netdev - [PATCH net-next] net: avoid unneeded atomic operation in ip*_append

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Date:   Wed,  4 Apr 2018 14:30:01 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next] net: avoid unneeded atomic operation in ip*_append_data()

After commit 694aba690de0 ("ipv4: factorize sk_wmem_alloc updates
done by __ip_append_data()") and commit 1f4c6eb24029 ("ipv6:
factorize sk_wmem_alloc updates done by __ip6_append_data()"),
when transmitting sub MTU datagram, an addtional, unneeded atomic
operation is performed in ip*_append_data() to update wmem_alloc:
in the above condition the delta is 0.

The above cause small but measurable performance regression in UDP
xmit tput test with packet size below MTU.

This change avoids such overhead updating wmem_alloc only if
wmem_alloc_delta is non zero.

The error path is left intentionally unmodified: it's a slow path
and simplicity is preferred to performances.

Fixes: 694aba690de0 ("ipv4: factorize sk_wmem_alloc updates done by __ip_append_data()")
Fixes: 1f4c6eb24029 ("ipv6: factorize sk_wmem_alloc updates done by __ip6_append_data()")
Signed-off-by: Paolo Abeni <pabeni@...hat.com>
---
 net/ipv4/ip_output.c  | 3 ++-
 net/ipv6/ip6_output.c | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 94cacae76aca..4c11b810a447 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -1090,7 +1090,8 @@ static int __ip_append_data(struct sock *sk,
 		length -= copy;
 	}

-	refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc);
+	if (wmem_alloc_delta)
+		refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc);
 	return 0;

 error_efault:
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index e6eaa4dd9f60..b4ae9b7941fa 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1536,7 +1536,8 @@ static int __ip6_append_data(struct sock *sk,
 		length -= copy;
 	}

-	refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc);
+	if (wmem_alloc_delta)
+		refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc);
 	return 0;

 error_efault:
-- 
2.14.3