[PATCH net-next v2] tcp: shrink per-packet memset in __tcp_transmit_skb()

[Keita Morisaki <kmta1236@gmail.com>]

Use struct_group() to group the three fields in tcp_out_options that are
read unconditionally by tcp_options_write() and bpf_skops_write_hdr_opt()
(mss, bpf_opt_len, num_sack_blocks), then replace the full-struct memset
with a targeted memset of only that group.

struct tcp_out_options is 40 bytes without MPTCP and 96 bytes with
CONFIG_MPTCP=y (typical distro config). Every remaining field is either
assigned before first use by tcp_established_options()/tcp_syn_options(),
or gated behind its OPTION_* flag in tcp_options_write(). This memset
runs on every transmitted TCP packet, so shrinking it from 96 (or 40)
bytes to 4 bytes reduces per-packet overhead on the hot path.

Assembly comparison (x86-64, GCC 13, CONFIG_MPTCP=y):

  Before: rep stos zeroing 96 bytes (5 instructions, 12 8-byte stores)
  After:  movl $0x0 zeroing 4 bytes (1 instruction, 1 store)

Also add opts->options = 0 at the top of tcp_syn_options(), which
already used |= without a prior clear. tcp_established_options() already
clears opts->options at its top.

Signed-off-by: Keita Morisaki <kmta1236@gmail.com>
---
 net/ipv4/tcp_output.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 326b58ff1118d..63ee037f46e50 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -429,14 +429,19 @@ static void smc_options_write(__be32 *ptr, u16 *options)
 }
 
 struct tcp_out_options {
+	/* Following group is cleared in __tcp_transmit_skb() */
+	struct_group(cleared,
+		u16 mss;		/* 0 to disable */
+		u8 bpf_opt_len;		/* length of BPF hdr option */
+		u8 num_sack_blocks;	/* number of SACK blocks to include */
+	);
+
+	/* Caution: following fields are not cleared in __tcp_transmit_skb() */
 	u16 options;		/* bit field of OPTION_* */
-	u16 mss;		/* 0 to disable */
 	u8 ws;			/* window scale, 0 to disable */
-	u8 num_sack_blocks;	/* number of SACK blocks to include */
 	u8 num_accecn_fields:7,	/* number of AccECN fields needed */
 	   use_synack_ecn_bytes:1; /* Use synack_ecn_bytes or not */
 	u8 hash_size;		/* bytes in hash_location */
-	u8 bpf_opt_len;		/* length of BPF hdr option */
 	__u8 *hash_location;	/* temporary pointer, overloaded */
 	__u32 tsval, tsecr;	/* need to include OPTION_TS */
 	struct tcp_fastopen_cookie *fastopen_cookie;	/* Fast open cookie */
@@ -965,6 +970,8 @@ static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
 	struct tcp_fastopen_request *fastopen = tp->fastopen_req;
 	bool timestamps;
 
+	opts->options = 0;
+
 	/* Better than switch (key.type) as it has static branches */
 	if (tcp_key_is_md5(key)) {
 		timestamps = false;
@@ -1549,7 +1556,7 @@ static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb,
 
 	inet = inet_sk(sk);
 	tcb = TCP_SKB_CB(skb);
-	memset(&opts, 0, sizeof(opts));
+	memset(&opts.cleared, 0, sizeof(opts.cleared));
 
 	tcp_get_current_key(sk, &key);
 	if (unlikely(tcb->tcp_flags & TCPHDR_SYN)) {

base-commit: af4e9ef3d78420feb8fe58cd9a1ab80c501b3c08
-- 
2.34.1