netdev - Re: [PATCH v15 net-next 11/14] tcp: accecn: AccECN option send control

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Message-ID: <CANn89i+qYjt45-qO11vu_v=TrK7tn-C=iA5q7bw9YbK-qe5KZA@mail.gmail.com>
Date: Thu, 21 Aug 2025 05:46:49 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: chia-yu.chang@...ia-bell-labs.com
Cc: pabeni@...hat.com, linux-doc@...r.kernel.org, corbet@....net, 
	horms@...nel.org, dsahern@...nel.org, kuniyu@...zon.com, bpf@...r.kernel.org, 
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	mirja.kuehlewind@...csson.com, cheshire@...le.com, rs.ietf@....at, 
	Jason_Livingood@...cast.com, vidhi_goel@...le.com
Subject: Re: [PATCH v15 net-next 11/14] tcp: accecn: AccECN option send control

On Fri, Aug 15, 2025 at 1:40 AM <chia-yu.chang@...ia-bell-labs.com> wrote:
>
> From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
>
> 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@...ia-bell-labs.com>
> Co-developed-by: Ilpo Järvinen <ij@...nel.org>
> Signed-off-by: Ilpo Järvinen <ij@...nel.org>
>

Reviewed-by: Eric Dumazet <edumazet@...gle.com>