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>

2 files changed, 9 insertions, 0 deletions

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index 1c206501b973..a06cb99d66dc 100644
--- 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 b941151f8c0a..d4dc01800945 100644
--- 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)