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Date: Tue, 9 Apr 2024 02:06:32 +0300
From: Vladimir Oltean <olteanv@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: kernel test robot <lkp@...el.com>, llvm@...ts.linux.dev,
oe-kbuild-all@...ts.linux.dev, netdev@...r.kernel.org
Subject: Re: [net-next:main 26/50] net/ipv4/tcp.c:4673:2: error: call to
'__compiletime_assert_1030' declared with 'error' attribute: BUILD_BUG_ON
failed: offsetof(struct tcp_sock,
__cacheline_group_end__tcp_sock_write_txrx) - offsetofend(struct tcp_sock,
__cacheline_group_begin__tcp_sock_...
Hi Eric,
On Mon, Apr 08, 2024 at 10:49:35PM +0800, kernel test robot wrote:
> >> net/ipv4/tcp.c:4673:2: error: call to '__compiletime_assert_1030' declared with 'error' attribute: BUILD_BUG_ON failed: offsetof(struct tcp_sock, __cacheline_group_end__tcp_sock_write_txrx) - offsetofend(struct tcp_sock, __cacheline_group_begin__tcp_sock_write_txrx) > 92
> > 4673 CACHELINE_ASSERT_GROUP_SIZE(struct tcp_sock, tcp_sock_write_txrx, 92);
I can confirm the same compile time assertion with an armv7 gcc 7.3.1 compiler.
If I revert commit 86dad9aebd0d ("Revert "tcp: more struct tcp_sock adjustments")
it goes away.
Before the change (actually with it reverted), I can see that the
tcp_sock_write_txrx cacheline group begins at offset 1821 with a 3 byte
hole, and ends at offset 1897 (it has 76 bytes).
$ pahole -C tcp_sock $KBUILD_OUTPUT/net/ipv4/tcp.o
struct tcp_sock {
struct inet_connection_sock inet_conn __attribute__((__aligned__(8))); /* 0 1568 */
/* --- cacheline 24 boundary (1536 bytes) was 32 bytes ago --- */
__u8 __cacheline_group_begin__tcp_sock_read_tx[0]; /* 1568 0 */
u32 max_window; /* 1568 4 */
u32 rcv_ssthresh; /* 1572 4 */
u32 reordering; /* 1576 4 */
u32 notsent_lowat; /* 1580 4 */
u16 gso_segs; /* 1584 2 */
/* XXX 2 bytes hole, try to pack */
struct sk_buff * lost_skb_hint; /* 1588 4 */
struct sk_buff * retransmit_skb_hint; /* 1592 4 */
__u8 __cacheline_group_end__tcp_sock_read_tx[0]; /* 1596 0 */
__u8 __cacheline_group_begin__tcp_sock_read_txrx[0]; /* 1596 0 */
u32 tsoffset; /* 1596 4 */
/* --- cacheline 25 boundary (1600 bytes) --- */
u32 snd_wnd; /* 1600 4 */
u32 mss_cache; /* 1604 4 */
u32 snd_cwnd; /* 1608 4 */
u32 prr_out; /* 1612 4 */
u32 lost_out; /* 1616 4 */
u32 sacked_out; /* 1620 4 */
u16 tcp_header_len; /* 1624 2 */
u8 scaling_ratio; /* 1626 1 */
u8 chrono_type:2; /* 1627: 0 1 */
u8 repair:1; /* 1627: 2 1 */
u8 tcp_usec_ts:1; /* 1627: 3 1 */
u8 is_sack_reneg:1; /* 1627: 4 1 */
u8 is_cwnd_limited:1; /* 1627: 5 1 */
/* XXX 2 bits hole, try to pack */
__u8 __cacheline_group_end__tcp_sock_read_txrx[0]; /* 1628 0 */
__u8 __cacheline_group_begin__tcp_sock_read_rx[0]; /* 1628 0 */
u32 copied_seq; /* 1628 4 */
u32 rcv_tstamp; /* 1632 4 */
u32 snd_wl1; /* 1636 4 */
u32 tlp_high_seq; /* 1640 4 */
u32 rttvar_us; /* 1644 4 */
u32 retrans_out; /* 1648 4 */
u16 advmss; /* 1652 2 */
u16 urg_data; /* 1654 2 */
u32 lost; /* 1656 4 */
struct minmax rtt_min; /* 1660 24 */
/* --- cacheline 26 boundary (1664 bytes) was 20 bytes ago --- */
struct rb_root out_of_order_queue; /* 1684 4 */
u32 snd_ssthresh; /* 1688 4 */
__u8 __cacheline_group_end__tcp_sock_read_rx[0]; /* 1692 0 */
/* XXX 36 bytes hole, try to pack */
/* --- cacheline 27 boundary (1728 bytes) --- */
__u8 __cacheline_group_begin__tcp_sock_write_tx[0] __attribute__((__aligned__(64))); /* 1728 0 */
u32 segs_out; /* 1728 4 */
u32 data_segs_out; /* 1732 4 */
u64 bytes_sent; /* 1736 8 */
u32 snd_sml; /* 1744 4 */
u32 chrono_start; /* 1748 4 */
u32 chrono_stat[3]; /* 1752 12 */
u32 write_seq; /* 1764 4 */
u32 pushed_seq; /* 1768 4 */
u32 lsndtime; /* 1772 4 */
u32 mdev_us; /* 1776 4 */
u32 rtt_seq; /* 1780 4 */
u64 tcp_wstamp_ns; /* 1784 8 */
/* --- cacheline 28 boundary (1792 bytes) --- */
u64 tcp_clock_cache; /* 1792 8 */
u64 tcp_mstamp; /* 1800 8 */
struct list_head tsorted_sent_queue; /* 1808 8 */
struct sk_buff * highest_sack; /* 1816 4 */
u8 ecn_flags; /* 1820 1 */
__u8 __cacheline_group_end__tcp_sock_write_tx[0]; /* 1821 0 */
__u8 __cacheline_group_begin__tcp_sock_write_txrx[0]; /* 1821 0 */
/* XXX 3 bytes hole, try to pack */
__be32 pred_flags; /* 1824 4 */
u32 rcv_nxt; /* 1828 4 */
u32 snd_nxt; /* 1832 4 */
u32 snd_una; /* 1836 4 */
u32 window_clamp; /* 1840 4 */
u32 srtt_us; /* 1844 4 */
u32 packets_out; /* 1848 4 */
u32 snd_up; /* 1852 4 */
/* --- cacheline 29 boundary (1856 bytes) --- */
u32 delivered; /* 1856 4 */
u32 delivered_ce; /* 1860 4 */
u32 app_limited; /* 1864 4 */
u32 rcv_wnd; /* 1868 4 */
struct tcp_options_received rx_opt; /* 1872 24 */
u8 nonagle:4; /* 1896: 0 1 */
u8 rate_app_limited:1; /* 1896: 4 1 */
/* XXX 3 bits hole, try to pack */
__u8 __cacheline_group_end__tcp_sock_write_txrx[0]; /* 1897 0 */
/* XXX 7 bytes hole, try to pack */
__u8 __cacheline_group_begin__tcp_sock_write_rx[0] __attribute__((__aligned__(8))); /* 1904 0 */
u64 bytes_received; /* 1904 8 */
u32 segs_in; /* 1912 4 */
u32 data_segs_in; /* 1916 4 */
/* --- cacheline 30 boundary (1920 bytes) --- */
u32 rcv_wup; /* 1920 4 */
u32 max_packets_out; /* 1924 4 */
u32 cwnd_usage_seq; /* 1928 4 */
u32 rate_delivered; /* 1932 4 */
u32 rate_interval_us; /* 1936 4 */
u32 rcv_rtt_last_tsecr; /* 1940 4 */
u64 first_tx_mstamp; /* 1944 8 */
u64 delivered_mstamp; /* 1952 8 */
u64 bytes_acked; /* 1960 8 */
struct {
u32 rtt_us; /* 1968 4 */
u32 seq; /* 1972 4 */
u64 time; /* 1976 8 */
} rcv_rtt_est; /* 1968 16 */
/* --- cacheline 31 boundary (1984 bytes) --- */
struct {
u32 space; /* 1984 4 */
u32 seq; /* 1988 4 */
u64 time; /* 1992 8 */
} rcvq_space; /* 1984 16 */
__u8 __cacheline_group_end__tcp_sock_write_rx[0]; /* 2000 0 */
u32 dsack_dups; /* 2000 4 */
u32 compressed_ack_rcv_nxt; /* 2004 4 */
struct list_head tsq_node; /* 2008 8 */
struct tcp_rack rack; /* 2016 24 */
/* XXX last struct has 2 bytes of padding */
u8 compressed_ack; /* 2040 1 */
u8 dup_ack_counter:2; /* 2041: 0 1 */
u8 tlp_retrans:1; /* 2041: 2 1 */
u8 unused:5; /* 2041: 3 1 */
u8 thin_lto:1; /* 2042: 0 1 */
u8 recvmsg_inq:1; /* 2042: 1 1 */
u8 fastopen_connect:1; /* 2042: 2 1 */
u8 fastopen_no_cookie:1; /* 2042: 3 1 */
u8 fastopen_client_fail:2; /* 2042: 4 1 */
u8 frto:1; /* 2042: 6 1 */
/* XXX 1 bit hole, try to pack */
u8 repair_queue; /* 2043 1 */
u8 save_syn:2; /* 2044: 0 1 */
u8 syn_data:1; /* 2044: 2 1 */
u8 syn_fastopen:1; /* 2044: 3 1 */
u8 syn_fastopen_exp:1; /* 2044: 4 1 */
u8 syn_fastopen_ch:1; /* 2044: 5 1 */
u8 syn_data_acked:1; /* 2044: 6 1 */
/* XXX 1 bit hole, try to pack */
u8 keepalive_probes; /* 2045 1 */
/* XXX 2 bytes hole, try to pack */
/* --- cacheline 32 boundary (2048 bytes) --- */
u32 tcp_tx_delay; /* 2048 4 */
u32 mdev_max_us; /* 2052 4 */
u32 reord_seen; /* 2056 4 */
u32 snd_cwnd_cnt; /* 2060 4 */
u32 snd_cwnd_clamp; /* 2064 4 */
u32 snd_cwnd_used; /* 2068 4 */
u32 snd_cwnd_stamp; /* 2072 4 */
u32 prior_cwnd; /* 2076 4 */
u32 prr_delivered; /* 2080 4 */
u32 last_oow_ack_time; /* 2084 4 */
struct hrtimer pacing_timer __attribute__((__aligned__(8))); /* 2088 48 */
/* XXX last struct has 4 bytes of padding */
/* --- cacheline 33 boundary (2112 bytes) was 24 bytes ago --- */
struct hrtimer compressed_ack_timer __attribute__((__aligned__(8))); /* 2136 48 */
/* XXX last struct has 4 bytes of padding */
/* --- cacheline 34 boundary (2176 bytes) was 8 bytes ago --- */
struct sk_buff * ooo_last_skb; /* 2184 4 */
struct tcp_sack_block duplicate_sack[1]; /* 2188 8 */
struct tcp_sack_block selective_acks[4]; /* 2196 32 */
struct tcp_sack_block recv_sack_cache[4]; /* 2228 32 */
/* --- cacheline 35 boundary (2240 bytes) was 20 bytes ago --- */
int lost_cnt_hint; /* 2260 4 */
u32 prior_ssthresh; /* 2264 4 */
u32 high_seq; /* 2268 4 */
u32 retrans_stamp; /* 2272 4 */
u32 undo_marker; /* 2276 4 */
int undo_retrans; /* 2280 4 */
/* XXX 4 bytes hole, try to pack */
u64 bytes_retrans; /* 2288 8 */
u32 total_retrans; /* 2296 4 */
u32 rto_stamp; /* 2300 4 */
/* --- cacheline 36 boundary (2304 bytes) --- */
u16 total_rto; /* 2304 2 */
u16 total_rto_recoveries; /* 2306 2 */
u32 total_rto_time; /* 2308 4 */
u32 urg_seq; /* 2312 4 */
unsigned int keepalive_time; /* 2316 4 */
unsigned int keepalive_intvl; /* 2320 4 */
int linger2; /* 2324 4 */
u8 bpf_sock_ops_cb_flags; /* 2328 1 */
u8 bpf_chg_cc_inprogress:1; /* 2329: 0 1 */
/* XXX 7 bits hole, try to pack */
u16 timeout_rehash; /* 2330 2 */
u32 rcv_ooopack; /* 2332 4 */
struct {
u32 probe_seq_start; /* 2336 4 */
u32 probe_seq_end; /* 2340 4 */
} mtu_probe; /* 2336 8 */
u32 plb_rehash; /* 2344 4 */
u32 mtu_info; /* 2348 4 */
struct tcp_fastopen_request * fastopen_req; /* 2352 4 */
struct request_sock * fastopen_rsk; /* 2356 4 */
struct saved_syn * saved_syn; /* 2360 4 */
/* size: 2368, cachelines: 37, members: 156 */
/* sum members: 2304, holes: 6, sum holes: 54 */
/* sum bitfield members: 34 bits, bit holes: 5, sum bit holes: 14 bits */
/* padding: 4 */
/* paddings: 3, sum paddings: 10 */
/* forced alignments: 5, forced holes: 2, sum forced holes: 43 */
} __attribute__((__aligned__(64)));
After the change, it begins at 1805 and ends at 1901, thus triggering
the assertion (96 > 92):
struct tcp_sock {
struct inet_connection_sock inet_conn __attribute__((__aligned__(8))); /* 0 1568 */
/* --- cacheline 24 boundary (1536 bytes) was 32 bytes ago --- */
__u8 __cacheline_group_begin__tcp_sock_read_tx[0]; /* 1568 0 */
u32 max_window; /* 1568 4 */
u32 rcv_ssthresh; /* 1572 4 */
u32 reordering; /* 1576 4 */
u32 notsent_lowat; /* 1580 4 */
u16 gso_segs; /* 1584 2 */
/* XXX 2 bytes hole, try to pack */
struct sk_buff * lost_skb_hint; /* 1588 4 */
struct sk_buff * retransmit_skb_hint; /* 1592 4 */
__u8 __cacheline_group_end__tcp_sock_read_tx[0]; /* 1596 0 */
__u8 __cacheline_group_begin__tcp_sock_read_txrx[0]; /* 1596 0 */
u32 tsoffset; /* 1596 4 */
/* --- cacheline 25 boundary (1600 bytes) --- */
u32 snd_wnd; /* 1600 4 */
u32 mss_cache; /* 1604 4 */
u32 snd_cwnd; /* 1608 4 */
u32 prr_out; /* 1612 4 */
u32 lost_out; /* 1616 4 */
u32 sacked_out; /* 1620 4 */
u16 tcp_header_len; /* 1624 2 */
u8 scaling_ratio; /* 1626 1 */
u8 chrono_type:2; /* 1627: 0 1 */
u8 repair:1; /* 1627: 2 1 */
u8 tcp_usec_ts:1; /* 1627: 3 1 */
u8 is_sack_reneg:1; /* 1627: 4 1 */
u8 is_cwnd_limited:1; /* 1627: 5 1 */
/* XXX 2 bits hole, try to pack */
__u8 __cacheline_group_end__tcp_sock_read_txrx[0]; /* 1628 0 */
__u8 __cacheline_group_begin__tcp_sock_read_rx[0]; /* 1628 0 */
u32 copied_seq; /* 1628 4 */
u32 rcv_tstamp; /* 1632 4 */
u32 snd_wl1; /* 1636 4 */
u32 tlp_high_seq; /* 1640 4 */
u32 rttvar_us; /* 1644 4 */
u32 retrans_out; /* 1648 4 */
u16 advmss; /* 1652 2 */
u16 urg_data; /* 1654 2 */
u32 lost; /* 1656 4 */
struct minmax rtt_min; /* 1660 24 */
/* --- cacheline 26 boundary (1664 bytes) was 20 bytes ago --- */
struct rb_root out_of_order_queue; /* 1684 4 */
u32 snd_ssthresh; /* 1688 4 */
u8 recvmsg_inq:1; /* 1692: 0 1 */
/* XXX 7 bits hole, try to pack */
__u8 __cacheline_group_end__tcp_sock_read_rx[0]; /* 1693 0 */
/* XXX 35 bytes hole, try to pack */
/* --- cacheline 27 boundary (1728 bytes) --- */
__u8 __cacheline_group_begin__tcp_sock_write_tx[0] __attribute__((__aligned__(64))); /* 1728 0 */
u32 segs_out; /* 1728 4 */
u32 data_segs_out; /* 1732 4 */
u64 bytes_sent; /* 1736 8 */
u32 snd_sml; /* 1744 4 */
u32 chrono_start; /* 1748 4 */
u32 chrono_stat[3]; /* 1752 12 */
u32 write_seq; /* 1764 4 */
u32 pushed_seq; /* 1768 4 */
u32 lsndtime; /* 1772 4 */
u32 mdev_us; /* 1776 4 */
u32 rtt_seq; /* 1780 4 */
u64 tcp_wstamp_ns; /* 1784 8 */
/* --- cacheline 28 boundary (1792 bytes) --- */
struct list_head tsorted_sent_queue; /* 1792 8 */
struct sk_buff * highest_sack; /* 1800 4 */
u8 ecn_flags; /* 1804 1 */
__u8 __cacheline_group_end__tcp_sock_write_tx[0]; /* 1805 0 */
__u8 __cacheline_group_begin__tcp_sock_write_txrx[0]; /* 1805 0 */
/* XXX 3 bytes hole, try to pack */
__be32 pred_flags; /* 1808 4 */
/* XXX 4 bytes hole, try to pack */
u64 tcp_clock_cache; /* 1816 8 */
u64 tcp_mstamp; /* 1824 8 */
u32 rcv_nxt; /* 1832 4 */
u32 snd_nxt; /* 1836 4 */
u32 snd_una; /* 1840 4 */
u32 window_clamp; /* 1844 4 */
u32 srtt_us; /* 1848 4 */
u32 packets_out; /* 1852 4 */
/* --- cacheline 29 boundary (1856 bytes) --- */
u32 snd_up; /* 1856 4 */
u32 delivered; /* 1860 4 */
u32 delivered_ce; /* 1864 4 */
u32 app_limited; /* 1868 4 */
u32 rcv_wnd; /* 1872 4 */
struct tcp_options_received rx_opt; /* 1876 24 */
u8 nonagle:4; /* 1900: 0 1 */
u8 rate_app_limited:1; /* 1900: 4 1 */
/* XXX 3 bits hole, try to pack */
__u8 __cacheline_group_end__tcp_sock_write_txrx[0]; /* 1901 0 */
/* XXX 3 bytes hole, try to pack */
__u8 __cacheline_group_begin__tcp_sock_write_rx[0] __attribute__((__aligned__(8))); /* 1904 0 */
u64 bytes_received; /* 1904 8 */
u32 segs_in; /* 1912 4 */
u32 data_segs_in; /* 1916 4 */
/* --- cacheline 30 boundary (1920 bytes) --- */
u32 rcv_wup; /* 1920 4 */
u32 max_packets_out; /* 1924 4 */
u32 cwnd_usage_seq; /* 1928 4 */
u32 rate_delivered; /* 1932 4 */
u32 rate_interval_us; /* 1936 4 */
u32 rcv_rtt_last_tsecr; /* 1940 4 */
u64 first_tx_mstamp; /* 1944 8 */
u64 delivered_mstamp; /* 1952 8 */
u64 bytes_acked; /* 1960 8 */
struct {
u32 rtt_us; /* 1968 4 */
u32 seq; /* 1972 4 */
u64 time; /* 1976 8 */
} rcv_rtt_est; /* 1968 16 */
/* --- cacheline 31 boundary (1984 bytes) --- */
struct {
u32 space; /* 1984 4 */
u32 seq; /* 1988 4 */
u64 time; /* 1992 8 */
} rcvq_space; /* 1984 16 */
__u8 __cacheline_group_end__tcp_sock_write_rx[0]; /* 2000 0 */
u32 dsack_dups; /* 2000 4 */
u32 compressed_ack_rcv_nxt; /* 2004 4 */
struct list_head tsq_node; /* 2008 8 */
struct tcp_rack rack; /* 2016 24 */
/* XXX last struct has 2 bytes of padding */
u8 compressed_ack; /* 2040 1 */
u8 dup_ack_counter:2; /* 2041: 0 1 */
u8 tlp_retrans:1; /* 2041: 2 1 */
u8 unused:5; /* 2041: 3 1 */
u8 thin_lto:1; /* 2042: 0 1 */
u8 fastopen_connect:1; /* 2042: 1 1 */
u8 fastopen_no_cookie:1; /* 2042: 2 1 */
u8 fastopen_client_fail:2; /* 2042: 3 1 */
u8 frto:1; /* 2042: 5 1 */
/* XXX 2 bits hole, try to pack */
u8 repair_queue; /* 2043 1 */
u8 save_syn:2; /* 2044: 0 1 */
u8 syn_data:1; /* 2044: 2 1 */
u8 syn_fastopen:1; /* 2044: 3 1 */
u8 syn_fastopen_exp:1; /* 2044: 4 1 */
u8 syn_fastopen_ch:1; /* 2044: 5 1 */
u8 syn_data_acked:1; /* 2044: 6 1 */
/* XXX 1 bit hole, try to pack */
u8 keepalive_probes; /* 2045 1 */
/* XXX 2 bytes hole, try to pack */
/* --- cacheline 32 boundary (2048 bytes) --- */
u32 tcp_tx_delay; /* 2048 4 */
u32 mdev_max_us; /* 2052 4 */
u32 reord_seen; /* 2056 4 */
u32 snd_cwnd_cnt; /* 2060 4 */
u32 snd_cwnd_clamp; /* 2064 4 */
u32 snd_cwnd_used; /* 2068 4 */
u32 snd_cwnd_stamp; /* 2072 4 */
u32 prior_cwnd; /* 2076 4 */
u32 prr_delivered; /* 2080 4 */
u32 last_oow_ack_time; /* 2084 4 */
struct hrtimer pacing_timer __attribute__((__aligned__(8))); /* 2088 48 */
/* XXX last struct has 4 bytes of padding */
/* --- cacheline 33 boundary (2112 bytes) was 24 bytes ago --- */
struct hrtimer compressed_ack_timer __attribute__((__aligned__(8))); /* 2136 48 */
/* XXX last struct has 4 bytes of padding */
/* --- cacheline 34 boundary (2176 bytes) was 8 bytes ago --- */
struct sk_buff * ooo_last_skb; /* 2184 4 */
struct tcp_sack_block duplicate_sack[1]; /* 2188 8 */
struct tcp_sack_block selective_acks[4]; /* 2196 32 */
struct tcp_sack_block recv_sack_cache[4]; /* 2228 32 */
/* --- cacheline 35 boundary (2240 bytes) was 20 bytes ago --- */
int lost_cnt_hint; /* 2260 4 */
u32 prior_ssthresh; /* 2264 4 */
u32 high_seq; /* 2268 4 */
u32 retrans_stamp; /* 2272 4 */
u32 undo_marker; /* 2276 4 */
int undo_retrans; /* 2280 4 */
/* XXX 4 bytes hole, try to pack */
u64 bytes_retrans; /* 2288 8 */
u32 total_retrans; /* 2296 4 */
u32 rto_stamp; /* 2300 4 */
/* --- cacheline 36 boundary (2304 bytes) --- */
u16 total_rto; /* 2304 2 */
u16 total_rto_recoveries; /* 2306 2 */
u32 total_rto_time; /* 2308 4 */
u32 urg_seq; /* 2312 4 */
unsigned int keepalive_time; /* 2316 4 */
unsigned int keepalive_intvl; /* 2320 4 */
int linger2; /* 2324 4 */
u8 bpf_sock_ops_cb_flags; /* 2328 1 */
u8 bpf_chg_cc_inprogress:1; /* 2329: 0 1 */
/* XXX 7 bits hole, try to pack */
u16 timeout_rehash; /* 2330 2 */
u32 rcv_ooopack; /* 2332 4 */
struct {
u32 probe_seq_start; /* 2336 4 */
u32 probe_seq_end; /* 2340 4 */
} mtu_probe; /* 2336 8 */
u32 plb_rehash; /* 2344 4 */
u32 mtu_info; /* 2348 4 */
struct tcp_fastopen_request * fastopen_req; /* 2352 4 */
struct request_sock * fastopen_rsk; /* 2356 4 */
struct saved_syn * saved_syn; /* 2360 4 */
/* size: 2368, cachelines: 37, members: 156 */
/* sum members: 2304, holes: 7, sum holes: 53 */
/* sum bitfield members: 34 bits, bit holes: 6, sum bit holes: 22 bits */
/* padding: 4 */
/* paddings: 3, sum paddings: 10 */
/* forced alignments: 5, forced holes: 2, sum forced holes: 38 */
} __attribute__((__aligned__(64)));
It gained 20 bytes in the change. Most notably, it gained a 4 byte hole
between pred_flags and tcp_clock_cache.
I haven't followed the development of these optimizations, and I tried a
few trivial things, some of which didn't work, and some of which did.
Of those that worked, the most notable one was letting the 2 u64 fields,
tcp_clock_cache and tcp_mstamp, be the first members of the group, and
moving the __be32 pred_flags right below them.
Obviously my level of confidence in the fix is quite low, so it would be
great if you could cast an expert eye onto this.
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