tcp: accecn: AccECN option send control

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>