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

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Message-ID: <CAE4R7bBeFg9v-CnUJU-0pe8ZS4AYXQNAGNTcvCrHdZ1ggYmOPA@mail.gmail.com>
Date:	Tue, 24 Mar 2015 22:06:03 -0700
From:	Scott Feldman <sfeldma@...il.com>
To:	Florian Fainelli <f.fainelli@...il.com>
Cc:	Guenter Roeck <linux@...ck-us.net>,
	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 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.
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