netdev - [PATCH v2 net-next 6/6] skbuff: micro-optimize {,__}skb_header

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Date:   Sat, 13 Mar 2021 11:38:10 +0000
From:   Alexander Lobakin <alobakin@...me>
To:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Alexander Lobakin <alobakin@...me>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Kevin Hao <haokexin@...il.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Marco Elver <elver@...gle.com>,
        Dexuan Cui <decui@...rosoft.com>,
        Vladimir Oltean <vladimir.oltean@....com>,
        Ariel Levkovich <lariel@...lanox.com>,
        Wang Qing <wangqing@...o.com>,
        Davide Caratti <dcaratti@...hat.com>,
        Guillaume Nault <gnault@...hat.com>,
        Eran Ben Elisha <eranbe@...dia.com>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org
Subject: [PATCH v2 net-next 6/6] skbuff: micro-optimize {,__}skb_header_pointer()

{,__}skb_header_pointer() helpers exist mainly for preventing
accesses-beyond-end of the linear data.
In the vast majorify of cases, they bail out on the first condition.
All code going after is mostly a fallback.
Mark the most common branch as 'likely' one to move it in-line.
Also, skb_copy_bits() can return negative values only when the input
arguments are invalid, e.g. offset is greater than skb->len. It can
be safely marked as 'unlikely' branch, assuming that hotpath code
provides sane input to not fail here.

These two bump the throughput with a single Flow Dissector pass on
every packet (e.g. with RPS or driver that uses eth_get_headlen())
on 20 Mbps per flow/core.

Signed-off-by: Alexander Lobakin <alobakin@...me>
---
 include/linux/skbuff.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 46c61e127e9f..ecc029674ae4 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3680,11 +3680,10 @@ static inline void * __must_check
 __skb_header_pointer(const struct sk_buff *skb, int offset, int len,
 		     const void *data, int hlen, void *buffer)
 {
-	if (hlen - offset >= len)
+	if (likely(hlen - offset >= len))
 		return (void *)data + offset;

-	if (!skb ||
-	    skb_copy_bits(skb, offset, buffer, len) < 0)
+	if (!skb || unlikely(skb_copy_bits(skb, offset, buffer, len) < 0))
 		return NULL;

 	return buffer;
--
2.30.2