From patchwork Mon Nov 18 13:14:58 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Justin Iurman X-Patchwork-Id: 13878571 Received: from serv108.segi.ulg.ac.be (serv108.segi.ulg.ac.be [139.165.32.111]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5FB9319AD7E; Mon, 18 Nov 2024 13:21:26 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=139.165.32.111 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1731936088; cv=none; b=TGA4n4TqlpzpRN8j74mpUTVNabmp908RM5ok9TvcUuY5+YemlOwCglWW0J7BSRbzhJDqMm/9DIv56l3qinSR1yyJc6+fQhMGAn3E2EU1OAsAVw+BpXvOXRtXjGqi4BD9abp/qSDJEh+Q4LLQ8UmqTFt9kodyibRtyTPyEVVLvXQ= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1731936088; c=relaxed/simple; bh=ukp9D4QNFnqQgIx33t+FEeznz5RbRDWC5wYe9t+Ofqw=; h=From:To:Cc:Subject:Date:Message-Id:MIME-Version; b=vBGxY7pnHEG8SpukE5S02LgM2aAdyZXfFQ22w68T+5a8W6m65XJg+tEaT3cBd48xIuGCYfs2uE1e4omA0aZ1r95EU5lJeolSYIzKwGcTMJuBt0zvOOCcU6TN65ocyCLulFN/Bj11aQURTTr0YvTKV7TyGOEAZzB6mgaTEcHk8Dw= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=uliege.be; spf=pass smtp.mailfrom=uliege.be; dkim=pass (2048-bit key) header.d=uliege.be header.i=@uliege.be header.b=kCOXYhhc; arc=none smtp.client-ip=139.165.32.111 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=uliege.be Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=uliege.be Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=uliege.be header.i=@uliege.be header.b="kCOXYhhc" Received: from localhost.localdomain (220.24-245-81.adsl-dyn.isp.belgacom.be [81.245.24.220]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by serv108.segi.ulg.ac.be (Postfix) with ESMTPSA id 8A251200DF80; Mon, 18 Nov 2024 14:15:17 +0100 (CET) DKIM-Filter: OpenDKIM Filter v2.11.0 serv108.segi.ulg.ac.be 8A251200DF80 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=uliege.be; s=ulg20190529; t=1731935717; bh=FdARQ9FDv6A1GuZWWmG8cFQIe3+wSYh7fXDonFFduzM=; h=From:To:Cc:Subject:Date:From; b=kCOXYhhchSCZXM7C9u7+2ApZ0nRp0slCAPLQYvYA/u8syf10ftoaJTktAKI1M/BEi nvK8aQ9wri7GDkWw0GyFkTDObD2CZ4WJDHY09BUcOApvpd+ToKY31rqpws4dzbkNM+ 8nvvg46ZXfI492pJcc2dFaVGp4BUvCAk+yzLtxAxvqzXNpjXNQHKnLnrSaWOEjQgGU vA4BT1+XOhluPsnRsavP9I2U0YfOF6PL0V9sqSCR2E/9IowauX5KfGTlfmBjhGdep1 NYC6ZozLuZgeQ0rIb1fdOuZ/B7KYjv6YjWJR7UgGq4I9tZEHUFncFCxIbWXk32pEEO nf1MJlcddHlsA== From: Justin Iurman To: netdev@vger.kernel.org Cc: davem@davemloft.net, dsahern@kernel.org, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, linux-kernel@vger.kernel.org, justin.iurman@uliege.be Subject: [PATCH net-next v4 0/4] Mitigate the two-reallocations issue for iptunnels Date: Mon, 18 Nov 2024 14:14:58 +0100 Message-Id: <20241118131502.10077-1-justin.iurman@uliege.be> X-Mailer: git-send-email 2.34.1 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Patchwork-Delegate: kuba@kernel.org v4: - move static inline function to include/net/dst.h v3: - fix compilation error in seg6_iptunnel v2: - add missing "static" keywords in seg6_iptunnel - use a static-inline function to return the dev overhead (as suggested by Olek, thanks) The same pattern is found in ioam6, rpl6, and seg6. Basically, it first makes sure there is enough room for inserting a new header: (1) err = skb_cow_head(skb, len + skb->mac_len); Then, when the insertion (encap or inline) is performed, the input and output handlers respectively make sure there is enough room for layer 2: (2) err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev)); skb_cow_head() does nothing when there is enough room. Otherwise, it reallocates more room, which depends on the architecture. Briefly, skb_cow_head() calls __skb_cow() which then calls pskb_expand_head() as follows: pskb_expand_head(skb, ALIGN(delta, NET_SKB_PAD), 0, GFP_ATOMIC); "delta" represents the number of bytes to be added. This value is aligned with NET_SKB_PAD, which is defined as follows: NET_SKB_PAD = max(32, L1_CACHE_BYTES) ... where L1_CACHE_BYTES also depends on the architecture. In our case (x86), it is defined as follows: L1_CACHE_BYTES = (1 << CONFIG_X86_L1_CACHE_SHIFT) ... where (again, in our case) CONFIG_X86_L1_CACHE_SHIFT equals 6 (=X86_GENERIC). All this to say, skb_cow_head() would reallocate to the next multiple of NET_SKB_PAD (in our case a 64-byte multiple) when there is not enough room. Back to the main issue with the pattern: in some cases, two reallocations are triggered, resulting in a performance drop (i.e., lines (1) and (2) would both trigger an implicit reallocation). How's that possible? Well, this is kind of bad luck as we hit an exact NET_SKB_PAD boundary and when skb->mac_len (=14) is smaller than LL_RESERVED_SPACE(dst->dev) (=16 in our case). For an x86 arch, it happens in the following cases (with the default needed_headroom): - ioam6: - (inline mode) pre-allocated data trace of 236 or 240 bytes - (encap mode) pre-allocated data trace of 196 or 200 bytes - seg6: - (encap mode) for 13, 17, 21, 25, 29, 33, ...(+4)... prefixes Let's illustrate the problem, i.e., when we fall on the exact NET_SKB_PAD boundary. In the case of ioam6, for the above problematic values, the total overhead is 256 bytes for both modes. Based on line (1), skb->mac_len (=14) is added, therefore passing 270 bytes to skb_cow_head(). At that moment, the headroom has 206 bytes available (in our case). Since 270 > 206, skb_cow_head() performs a reallocation and the new headroom is now 206 + 64 (NET_SKB_PAD) = 270. Which is exactly the room we needed. After the insertion, the headroom has 0 byte available. But, there's line (2) where 16 bytes are still needed. Which, again, triggers another reallocation. The same logic is applied to seg6 (although it does not happen with the inline mode, i.e., -40 bytes). It happens with other L1 cache shifts too (the larger the cache shift, the less often it happens). For example, with a +32 cache shift (instead of +64), the following number of segments would trigger two reallocations: 11, 15, 19, ... With a +128 cache shift, the following number of segments would trigger two reallocations: 17, 25, 33, ... And so on and so forth. Note that it is the same for both the "encap" and "l2encap" modes. For the "encap.red" and "l2encap.red" modes, it is the same logic but with "segs+1" (e.g., 14, 18, 22, 26, etc for a +64 cache shift). Note also that it may happen with rpl6 (based on some calculations), although it did not in our case. This series provides a solution to mitigate the aforementioned issue for ioam6, seg6, and rpl6. It provides the dst_entry (in the cache) to skb_cow_head() **before** the insertion (line (1)). As a result, the very first iteration would still trigger two reallocations (i.e., empty cache), while next iterations would only trigger a single reallocation. Justin Iurman (4): include: net: add static inline dst_dev_overhead() to dst.h net: ipv6: ioam6_iptunnel: mitigate 2-realloc issue net: ipv6: seg6_iptunnel: mitigate 2-realloc issue net: ipv6: rpl_iptunnel: mitigate 2-realloc issue include/net/dst.h | 8 +++ net/ipv6/ioam6_iptunnel.c | 82 ++++++++++++++--------------- net/ipv6/rpl_iptunnel.c | 59 ++++++++++----------- net/ipv6/seg6_iptunnel.c | 106 +++++++++++++++++++++----------------- 4 files changed, 137 insertions(+), 118 deletions(-)