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Date:	Thu, 21 Feb 2008 19:51:52 +0100
From:	Eric Dumazet <>
To:	"David S. Miller" <>
Subject: [RFC,PATCH] loopback: calls netif_receive_skb() instead of netif_rx()

Hi David

This is an RFC, based on net-2.6 for convenience only.

Thank you

[RFC,PATCH] loopback: calls netif_receive_skb() instead of netif_rx()

Loopback transmit function loopback_xmit() actually calls netif_rx() to 
a skb to the softnet queue, and arms a softirq so that this skb can be 
handled later.

This has a cost on SMP, because we need to hold a reference on the 
device, and free this
reference when softirq dequeues packet.

Following patch directly calls netif_receive_skb() and avoids lot of 
atomic operations.
(atomic_inc(&dev->refcnt), set_and_set_bit(NAPI_STATE_SCHED, &n->state), ...
  atomic_dec(&dev->refcnt)...), cache line ping-pongs on device refcnt, 
but also softirq overhead.

This gives a nice boost on tbench for example (5 % on my machine)

Signed-off-by: Eric Dumazet <>

View attachment "loopback_xmit.patch" of type "text/plain" (356 bytes)

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