netdev - Re: [PATCH net-next RFC v2] switchdev: bridge: drop hardware forwarded packets

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Date:	Tue, 24 Mar 2015 11:14:28 -0700
From:	Scott Feldman <sfeldma@...il.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	roopa <roopa@...ulusnetworks.com>, Jiri Pirko <jiri@...nulli.us>,
	John Fastabend <john.fastabend@...il.com>,
	Andrew Lunn <andrew@...n.ch>,
	David Miller <davem@...emloft.net>,
	"Arad, Ronen" <ronen.arad@...el.com>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next RFC v2] switchdev: bridge: drop hardware
 forwarded packets

On Tue, Mar 24, 2015 at 10:58 AM, Guenter Roeck <linux@...ck-us.net> wrote:
> On Tue, Mar 24, 2015 at 10:45:03AM -0700, roopa wrote:
>> On 3/24/15, 9:01 AM, Guenter Roeck wrote:
>> >On Tue, Mar 24, 2015 at 03:29:21PM +0100, Jiri Pirko wrote:
>> >>>diff --git a/drivers/net/ethernet/rocker/rocker.c
>> >>>b/drivers/net/ethernet/rocker/rocker.c
>> >>>index aab962c..0f7217f7 100644
>> >>>--- a/drivers/net/ethernet/rocker/rocker.c
>> >>>+++ b/drivers/net/ethernet/rocker/rocker.c
>> >>>@@ -3931,15 +3931,28 @@ unmap_frag:
>> >>>        return -EMSGSIZE;
>> >>>}
>> >>>
>> >>>+static bool rocker_port_dev_check(struct net_device *dev);
>> >>>+
>> >>>static netdev_tx_t rocker_port_xmit(struct sk_buff *skb, struct
>> >>>net_device *dev)
>> >>>{
>> >>>        struct rocker_port *rocker_port = netdev_priv(dev);
>> >>>        struct rocker *rocker = rocker_port->rocker;
>> >>>        struct rocker_desc_info *desc_info;
>> >>>        struct rocker_tlv *frags;
>> >>>+       struct net_device *in_dev;
>> >>>        int i;
>> >>>        int err;
>> >>>
>> >>>+       if (rocker_port_is_bridged(rocker_port)) {
>> >>>+               rcu_read_lock();
>> >>>+               in_dev = dev_get_by_index_rcu(dev_net(dev), skb->skb_iif);
>> >>Hmm, you iterate over all ports for every xmit call :/
>> >>Would be nicer if skb_iif would be netdev poiter. Not sure it is doable.
>> >>
>> >It may be easier (and faster) to loop through all rocker ports and try to find
>> >one with the same ifindex. Then the dev_check call would not be necessary.
>> >
>> This is still overhead for every packet on the switches we support. The
>> number of ports can go close to 128
>> (40G ports can be broken into 4x10G ports).
>>
> Agreed. Given that, and since dev_get_by_index_rcu uses a hash to find the
> device pointer, it may actually be (much) faster (and the above "iterate
> over all ports" is a bit misleading).
>
> I tested the above approach with DSA and a Marvell switch chip. It works,
> but I am a bit concerned about the per-packet overhead, especially
> in larger networks. I would prefer if there would be a means to 'catch'
> duplicate packets earlier - before they are even created, if that is
> possible.

I'm not so concerned about the per-packet overhead.  For multicast, we
have IGMP snooping.  And big switches are going to have rate controls
on CPU bound traffic, so the CPU should be able to handle the
per-packet overhead with ease.
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