Query on Rx Frame processing

Query on Rx Frame processing

[amit mehta]

I'm trying to understand Linux Network subsystem and after
reading some documents, my broad level understanding for
Rx Path is as follows:

1: Card receives the packet.
2: Assume everything is fine, the card DMAs the frame for
the driver to process it further.
3: Driver allocate an skb buffer, adjusts the headroom.
4: Fills the buffer and pass it to the Network layer.

Now in step 4 above, driver passes this packet to the
network layer by calling netif_receive_skb()  which may return
NET_RX_DROP (packet was dropped), but most
(I checked some of the network driver code) of the network
drivers do not do this check.
Hence,
Q1: Shouldn't drivers be doing that ?

Q2: Suppose the Card keeps pushing the frames at very high rate
(10 Gig adapters are already in the market and maybe in near future
we might have even faster ethernet adapters) then how the driver
and the networking stack will handle such rapid frames arrivals?
Will NAPI, interrupt coalescing be enough ?

I'm referring kernel version 2.6.31

-Amit

Query on Rx Frame processing

[michi1 at michaelblizek.twilightparadox.com]

Hi!

On 19:45 Sat 15 Oct , amit mehta wrote:
...
> Now in step 4 above, driver passes this packet to the
> network layer by calling netif_receive_skb()  which may return
> NET_RX_DROP (packet was dropped), but most
> (I checked some of the network driver code) of the network
> drivers do not do this check.
> Hence,
> Q1: Shouldn't drivers be doing that ?

The return value NET_RX_DROP seems to be used in various places where no
congestion exists:
http://lxr.linux.no/linux+v3.0.4/net/ipv4/ip_forward.c#L131
http://lxr.linux.no/linux+v3.0.4/net/ipv4/ip_input.c#L369
http://lxr.linux.no/linux+v3.0.4/net/ipv4/ip_input.c#L451

What should network drivers do with this info?

Also, there might be multiple layer 3 protocols. If one of them would signal
congestion, should the driver slow down receiving?

Maybe the question is rather why does this variable exist at all. I guess it
is just to got statistic data.

> Q2: Suppose the Card keeps pushing the frames at very high rate
> (10 Gig adapters are already in the market and maybe in near future
> we might have even faster ethernet adapters) then how the driver
> and the networking stack will handle such rapid frames arrivals?
> Will NAPI, interrupt coalescing be enough ?

It seems like net_rx_action does some throttling:
http://lxr.linux.no/linux+v3.0.4/net/core/dev.c#L3764

	-Michi
-- 
programing a layer 3+4 network protocol for mesh networks
see http://michaelblizek.twilightparadox.com