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

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Date:	Wed, 25 Mar 2015 10:01:32 -0700
From:	roopa <roopa@...ulusnetworks.com>
To:	Scott Feldman <sfeldma@...il.com>
CC:	Florian Fainelli <f.fainelli@...il.com>,
	Guenter Roeck <linux@...ck-us.net>,
	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 3/24/15, 10:06 PM, Scott Feldman wrote:
> On Tue, Mar 24, 2015 at 8:46 PM, Florian Fainelli <f.fainelli@...il.com> wrote:
>> 2015-03-24 11:14 GMT-07:00 Scott Feldman <sfeldma@...il.com>:
>>> 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.
>> Ok, that works for a model where you are only processing exception
>> traffic, but this may not always be the case, there are things you
>> don't/can't offload on smaller devices, such that you still want the
>> overhead to be a lightweight as possible.
> Ya, that makes sense.  Well, I'll concede this solution.  It has
> helped to draw out the requirements, so that's a positive.
indeed, thanks scott.
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