netdev - [PATCH 01/14] ipv6: optimise dst referencing

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Message-Id: <6ed8db52c96c8775fd311715d2b6d5e20d5dd9a5.1641863490.git.asml.silence@gmail.com>
Date:   Tue, 11 Jan 2022 01:21:33 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     netdev@...r.kernel.org, "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        linux-kernel@...r.kernel.org,
        Pavel Begunkov <asml.silence@...il.com>
Subject: [PATCH 01/14] ipv6: optimise dst referencing

__ip6_make_skb() initialises skb's dst by taking an additional reference
to cork->dst. However, cork->dst comes into the function holding a ref,
which will be put shortly at the end of the function in
ip6_cork_release().

Avoid this extra pair of get/put atomics by stealing cork->dst and
NULL'ing the field, ip6_cork_release() already handles zero values.

Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
---
 net/ipv6/ip6_output.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 2995f8d89e7e..14d607ccfeea 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1807,6 +1807,15 @@ int ip6_append_data(struct sock *sk,
 }
 EXPORT_SYMBOL_GPL(ip6_append_data);
 
+static void ip6_cork_steal_dst(struct sk_buff *skb, struct inet_cork_full *cork)
+{
+	struct dst_entry *dst = cork->base.dst;
+
+	cork->base.dst = NULL;
+	cork->base.flags &= ~IPCORK_ALLFRAG;
+	skb_dst_set(skb, dst);
+}
+
 static void ip6_cork_release(struct inet_cork_full *cork,
 			     struct inet6_cork *v6_cork)
 {
@@ -1889,7 +1898,7 @@ struct sk_buff *__ip6_make_skb(struct sock *sk,
 
 	skb->tstamp = cork->base.transmit_time;
 
-	skb_dst_set(skb, dst_clone(&rt->dst));
+	ip6_cork_steal_dst(skb, cork);
 	IP6_UPD_PO_STATS(net, rt->rt6i_idev, IPSTATS_MIB_OUT, skb->len);
 	if (proto == IPPROTO_ICMPV6) {
 		struct inet6_dev *idev = ip6_dst_idev(skb_dst(skb));
-- 
2.34.1