netdev - Re: dsa traffic priorization

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Message-ID: <CA+h21hrca8dhtr4CD2KaXRgAY5TVco_ijUx_+5Z=EM5DgYrTBQ@mail.gmail.com>
Date:   Thu, 19 Sep 2019 11:42:41 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Sascha Hauer <s.hauer@...gutronix.de>,
        netdev <netdev@...r.kernel.org>,
        Vivien Didelot <vivien.didelot@...oirfairelinux.com>,
        Florian Fainelli <f.fainelli@...il.com>, kernel@...gutronix.de,
        Andrew Lunn <andrew@...n.ch>
Subject: Re: dsa traffic priorization

Hi Uwe,

On Thu, 19 Sep 2019 at 11:36, Uwe Kleine-König
<u.kleine-koenig@...gutronix.de> wrote:
>
> On Thu, Sep 19, 2019 at 10:00:51AM +0200, Sascha Hauer wrote:
> > Hi Vladimir,
> >
> > On Wed, Sep 18, 2019 at 05:36:08PM +0300, Vladimir Oltean 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?
> >
> > If I understand correctly then the switch has multiple output queues per
> > port. The Ethercat traffic will go to a higher priority queue and on
> > congestion on other queues, frames designated for that queue will be
> > dropped. I just talked to our customer and he verified that their
> > Ethercat traffic still goes through even when the ports with the general
> > traffic are jammed with packets. So yes, I think this is what I want.
>
> Moreover egressing the cpu port has the advantage that network
> participants on other ports that might be able to process packet quicker
> are not limited.
>

Yes, that is true if you apply port-based policers (matchall), but not
necessarily so if your switch is able to do ingress classification (a
common key for switches being the {DMAC, VLAN} pair). Again, there is
a model for configuring even that, if the hardware is capable. Just
not for egress shaping on the CPU port.


> Best regards
> Uwe
>
> --
> Pengutronix e.K.                           | Uwe Kleine-König            |
> Industrial Linux Solutions                 | http://www.pengutronix.de/  |