On Fri, Aug 15, 2025 at 1:40 AM <chia-yu.chang@xxxxxxxxxxxxxxxxxxx> wrote:
>
> From: Chia-Yu Chang <chia-yu.chang@xxxxxxxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxxxxxx>
> Co-developed-by: Ilpo Järvinen <ij@xxxxxxxxxx>
> Signed-off-by: Ilpo Järvinen <ij@xxxxxxxxxx>
>
Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
