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Date: Fri, 6 Mar 2026 15:25:39 -0800
From: Jakub Kicinski <kuba@kernel.org>
To: Keita Morisaki <kmta1236@gmail.com>
Cc: Eric Dumazet <edumazet@google.com>, Neal Cardwell
 <ncardwell@google.com>, "David S . Miller" <davem@davemloft.net>, David
 Ahern <dsahern@kernel.org>, Paolo Abeni <pabeni@redhat.com>, Kuniyuki
 Iwashima <kuniyu@google.com>, Simon Horman <horms@kernel.org>, Jakub
 Sitnicki <jakub@cloudflare.com>, Matthieu Baerts <matttbe@kernel.org>, Mat
 Martineau <martineau@kernel.org>, Geliang Tang <geliang@kernel.org>,
 netdev@vger.kernel.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
 mptcp@lists.linux.dev
Subject: Re: [PATCH net-next V2 RESEND] tcp: shrink per-packet memset in
 __tcp_transmit_skb()
Message-ID: <20260306152539.28f5a6f3@kernel.org>
In-Reply-To: <20260304111517.2088694-1-kmta1236@gmail.com>
References: <20260304111517.2088694-1-kmta1236@gmail.com>
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On Wed,  4 Mar 2026 20:15:17 +0900 Keita Morisaki wrote:
> 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 <kmta1236@gmail.com>

Applied (cfcceb7a39f), thanks!
-- 
pw-bot: accept