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

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Date:	Tue, 24 Mar 2015 20:46:23 -0700
From:	Florian Fainelli <f.fainelli@...il.com>
To:	Scott Feldman <sfeldma@...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

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.
--
Florian
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