Daniel Lezcano a écrit :
Eric Dumazet wrote:
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 queue
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)
I understand this is interesting for the loopback when there is no
multiple instances of it and it can't be unregistered. But now with the
network namespaces, we can have multiple instances of the loopback and
it can to be unregistered. Shouldn't we still use netif_rx ?
Perhaps we can do something like:
if (dev->nd_net == &init_net)
netif_receive_skb(skb);
else
netif_rx(skb);
or
#ifdef CONFIG_NET_NS
if (dev->nd_net != &init_net)
netif_rx(skb);
else
#endif
netif_receive_skb(skb);
Or we create:
init_loopback_xmit() calling netif_receive_skb(skb);
and setup this function when creating the loopback for init_net,
otherwise we setup the usual loopback_xmit.
We are still safe for multiple network namespaces and we have the
improvement for init_net loopback.
I dont understand how my patch could degrade loopbackdev unregister logic. It
should only help it, by avoiding a queue of 'pending packets' per cpu.
When we want to unregister a network device, stack makes sure that no more
calls to dev->hard_start_xmit() can occur.
If no more loopback_xmit() calls are done on this device, it doesnt matter if
it internally uses netif_rx() or netif_receive_skb(skb)
loopback device has no queue, its really unfortunate to use the 'softirq'
internal queue.
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