct ctl_table_header *ipv4_hdr;
struct ctl_table_header *route_hdr;
struct ctl_table_header *xfrm4_hdr;
#endif
struct ipv4_devconf *devconf_all;
struct ipv4_devconf *devconf_dflt;
struct ip_ra_chain __rcu *ra_chain;
struct mutex ra_mutex;
#ifdef CONFIG_IP_MULTIPLE_TABLES
struct fib_rules_ops *rules_ops;
struct fib_table __rcu *fib_main;
struct fib_table __rcu *fib_default;
unsigned int fib_rules_require_fldissect;
bool fib_has_custom_rules;
#endif
bool fib_has_custom_local_routes;
bool fib_offload_disabled;
u8 sysctl_tcp_shrink_window;
#ifdef CONFIG_IP_ROUTE_CLASSID
atomic_t fib_num_tclassid_users;
#endif
struct hlist_head *fib_table_hash;
struct sock *fibnl;
struct hlist_head *fib_info_hash;
unsigned int fib_info_hash_bits;
unsigned int fib_info_cnt;
struct sock *mc_autojoin_sk;
struct inet_peer_base *peers;
struct fqdir *fqdir;
u8 sysctl_icmp_echo_ignore_all;
u8 sysctl_icmp_echo_enable_probe;
u8 sysctl_icmp_echo_ignore_broadcasts;
u8 sysctl_icmp_ignore_bogus_error_responses;
u8 sysctl_icmp_errors_use_inbound_ifaddr;
u8 sysctl_icmp_errors_extension_mask;
int sysctl_icmp_ratelimit;
int sysctl_icmp_ratemask;
int sysctl_icmp_msgs_per_sec;
int sysctl_icmp_msgs_burst;
atomic_t icmp_global_credit;
u32 icmp_global_stamp;
u32 ip_rt_min_pmtu;
int ip_rt_mtu_expires;
int ip_rt_min_advmss;
struct local_ports ip_local_ports;
u8 sysctl_tcp_ecn;
u8 sysctl_tcp_ecn_option;
u8 sysctl_tcp_ecn_option_beacon;
u8 sysctl_tcp_ecn_fallback;
u8 sysctl_ip_default_ttl;
u8 sysctl_ip_no_pmtu_disc;
u8 sysctl_ip_fwd_update_priority;
u8 sysctl_ip_nonlocal_bind;
u8 sysctl_ip_autobind_reuse;
/* Shall we try to damage output packets if routing dev changes? */
u8 sysctl_ip_dynaddr;
#ifdef CONFIG_NET_L3_MASTER_DEV
u8 sysctl_raw_l3mdev_accept;
#endif
u8 sysctl_udp_early_demux;
u8 sysctl_nexthop_compat_mode;
u8 sysctl_fwmark_reflect;
u8 sysctl_tcp_fwmark_accept;
u8 sysctl_tcp_mtu_probing;
int sysctl_tcp_mtu_probe_floor;
int sysctl_tcp_base_mss;
int sysctl_tcp_probe_threshold;
u32 sysctl_tcp_probe_interval;
int sysctl_tcp_keepalive_time;
int sysctl_tcp_keepalive_intvl;
u8 sysctl_tcp_keepalive_probes;
u8 sysctl_tcp_syn_retries;
u8 sysctl_tcp_synack_retries;
u8 sysctl_tcp_syncookies;
u8 sysctl_tcp_migrate_req;
u8 sysctl_tcp_comp_sack_nr;
u8 sysctl_tcp_backlog_ack_defer;
u8 sysctl_tcp_pingpong_thresh;
u8 sysctl_tcp_retries1;
u8 sysctl_tcp_retries2;
u8 sysctl_tcp_orphan_retries;
u8 sysctl_tcp_tw_reuse;
unsigned int sysctl_tcp_tw_reuse_delay;
int sysctl_tcp_fin_timeout;
u8 sysctl_tcp_sack;
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;
u8 sysctl_tcp_retrans_collapse;
u8 sysctl_tcp_stdurg;
u8 sysctl_tcp_rfc1337;
u8 sysctl_tcp_abort_on_overflow;
u8 sysctl_tcp_fack; /* obsolete */
int sysctl_tcp_max_reordering;
int sysctl_tcp_adv_win_scale; /* obsolete */
u8 sysctl_tcp_dsack;
u8 sysctl_tcp_app_win;
u8 sysctl_tcp_frto;
u8 sysctl_tcp_nometrics_save;
u8 sysctl_tcp_no_ssthresh_metrics_save;
u8 sysctl_tcp_workaround_signed_windows;
int sysctl_tcp_challenge_ack_limit;
u8 sysctl_tcp_min_tso_segs;
u8 sysctl_tcp_reflect_tos;
int sysctl_tcp_invalid_ratelimit;
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;
int sysctl_tcp_fastopen;
const struct tcp_congestion_ops __rcu *tcp_congestion_control;
struct tcp_fastopen_context __rcu *tcp_fastopen_ctx;
unsigned int sysctl_tcp_fastopen_blackhole_timeout;
atomic_t tfo_active_disable_times;
unsigned long tfo_active_disable_stamp;
u32 tcp_challenge_timestamp;
u32 tcp_challenge_count;
u8 sysctl_tcp_plb_enabled;
u8 sysctl_tcp_plb_idle_rehash_rounds;
u8 sysctl_tcp_plb_rehash_rounds;
u8 sysctl_tcp_plb_suspend_rto_sec;
int sysctl_tcp_plb_cong_thresh;
int sysctl_udp_wmem_min;
int sysctl_udp_rmem_min;
u8 sysctl_fib_notify_on_flag_change;
u8 sysctl_tcp_syn_linear_timeouts;
#ifdef CONFIG_NET_L3_MASTER_DEV
u8 sysctl_udp_l3mdev_accept;
#endif
u8 sysctl_igmp_llm_reports;
int sysctl_igmp_max_memberships;
int sysctl_igmp_max_msf;
int sysctl_igmp_qrv;
struct ping_group_range ping_group_range;
u16 ping_port_rover;
atomic_t dev_addr_genid;
unsigned int sysctl_udp_child_hash_entries;
#ifdef CONFIG_SYSCTL
unsigned long *sysctl_local_reserved_ports;
int sysctl_ip_prot_sock;
#endif
#ifdef CONFIG_IP_MROUTE
#ifndef CONFIG_IP_MROUTE_MULTIPLE_TABLES
struct mr_table *mrt;
#else
struct list_head mr_tables;
struct fib_rules_ops *mr_rules_ops;
#endif
#endif
#ifdef CONFIG_IP_ROUTE_MULTIPATH
struct sysctl_fib_multipath_hash_seed sysctl_fib_multipath_hash_seed;
u32 sysctl_fib_multipath_hash_fields;
u8 sysctl_fib_multipath_use_neigh;
u8 sysctl_fib_multipath_hash_policy;
#endif
struct fib_notifier_ops *notifier_ops;
unsigned int fib_seq; /* writes protected by rtnl_mutex */
struct fib_notifier_ops *ipmr_notifier_ops;
unsigned int ipmr_seq; /* protected by rtnl_mutex */
atomic_t rt_genid;
siphash_key_t ip_id_key;
struct hlist_head *inet_addr_lst;
struct delayed_work addr_chk_work;
};
#endif
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