On Mon, Sep 8, 2025 at 10:34 AM <chia-yu.ch...@nokia-bell-labs.com> wrote: > > From: Ilpo Järvinen <i...@kernel.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 <i...@kernel.org> > Signed-off-by: Neal Cardwell <ncardw...@google.com>
> Co-developed-by: Chia-Yu Chang <chia-yu.ch...@nokia-bell-labs.com> > Signed-off-by: Chia-Yu Chang <chia-yu.ch...@nokia-bell-labs.com> > Reviewed-by: Eric Dumazet <eduma...@google.com>
