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tcp: accecn: AccECN option send control 

Instead of sending the option in every ACK, limit sending to
those ACKs where the option is necessary:
- Handshake
- "Change-triggered ACK" + the ACK following it. The
  2nd ACK is necessary to unambiguously indicate which
  of the ECN byte counters in increasing. The first
  ACK has two counters increasing due to the ecnfield
  edge.
- ACKs with CE to allow CEP delta validations to take
  advantage of the option.
- Force option to be sent every at least once per 2^22
  bytes. The check is done using the bit edges of the
  byte counters (avoids need for extra variables).
- AccECN option beacon to send a few times per RTT even if
  nothing in the ECN state requires that. The default is 3
  times per RTT, and its period can be set via
  sysctl_tcp_ecn_option_beacon.

Below are the pahole outcomes before and after this patch,
in which the group size of tcp_sock_write_tx is increased
from 89 to 97 due to the new u64 accecn_opt_tstamp member:

[BEFORE THIS PATCH]
struct tcp_sock {
    [...]
    u64                        tcp_wstamp_ns;        /*  2488     8 */
    struct list_head           tsorted_sent_queue;   /*  2496    16 */

    [...]
    __cacheline_group_end__tcp_sock_write_tx[0];     /*  2521     0 */
    __cacheline_group_begin__tcp_sock_write_txrx[0]; /*  2521     0 */
    u8                         nonagle:4;            /*  2521: 0  1 */
    u8                         rate_app_limited:1;   /*  2521: 4  1 */
    /* XXX 3 bits hole, try to pack */

    /* Force alignment to the next boundary: */
    u8                         :0;
    u8                         received_ce_pending:4;/*  2522: 0  1 */
    u8                         unused2:4;            /*  2522: 4  1 */
    u8                         accecn_minlen:2;      /*  2523: 0  1 */
    u8                         est_ecnfield:2;       /*  2523: 2  1 */
    u8                         unused3:4;            /*  2523: 4  1 */

    [...]
    __cacheline_group_end__tcp_sock_write_txrx[0];   /*  2628     0 */

    [...]
    /* size: 3200, cachelines: 50, members: 171 */
}

[AFTER THIS PATCH]
struct tcp_sock {
    [...]
    u64                        tcp_wstamp_ns;        /*  2488     8 */
    u64                        accecn_opt_tstamp;    /*  2596     8 */
    struct list_head           tsorted_sent_queue;   /*  2504    16 */

    [...]
    __cacheline_group_end__tcp_sock_write_tx[0];     /*  2529     0 */
    __cacheline_group_begin__tcp_sock_write_txrx[0]; /*  2529     0 */
    u8                         nonagle:4;            /*  2529: 0  1 */
    u8                         rate_app_limited:1;   /*  2529: 4  1 */
    /* XXX 3 bits hole, try to pack */

    /* Force alignment to the next boundary: */
    u8                         :0;
    u8                         received_ce_pending:4;/*  2530: 0  1 */
    u8                         unused2:4;            /*  2530: 4  1 */
    u8                         accecn_minlen:2;      /*  2531: 0  1 */
    u8                         est_ecnfield:2;       /*  2531: 2  1 */
    u8                         accecn_opt_demand:2;  /*  2531: 4  1 */
    u8                         prev_ecnfield:2;      /*  2531: 6  1 */

    [...]
    __cacheline_group_end__tcp_sock_write_txrx[0];   /*  2636     0 */

    [...]
    /* size: 3200, cachelines: 50, members: 173 */
}

Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
Co-developed-by: Ilpo Järvinen <ij@kernel.org>
Signed-off-by: Ilpo Järvinen <ij@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250916082434.100722-8-chia-yu.chang@nokia-bell-labs.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Chia-Yu Chang 2025-09-16 10:24:31 +02:00 committed by Paolo Abeni
parent b5e74132df
commit aa55a7dde7
12 changed files with 107 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
Documentation/networking/ip-sysctl.rst
View File

@ -487,6 +487,12 @@ tcp_ecn_option - INTEGER
Default: 2
tcp_ecn_option_beacon - INTEGER
Control Accurate ECN (AccECN) option sending frequency per RTT and it
takes effect only when tcp_ecn_option is set to 2.
Default: 3 (AccECN will be send at least 3 times per RTT)
tcp_ecn_fallback - BOOLEAN
If the kernel detects that ECN connection misbehaves, enable fall
back to non-ECN. Currently, this knob implements the fallback

3
Documentation/networking/net_cachelines/tcp_sock.rst
View File

@ -109,6 +109,9 @@ u8:2 syn_ect_snt write_mostly read_w
u8:2 syn_ect_rcv read_mostly read_write
u8:2 accecn_minlen write_mostly read_write
u8:2 est_ecnfield read_write
u8:2 accecn_opt_demand read_mostly read_write
u8:2 prev_ecnfield read_write
u64 accecn_opt_tstamp read_write
u8:4 accecn_fail_mode
u32 lost read_mostly tcp_ack
u32 app_limited read_write read_mostly tcp_rate_check_app_limited,tcp_rate_skb_sent(tx);tcp_rate_gen(rx)

4
include/linux/tcp.h
View File

@ -275,6 +275,7 @@ struct tcp_sock {
u32 mdev_us; /* medium deviation */
u32 rtt_seq; /* sequence number to update rttvar */
u64 tcp_wstamp_ns; /* departure time for next sent data packet */
u64 accecn_opt_tstamp; /* Last AccECN option sent timestamp */
struct list_head tsorted_sent_queue; /* time-sorted sent but un-SACKed skbs */
struct sk_buff *highest_sack; /* skb just after the highest
* skb with SACKed bit set
@ -296,7 +297,8 @@ struct tcp_sock {
unused2:4;
u8 accecn_minlen:2,/* Minimum length of AccECN option sent */
est_ecnfield:2,/* ECN field for AccECN delivered estimates */
unused3:4;
accecn_opt_demand:2,/* Demand AccECN option for n next ACKs */
prev_ecnfield:2; /* ECN bits from the previous segment */
__be32 pred_flags;
u64 tcp_clock_cache; /* cache last tcp_clock_ns() (see tcp_mstamp_refresh()) */
u64 tcp_mstamp; /* most recent packet received/sent */

1
include/net/netns/ipv4.h
View File

@ -149,6 +149,7 @@ struct netns_ipv4 {
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;

3
include/net/tcp.h
View File

@ -100,6 +100,9 @@ void tcp_time_wait(struct sock *sk, int state, int timeo);
/* Maximal number of window scale according to RFC1323 */
#define TCP_MAX_WSCALE 14U
/* Default sending frequency of accurate ECN option per RTT */
#define TCP_ACCECN_OPTION_BEACON 3
/* urg_data states */
#define TCP_URG_VALID 0x0100
#define TCP_URG_NOTYET 0x0200

52
include/net/tcp_ecn.h
View File

@ -176,6 +176,17 @@ static inline void tcp_accecn_third_ack(struct sock *sk,
}
}
/* Demand the minimum # to send AccECN optnio */
static inline void tcp_accecn_opt_demand_min(struct sock *sk,
u8 opt_demand_min)
{
struct tcp_sock *tp = tcp_sk(sk);
u8 opt_demand;
opt_demand = max_t(u8, opt_demand_min, tp->accecn_opt_demand);
tp->accecn_opt_demand = opt_demand;
}
/* Maps IP ECN field ECT/CE code point to AccECN option field number, given
* we are sending fields with Accurate ECN Order 1: ECT(1), CE, ECT(0).
*/
@ -256,6 +267,7 @@ static inline void tcp_ecn_received_counters(struct sock *sk,
u8 ecnfield = TCP_SKB_CB(skb)->ip_dsfield & INET_ECN_MASK;
u8 is_ce = INET_ECN_is_ce(ecnfield);
struct tcp_sock *tp = tcp_sk(sk);
bool ecn_edge;
if (!INET_ECN_is_not_ect(ecnfield)) {
u32 pcount = is_ce * max_t(u16, 1, skb_shinfo(skb)->gso_segs);
@ -274,9 +286,34 @@ static inline void tcp_ecn_received_counters(struct sock *sk,
if (len > 0) {
u8 minlen = tcp_ecnfield_to_accecn_optfield(ecnfield);
u32 oldbytes = tp->received_ecn_bytes[ecnfield - 1];
u32 bytes_mask = GENMASK_U32(31, 22);
tp->received_ecn_bytes[ecnfield - 1] += len;
tp->accecn_minlen = max_t(u8, tp->accecn_minlen,
minlen);
/* Send AccECN option at least once per 2^22-byte
* increase in any ECN byte counter.
*/
if ((tp->received_ecn_bytes[ecnfield - 1] ^ oldbytes) &
bytes_mask) {
tcp_accecn_opt_demand_min(sk, 1);
}
}
}
ecn_edge = tp->prev_ecnfield != ecnfield;
if (ecn_edge || is_ce) {
tp->prev_ecnfield = ecnfield;
/* Demand Accurate ECN change-triggered ACKs. Two ACK are
* demanded to indicate unambiguously the ecnfield value
* in the latter ACK.
*/
if (tcp_ecn_mode_accecn(tp)) {
if (ecn_edge)
inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOW;
tp->accecn_opt_demand = 2;
}
}
}
@ -349,6 +386,7 @@ static inline void tcp_accecn_init_counters(struct tcp_sock *tp)
__tcp_accecn_init_bytes_counters(tp->received_ecn_bytes);
__tcp_accecn_init_bytes_counters(tp->delivered_ecn_bytes);
tp->accecn_minlen = 0;
tp->accecn_opt_demand = 0;
tp->est_ecnfield = 0;
}
@ -431,6 +469,7 @@ static inline void tcp_ecn_rcv_synack(struct sock *sk, const struct tcphdr *th,
default:
tcp_ecn_mode_set(tp, TCP_ECN_MODE_ACCECN);
tp->syn_ect_rcv = ip_dsfield & INET_ECN_MASK;
tp->accecn_opt_demand = 2;
if (INET_ECN_is_ce(ip_dsfield) &&
tcp_accecn_validate_syn_feedback(sk, ace,
tp->syn_ect_snt)) {
@ -451,6 +490,7 @@ static inline void tcp_ecn_rcv_syn(struct tcp_sock *tp, const struct tcphdr *th,
} else {
tp->syn_ect_rcv = TCP_SKB_CB(skb)->ip_dsfield &
INET_ECN_MASK;
tp->prev_ecnfield = tp->syn_ect_rcv;
tcp_ecn_mode_set(tp, TCP_ECN_MODE_ACCECN);
}
}
@ -542,4 +582,16 @@ tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr *th)
th->ece = 1;
}
static inline bool tcp_accecn_option_beacon_check(const struct sock *sk)
{
u32 ecn_beacon = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn_option_beacon);
const struct tcp_sock *tp = tcp_sk(sk);
if (!ecn_beacon)
return false;
return tcp_stamp_us_delta(tp->tcp_mstamp, tp->accecn_opt_tstamp) * ecn_beacon >=
(tp->srtt_us >> 3);
}
#endif /* _LINUX_TCP_ECN_H */

9
net/ipv4/sysctl_net_ipv4.c
View File

@ -740,6 +740,15 @@ static struct ctl_table ipv4_net_table[] = {
.extra1 = SYSCTL_ZERO,
.extra2 = SYSCTL_TWO,
},
{
.procname = "tcp_ecn_option_beacon",
.data = &init_net.ipv4.sysctl_tcp_ecn_option_beacon,
.maxlen = sizeof(u8),
.mode = 0644,
.proc_handler = proc_dou8vec_minmax,
.extra1 = SYSCTL_ZERO,
.extra2 = SYSCTL_THREE,
},
{
.procname = "tcp_ecn_fallback",
.data = &init_net.ipv4.sysctl_tcp_ecn_fallback,

5
net/ipv4/tcp.c
View File

@ -3410,6 +3410,8 @@ int tcp_disconnect(struct sock *sk, int flags)
tp->delivered_ce = 0;
tp->accecn_fail_mode = 0;
tcp_accecn_init_counters(tp);
tp->prev_ecnfield = 0;
tp->accecn_opt_tstamp = 0;
if (icsk->icsk_ca_initialized && icsk->icsk_ca_ops->release)
icsk->icsk_ca_ops->release(sk);
memset(icsk->icsk_ca_priv, 0, sizeof(icsk->icsk_ca_priv));
@ -5134,11 +5136,12 @@ static void __init tcp_struct_check(void)
CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_tx, lsndtime);
CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_tx, mdev_us);
CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_tx, tcp_wstamp_ns);
CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_tx, accecn_opt_tstamp);
CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_tx, rtt_seq);
CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_tx, tsorted_sent_queue);
CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_tx, highest_sack);
CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_tx, ecn_flags);
CACHELINE_ASSERT_GROUP_SIZE(struct tcp_sock, tcp_sock_write_tx, 89);
CACHELINE_ASSERT_GROUP_SIZE(struct tcp_sock, tcp_sock_write_tx, 97);
/* TXRX read-write hotpath cache lines */
CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_txrx, pred_flags);

4
net/ipv4/tcp_input.c
View File

@ -6121,8 +6121,10 @@ step1:
* RFC 5961 4.2 : Send a challenge ack
*/
if (th->syn) {
if (tcp_ecn_mode_accecn(tp))
if (tcp_ecn_mode_accecn(tp)) {
accecn_reflector = true;
tcp_accecn_opt_demand_min(sk, 1);
}
if (sk->sk_state == TCP_SYN_RECV && sk->sk_socket && th->ack &&
TCP_SKB_CB(skb)->seq + 1 == TCP_SKB_CB(skb)->end_seq &&
TCP_SKB_CB(skb)->seq + 1 == tp->rcv_nxt &&

1
net/ipv4/tcp_ipv4.c
View File

@ -3562,6 +3562,7 @@ static int __net_init tcp_sk_init(struct net *net)
{
net->ipv4.sysctl_tcp_ecn = TCP_ECN_IN_ECN_OUT_NOECN;
net->ipv4.sysctl_tcp_ecn_option = TCP_ACCECN_OPTION_FULL;
net->ipv4.sysctl_tcp_ecn_option_beacon = TCP_ACCECN_OPTION_BEACON;
net->ipv4.sysctl_tcp_ecn_fallback = 1;
net->ipv4.sysctl_tcp_base_mss = TCP_BASE_MSS;

2
net/ipv4/tcp_minisocks.c
View File

@ -463,6 +463,8 @@ static void tcp_ecn_openreq_child(struct sock *sk,
tcp_ecn_mode_set(tp, TCP_ECN_MODE_ACCECN);
tp->syn_ect_snt = treq->syn_ect_snt;
tcp_accecn_third_ack(sk, skb, treq->syn_ect_snt);
tp->prev_ecnfield = treq->syn_ect_rcv;
tp->accecn_opt_demand = 1;
tcp_ecn_received_counters_payload(sk, skb);
} else {
tcp_ecn_mode_set(tp, inet_rsk(req)->ecn_ok ?

26
net/ipv4/tcp_output.c
View File

@ -705,8 +705,12 @@ static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
*ptr++ = htonl(((e0b & 0xffffff) << 8) |
TCPOPT_NOP);
}
if (tp)
if (tp) {
tp->accecn_minlen = 0;
tp->accecn_opt_tstamp = tp->tcp_mstamp;
if (tp->accecn_opt_demand)
tp->accecn_opt_demand--;
}
}
if (unlikely(OPTION_SACK_ADVERTISE & options)) {
@ -1149,11 +1153,16 @@ static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb
opts->num_sack_blocks = 0;
}
if (tcp_ecn_mode_accecn(tp) &&
READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn_option)) {
opts->use_synack_ecn_bytes = 0;
size += tcp_options_fit_accecn(opts, tp->accecn_minlen,
MAX_TCP_OPTION_SPACE - size);
if (tcp_ecn_mode_accecn(tp)) {
int ecn_opt = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn_option);
if (ecn_opt &&
(ecn_opt >= TCP_ACCECN_OPTION_FULL || tp->accecn_opt_demand ||
tcp_accecn_option_beacon_check(sk))) {
opts->use_synack_ecn_bytes = 0;
size += tcp_options_fit_accecn(opts, tp->accecn_minlen,
MAX_TCP_OPTION_SPACE - size);
}
}
if (unlikely(BPF_SOCK_OPS_TEST_FLAG(tp,
@ -2863,6 +2872,11 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
sent_pkts = 0;
tcp_mstamp_refresh(tp);
/* AccECN option beacon depends on mstamp, it may change mss */
if (tcp_ecn_mode_accecn(tp) && tcp_accecn_option_beacon_check(sk))
mss_now = tcp_current_mss(sk);
if (!push_one) {
/* Do MTU probing. */
result = tcp_mtu_probe(sk);