[PATCH net-next V2 RESEND] tcp: shrink per-packet memset in __tcp_transmit_skb()
flat view
STALE226d
LANDED: 1 (1M)
From: Keita Morisaki <hidden>
Date: 2026-03-04 11:15:28
Also in:
bpf, lkml, mptcp
Subsystem:
networking [general], networking [tcp], the rest · Maintainers:
"David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Neal Cardwell, Linus Torvalds
1 review trailer; landed in mainline as cfcceb7a39fc on 2026-03-06.
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. Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com> Signed-off-by: Keita Morisaki <redacted> --- 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