tcp: reorganize tcp_sock_write_txrx group for variables later

Use the first 3-byte hole at the beginning of the tcp_sock_write_txrx
group for 'noneagle'/'rate_app_limited' to fill in the existing hole
in later patches. Therefore, the group size of tcp_sock_write_txrx is
reduced from 92 + 4 to 91 + 4. In addition, the group size of
tcp_sock_write_rx is changed to 96 to fit in the pahole outcome.
Below are the trimmed pahole outcomes before and after this patch:

[BEFORE THIS PATCH]
struct tcp_sock {
    [...]
    __cacheline_group_begin__tcp_sock_write_txrx[0]; /*  2521     0 */
    /* XXX 3 bytes hole, try to pack */

    [...]
    struct tcp_options_received rx_opt;              /*  2588    24 */
    u8                         nonagle:4;            /*  2612: 0  1 */
    u8                         rate_app_limited:1;   /*  2612: 4  1 */
    /* XXX 3 bits hole, try to pack */

    __cacheline_group_end__tcp_sock_write_txrx[0];   /*  2613     0 */
    /* XXX 3 bytes hole, try to pack */

    __cacheline_group_begin__tcp_sock_write_rx[0] __attribute__((__aligned__(8))); /*  2616     0 */

    [...]
    __cacheline_group_end__tcp_sock_write_rx[0];     /*  2712     0 */

    [...]
    /* size: 3200, cachelines: 50, members: 161 */
}

[AFTER THIS PATCH]
struct tcp_sock {
    [...]
    __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 */
    /* XXX 2 bytes hole, try to pack */

    [...]
    struct tcp_options_received rx_opt;              /*  2588    24 */

    __cacheline_group_end__tcp_sock_write_txrx[0];   /*  2612     0 */
    /* XXX 4 bytes hole, try to pack */

    __cacheline_group_begin__tcp_sock_write_rx[0] __attribute__((__aligned__(8))); /*  2616     0 */

    [...]
    __cacheline_group_end__tcp_sock_write_rx[0];     /*  2712     0 */

    [...]
    /* size: 3200, cachelines: 50, members: 161 */
}

Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250911110642.87529-4-chia-yu.chang@nokia-bell-labs.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 57e478bfaef20369f5dba1cff540e52c9302ebf4..d103cc0e7a3542ff894e4cdbefd478810f54e36a 100644 (file)
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -285,6 +285,8 @@ struct tcp_sock {
  *     Header prediction flags
  *     0x5?10 << 16 + snd_wnd in net byte order
  */
+       u8      nonagle     : 4,/* Disable Nagle algorithm?             */
+               rate_app_limited:1;  /* rate_{delivered,interval_us} limited? */
        __be32  pred_flags;
        u64     tcp_clock_cache; /* cache last tcp_clock_ns() (see tcp_mstamp_refresh()) */
        u64     tcp_mstamp;     /* most recent packet received/sent */
@@ -303,8 +305,6 @@ struct tcp_sock {
  *      Options received (usually on last packet, some only on SYN packets).
  */
        struct tcp_options_received rx_opt;
-       u8      nonagle     : 4,/* Disable Nagle algorithm?             */
-               rate_app_limited:1;  /* rate_{delivered,interval_us} limited? */
        __cacheline_group_end(tcp_sock_write_txrx);
 
        /* RX read-write hotpath cache lines */

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 7f9c671b1ee0b1736d905efd80e57675cedf1603..1f643faa8b93e1c257317027292a5962f720eb08 100644 (file)
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -5145,7 +5145,7 @@ static void __init tcp_struct_check(void)
        /* 32bit arches with 8byte alignment on u64 fields might need padding
         * before tcp_clock_cache.
         */
-       CACHELINE_ASSERT_GROUP_SIZE(struct tcp_sock, tcp_sock_write_txrx, 92 + 4);
+       CACHELINE_ASSERT_GROUP_SIZE(struct tcp_sock, tcp_sock_write_txrx, 91 + 4);
 
        /* RX read-write hotpath cache lines */
        CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_rx, bytes_received);
@@ -5162,7 +5162,7 @@ static void __init tcp_struct_check(void)
        CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_rx, bytes_acked);
        CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_rx, rcv_rtt_est);
        CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_rx, rcvq_space);
-       CACHELINE_ASSERT_GROUP_SIZE(struct tcp_sock, tcp_sock_write_rx, 99);
+       CACHELINE_ASSERT_GROUP_SIZE(struct tcp_sock, tcp_sock_write_rx, 96);
 }
 
 void __init tcp_init(void)