netdev - [PATCH net-next] net: do not read sk_drops if application does not care

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Date:   Wed, 07 Dec 2016 10:05:36 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>, Paolo Abeni <pabeni@...hat.com>
Subject: [PATCH net-next] net: do not read sk_drops if application does not
 care

From: Eric Dumazet <edumazet@...gle.com>

sk_drops can be an often written field, do not read it unless
application showed interest.

Note that sk_drops can be read via inet_diag, so applications
can avoid getting this info from every received packet.

In the future, 'reading' sk_drops might require folding per node or per
cpu fields, and thus become even more expensive than today.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Paolo Abeni <pabeni@...hat.com>
---
 include/net/sock.h |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index 1749e38d03014558ac882b5d1fb37b11ac5e6705..be167c1483f4a5a74b466f135bbfdf4281e5bef4 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -2160,7 +2160,8 @@ struct sock_skb_cb {
 static inline void
 sock_skb_set_dropcount(const struct sock *sk, struct sk_buff *skb)
 {
-	SOCK_SKB_CB(skb)->dropcount = atomic_read(&sk->sk_drops);
+	SOCK_SKB_CB(skb)->dropcount = sock_flag(sk, SOCK_RXQ_OVFL) ?
+						atomic_read(&sk->sk_drops) : 0;
 }
 
 static inline void sk_drops_add(struct sock *sk, const struct sk_buff *skb)