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>

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index 1c20650..a06cb99 100644 (file)
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -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

diff --git a/Documentation/networking/net_cachelines/tcp_sock.rst b/Documentation/networking/net_cachelines/tcp_sock.rst
index b941151..d4dc018 100644 (file)
--- a/Documentation/networking/net_cachelines/tcp_sock.rst
+++ b/Documentation/networking/net_cachelines/tcp_sock.rst
@@ -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)

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 7355765..f637b65 100644 (file)
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -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 */

diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
index acbb7dd..34eb3ae 100644 (file)
--- a/include/net/netns/ipv4.h
+++ b/include/net/netns/ipv4.h
@@ -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;

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 6be2912..78dd7b8 100644 (file)
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -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

diff --git a/include/net/tcp_ecn.h b/include/net/tcp_ecn.h
index 08c7f47..133fb6b 100644 (file)
--- a/include/net/tcp_ecn.h
+++ b/include/net/tcp_ecn.h
@@ -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 */

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 4a697ac..24dbc60 100644 (file)
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -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,

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 8c4a4b8..090f9ac 100644 (file)
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -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);

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index e898a76..87154fd 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -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 &&

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index aa8dbfe..6a63be1 100644 (file)
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -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;

diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index 1dbcc09..1933434 100644 (file)
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -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 ?

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 34e5c83..f897c25 100644 (file)
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -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);