[PATCH net-next] udp: prefetch rmem_alloc in udp_queue_rcv_skb()

On UDP packets processing, if the BH is the bottle-neck, it
always sees a cache miss while updating rmem_alloc; try to
avoid it prefetching the value as soon as we have the socket
available.

Performances under flood with multiple NIC rx queues used are
unaffected, but when a single NIC rx queue is in use, this
gives ~10% performance improvement.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/ipv4/udp.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index f3450f0..067a607 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c

@@ -1949,6 +1949,7 @@ static int udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
 		}
 	}
 
+	prefetch(&sk->sk_rmem_alloc);
 	if (rcu_access_pointer(sk->sk_filter) &&
 	    udp_lib_checksum_complete(skb))
 			goto csum_error;

-- 
2.9.4

From: Paolo Abeni <pabeni@redhat.com>
Date: Wed, 21 Jun 2017 10:24:40 +0200

On UDP packets processing, if the BH is the bottle-neck, it
always sees a cache miss while updating rmem_alloc; try to
avoid it prefetching the value as soon as we have the socket
available.

Performances under flood with multiple NIC rx queues used are
unaffected, but when a single NIC rx queue is in use, this
gives ~10% performance improvement.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Applied, but what about ipv6?

On Wed, 2017-06-21 at 11:41 -0400, David Miller wrote:

From: Paolo Abeni <pabeni@redhat.com>
Date: Wed, 21 Jun 2017 10:24:40 +0200

quoted

On UDP packets processing, if the BH is the bottle-neck, it
always sees a cache miss while updating rmem_alloc; try to
avoid it prefetching the value as soon as we have the socket
available.

Performances under flood with multiple NIC rx queues used are
unaffected, but when a single NIC rx queue is in use, this
gives ~10% performance improvement.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Applied, but what about ipv6?

You are right, that was unintentionally left out. I'll do some tests
and send another patch for ipv6, too.

Thanks,

Paolo