From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=195.10.208.50 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794495989; b=lRcltj65vxiiicqp/JY1hp1ovJGN6FtsgLtI4/3pjigBRb09PD1G7uyGeP71u21cA0pf mwbVx5Zh9kR1dg/kD5PKdwZ8m9ZhNzW3rZQ04+fGFS9bEMYTYBvZh74c0gcYVpRFJUgaa 3ZuVszj6EFYdgrg3A1Bs2tIeHM8lAIpXNtlEo+wCx8v/abPDNSJvNK+xVKWLoCdXnl4W+ 1lm+AQXUr0AcEcZy70fw8/4uG7ohlkc4U9j/7idQHvLHDE0hjTEk7dAAiYLTT4K7Qi1N8 BZLUjpQJ5Hm2wvuaadLGqeYY8KQCD8yA53IEaASRBxakljUmXEj234QlW6HG3MPBRlA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794495989; h=DMARC-Filter:Received:DKIM-Signature:From:To:Cc:Subject:Date: Message-ID:X-Mailer:In-Reply-To:References:MIME-Version: Content-Transfer-Encoding:X-Rspamd-Queue-Id; bh=N89g4t/mMALSVLoXHEkCFejzYZ4ZVRJdwJdi6jm2Jn0=; b=MYDfxS54HGPxL90mPtSp3wTnUsN9ngGaFDNimrOAeiKgcLz5sEMVaRaK+Kgkea7rQxCa z7LHDB9l5sh3FS9RzubL1D9CfeWxdXkxY0sEY5JIVB+GZhDTub92e1yBnuJbp4tVEUjR5 DROJuCGUWZ6e7t8xxymD2Qth5m/uv+tMPPk/34WpAcoXIWfyHE7rizDKp4v4s30ybBqDr 2HeyvFPqvefC9JLRx+Zf2nAXvbjqOOjcfGBhND1JfnemxnOMixCkWwRLnDVcJA/xuJNCq /ITg+h0cI/B1GA+4fwWHGF/e6AcML+GmZfJ19ZuZ9FaWux8jXKnfmuGihfkWnkEKDpQ== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=none header.from=mandelbit.com; spf=pass smtp.mailfrom=mandelbit.com; dkim=pass header.d=mandelbit.com header.i=@mandelbit.com header.a=rsa-sha256 header.s=MBO0001 header.b="z/7qU7Nr"; arc=none smtp.remote-ip=195.10.208.50 Received: from mout-b-105.mailbox.org (mout-b-105.mailbox.org [195.10.208.50]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BF07E606A330; Thu, 12 Nov 2026 15:06:28 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org BF07E606A330 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=mandelbit.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=mandelbit.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=mandelbit.com header.i=@mandelbit.com header.a=rsa-sha256 header.s=MBO0001 header.b=z/7qU7Nr Received: from smtp1.mailbox.org (smtp1.mailbox.org [IPv6:2001:67c:2050:b231:465::1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519MLKEM768 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by mout-b-105.mailbox.org (Postfix) with ESMTPS id 4jQNy32Brhz9syM; Thu, 12 Nov 2026 16:06:19 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=mandelbit.com; s=MBO0001; t=1794495979; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=N89g4t/mMALSVLoXHEkCFejzYZ4ZVRJdwJdi6jm2Jn0=; b=z/7qU7NrL3l06UU8hnZW2s4un7PobbSwRqNQIIRfBLRwCHmIJYNj5Nfd96s5WmkRrN7DKE cSK+eYWGcETnpz6Snu+Z7go3mv/ogAKI89iz7vjA8ZXgAnkfmlPMbIVW5xoGd5EAn+xMe5 zvJqKZ1XW5YA5lnyx58+FlZBLKutM6A6kccEzX7L7aafQvuFROJixywxMGfJ8MHnBUTl+K eRWrH5QPr5gnL1SS5dHKaMe2XAJq7TSPYs0sPbb4aCY9n7JQzPW97+hUtYiXC9BxZz5Ze/ uE/7LLAquO+/o8OcbbBGLvG2e4617ItnNdh8nou6MzILJAh0EPOTonFLu1h0Mw== Authentication-Results: outgoing_mbo_mout; dkim=none; spf=pass (outgoing_mbo_mout: domain of antonio@mandelbit.com designates 2001:67c:2050:b231:465::1 as permitted sender) smtp.mailfrom=antonio@mandelbit.com From: Antonio Quartulli To: willemdebruijn.kernel@gmail.com, jasowangio@gmail.com Cc: Antonio Quartulli , andrew+netdev@lunn.ch, davem@davemloft.net, edumazet@kernel.org, kuba@kernel.org, pabeni@redhat.com, ast@kernel.org, daniel@iogearbox.net, hawk@kernel.org, john.fastabend@gmail.com, sdf@fomichev.me, netdev@vger.kernel.org, bpf@vger.kernel.org Subject: [PATCH net-next 2/3] tun: don't disable BHs for the XDP program check of every packet Date: Thu, 12 Nov 2026 16:06:04 +0100 Message-ID: <20261112150612.3866546-3-antonio@mandelbit.com> X-Mailer: git-send-email 2.56.0 In-Reply-To: <20261112150612.3866546-1-antonio@mandelbit.com> References: <20261112150612.3866546-1-antonio@mandelbit.com> X-Mailing-List: netdev@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 4jQNy32Brhz9syM For every packet, tun_get_user() disables BHs and enters an RCU section only to find out whether an XDP program is attached, which only TAP devices can have. Check with rcu_access_pointer() first, in tun_build_skb() too, and enter the BH-disabled RCU section only with a program attached. A program attached concurrently could already miss the packets in flight. -2.3% instructions per 1400-byte packet dropped at the raw table, -3.0% when written 64 per write() (emulated x86). Signed-off-by: Antonio Quartulli --- drivers/net/tun.c | 14 ++++++++------ 1 file changed, 8 insertions(+), 6 deletions(-) diff --git a/drivers/net/tun.c b/drivers/net/tun.c index 9fc60a73fb96..395923651f31 100644 --- a/drivers/net/tun.c +++ b/drivers/net/tun.c @@ -1702,13 +1702,12 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun, int pad = TUN_RX_PAD; int metasize = 0; int err = 0; + bool xdp; - rcu_read_lock(); - xdp_prog = rcu_dereference(tun->xdp_prog); - if (xdp_prog) + xdp = rcu_access_pointer(tun->xdp_prog); + if (xdp) pad += XDP_PACKET_HEADROOM; buflen += SKB_DATA_ALIGN(len + pad); - rcu_read_unlock(); alloc_frag->offset = ALIGN((u64)alloc_frag->offset, SMP_CACHE_BYTES); if (unlikely(!skb_page_frag_refill(buflen, alloc_frag, GFP_KERNEL))) @@ -1725,7 +1724,7 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun, * of xdp_prog above, this should be rare and for simplicity * we do XDP on skb in case the headroom is not enough. */ - if (hdr->gso_type || !xdp_prog) { + if (hdr->gso_type || !xdp) { *skb_xdp = 1; return __tun_build_skb(tfile, alloc_frag, buf, buflen, len, pad, metasize); @@ -1962,7 +1961,10 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile, skb_reset_network_header(skb); skb_record_rx_queue(skb, tfile->queue_index); - if (skb_xdp) { + /* Only TAP devices can have an XDP program: don't disable BHs and + * enter an RCU section for every packet just to find out it's unset. + */ + if (skb_xdp && rcu_access_pointer(tun->xdp_prog)) { struct bpf_prog *xdp_prog; int ret; -- 2.56.0