netdev - Re: dsa traffic priorization

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Date:   Wed, 18 Sep 2019 10:27:32 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Dave Taht <dave.taht@...il.com>,
        Vladimir Oltean <olteanv@...il.com>
Cc:     Sascha Hauer <s.hauer@...gutronix.de>,
        netdev <netdev@...r.kernel.org>, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...oirfairelinux.com>,
        kernel@...gutronix.de
Subject: Re: dsa traffic priorization

On 9/18/19 8:03 AM, Dave Taht wrote:
> On Wed, Sep 18, 2019 at 7:37 AM Vladimir Oltean <olteanv@...il.com> wrote:
>>
>> Hi Sascha,
>>
>> On Wed, 18 Sep 2019 at 17:03, Sascha Hauer <s.hauer@...gutronix.de> wrote:
>>>
>>> Hi All,
>>>
>>> We have a customer using a Marvell 88e6240 switch with Ethercat on one port and
>>> regular network traffic on another port. The customer wants to configure two things
>>> on the switch: First Ethercat traffic shall be priorized over other network traffic
>>> (effectively prioritizing traffic based on port). Second the ethernet controller
>>> in the CPU is not able to handle full bandwidth traffic, so the traffic to the CPU
>>> port shall be rate limited.
>>>
>>
>> You probably already know this, but egress shaping will not drop
>> frames, just let them accumulate in the egress queue until something
>> else happens (e.g. queue occupancy threshold triggers pause frames, or
>> tail dropping is enabled, etc). Is this what you want? It sounds a bit
> 
> Dropping in general is a basic attribute of the fq_codel algorithm which is
> enabled by default on many boxes. It's latency sensitive, so it responds well
> to pause frame (over) use.
> 
> Usually the cpu to switch port is exposed via vlan (e.g eth0:2), and
> while you can inbound and
> outbound shape on that - using htb/hfsc +  fq_codel, or cake

That may be true with swconfig in OpenWrt, but this is not true with DSA
unless DSA_TAG_PROTO_8021Q is used which happens to be on just one
driver at the moment. With other switches that support a proprietary
switch tag format, there is not a particular VLAN or even a network
interface that describes the CPU port, other than the DSA master network
device which is the side facing the host system (not the switch itself).

> 
> But, also, most usually what happens when the cpu cannot keep up with
> the switch is we drop packets on the rx ring for receive, and in
> fq-codel on send.

Dave, you seem to have a tendency to just pattern match on specific QoS-
related topics appearing on netdev and throwing the wonderful tool that
fq_codel without necessarily considering whether this is applicable or
not to the people raising the questions.

Since we are talking about hardware switches here and not simply
stations on a network (although the Ethernet MAC behind the CPU port
ends up being one), there is the possibility of using the HW to do
ingress and/or egress policing. The question raised by Sascha is how to
avoid statically configuring and instead using possibly existing tools
to achieve the same configuration, from user-space, that is, not encode
policy in the driver, but just the mechanism.
-- 
Florian