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Message-ID: <CANn89i+LVRbmGq8Ft3WrsCoQmP8YfMYLM8-NQbaVT2bXnCWQ3g@mail.gmail.com>
Date: Thu, 11 Sep 2025 03:23:31 -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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shuah@...nel.org, linux-kselftest@...r.kernel.org, ij@...nel.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 v17 net-next 10/14] tcp: accecn: AccECN option
On Mon, Sep 8, 2025 at 10:34 AM <chia-yu.chang@...ia-bell-labs.com> wrote:
>
> From: Ilpo Järvinen <ij@...nel.org>
>
> The Accurate ECN allows echoing back the sum of bytes for
> each IP ECN field value in the received packets using
> AccECN option. This change implements AccECN option tx & rx
> side processing without option send control related features
> that are added by a later change.
>
> Based on specification:
> https://tools.ietf.org/id/draft-ietf-tcpm-accurate-ecn-28.txt
> (Some features of the spec will be added in the later changes
> rather than in this one).
>
> A full-length AccECN option is always attempted but if it does
> not fit, the minimum length is selected based on the counters
> that have changed since the last update. The AccECN option
> (with 24-bit fields) often ends in odd sizes so the option
> write code tries to take advantage of some nop used to pad
> the other TCP options.
>
> The delivered_ecn_bytes pairs with received_ecn_bytes similar
> to how delivered_ce pairs with received_ce. In contrast to
> ACE field, however, the option is not always available to update
> delivered_ecn_bytes. For ACK w/o AccECN option, the delivered
> bytes calculated based on the cumulative ACK+SACK information
> are assigned to one of the counters using an estimation
> heuristic to select the most likely ECN byte counter. Any
> estimation error is corrected when the next AccECN option
> arrives. It may occur that the heuristic gets too confused
> when there are enough different byte counter deltas between
> ACKs with the AccECN option in which case the heuristic just
> gives up on updating the counters for a while.
>
> tcp_ecn_option sysctl can be used to select option sending
> mode for AccECN: TCP_ECN_OPTION_DISABLED, TCP_ECN_OPTION_MINIMUM,
> and TCP_ECN_OPTION_FULL.
>
> This patch increases the size of tcp_info struct, as there is
> no existing holes for new u32 variables. Below are the pahole
> outcomes before and after this patch:
>
> [BEFORE THIS PATCH]
> struct tcp_info {
> [...]
> __u32 tcpi_total_rto_time; /* 244 4 */
>
> /* size: 248, cachelines: 4, members: 61 */
> }
>
> [AFTER THIS PATCH]
> struct tcp_info {
> [...]
> __u32 tcpi_total_rto_time; /* 244 4 */
> __u32 tcpi_received_ce; /* 248 4 */
> __u32 tcpi_delivered_e1_bytes; /* 252 4 */
> __u32 tcpi_delivered_e0_bytes; /* 256 4 */
> __u32 tcpi_delivered_ce_bytes; /* 260 4 */
> __u32 tcpi_received_e1_bytes; /* 264 4 */
> __u32 tcpi_received_e0_bytes; /* 268 4 */
> __u32 tcpi_received_ce_bytes; /* 272 4 */
>
> /* size: 280, cachelines: 5, members: 68 */
> }
>
> This patch uses the existing 1-byte holes in the tcp_sock_write_txrx
> group for new u8 members, but adds a 4-byte hole in tcp_sock_write_rx
> group after the new u32 delivered_ecn_bytes[3] member. Therefore, the
> group size of tcp_sock_write_rx is increased from 96 to 112. Below
> are the pahole outcomes before and after this patch:
>
> [BEFORE THIS PATCH]
> struct tcp_sock {
> [...]
> u8 received_ce_pending:4; /* 2522: 0 1 */
> u8 unused2:4; /* 2522: 4 1 */
> /* XXX 1 byte hole, try to pack */
>
> [...]
> u32 rcv_rtt_last_tsecr; /* 2668 4 */
>
> [...]
> __cacheline_group_end__tcp_sock_write_rx[0]; /* 2728 0 */
>
> [...]
> /* size: 3200, cachelines: 50, members: 167 */
> }
>
> [AFTER THIS PATCH]
> struct tcp_sock {
> [...]
> 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 */
>
> [...]
> u32 rcv_rtt_last_tsecr; /* 2668 4 */
> u32 delivered_ecn_bytes[3];/* 2672 12 */
> /* XXX 4 bytes hole, try to pack */
>
> [...]
> __cacheline_group_end__tcp_sock_write_rx[0]; /* 2744 0 */
>
> [...]
> /* size: 3200, cachelines: 50, members: 171 */
> }
>
> Signed-off-by: Ilpo Järvinen <ij@...nel.org>
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> Co-developed-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
> Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
>
Reviewed-by: Eric Dumazet <edumazet@...gle.com>
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