netdev - [PATCH v6 net-next 03/15] tcp: reorganize tcp_sock_write

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Message-Id: <20250509211820.36880-4-chia-yu.chang@nokia-bell-labs.com>
Date: Fri,  9 May 2025 23:18:08 +0200
From: chia-yu.chang@...ia-bell-labs.com
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Cc: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
Subject: [PATCH v6 net-next 03/15] tcp: reorganize tcp_sock_write_txrx group for variables later

From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>

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 exisintg hole
in later patches. 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]; /*  2585     0 */
    /* XXX 3 bytes hole, try to pack */

    [...]
    struct tcp_options_received rx_opt;              /*  2652    24 */
    u8                         nonagle:4;            /*  2676: 0  1 */
    u8                         rate_app_limited:1;   /*  2676: 4  1 */
    /* XXX 3 bits hole, try to pack */

    __cacheline_group_end__tcp_sock_write_txrx[0];   /*  2677     0 */
    /* XXX 3 bytes hole, try to pack */
    [...]

    /* size: 3264, cachelines: 51, members: 163 */
}

[AFTER THIS PATCH]
struct tcp_sock {
    [...]
    __cacheline_group_begin__tcp_sock_write_txrx[0]; /*  2585     0 */
    u8                         nonagle:4;            /*  2585: 0  1 */
    u8                         rate_app_limited:1;   /*  2585: 4  1 */
    /* XXX 3 bits hole, try to pack */
    /* XXX 2 bytes hole, try to pack */

    [...]
    struct tcp_options_received rx_opt;              /*  2652    24 */

    __cacheline_group_end__tcp_sock_write_txrx[0];   /*  2676     0 */
    /* XXX 4 bytes hole, try to pack */
    [...]

    /* size: 3264, cachelines: 51, members: 163 */
}

Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
---
 include/linux/tcp.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index a8af71623ba7..0c82a8c942dc 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -286,6 +286,8 @@ 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 */
@@ -304,8 +306,6 @@ 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 */
-- 
2.34.1