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Date:   Wed, 20 May 2020 15:10:42 +0000
From:   Ioana Ciornei <ioana.ciornei@....com>
To:     Jakub Kicinski <kuba@...nel.org>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH v2 net-next 0/7] dpaa2-eth: add support for Rx traffic
 classes

> Subject: Re: [PATCH v2 net-next 0/7] dpaa2-eth: add support for Rx traffic
> classes
> 
> On Tue, 19 May 2020 20:58:50 +0000 Ioana Ciornei wrote:
> > > This needs to be well
> > > integrated with the rest of the stack, but I don't think TC qdisc offload is a fit.
> > > Given we don't have qdiscs on ingress. As I said a new API for this
> > > would most likely have to be created.
> >
> > For just assigning a traffic class based on packet headers a tc filter
> > with the skbedit priority action on ingress is enough (maybe even too
> > much since there are other drivers that have the same default prioritization
> based on VLAN PCP).
> >
> > But you are correct that this would not be enough to cover all
> > possible use cases except for the most simple ones. There are
> > per-traffic class ingress policers, and those become tricky to support
> > since there's nothing that denotes the traffic class to match on, currently. I see
> 2 possible approaches, each with its own drawbacks:
> > - Allow clsact to be classful, similar to mqprio, and attach filters to its classes
> (each
> >   class would correspond to an ingress traffic class). But this would make the
> skbedit
> >   action redundant, since QoS classification with a classful clsact should be
> done
> >   completely differently now. Also, the classful clsact would have to deny
> qdiscs attached
> >   to it that don't make sense, because all of those were written with egress in
> mind.
> > - Try to linearize the ingress filter rules under the classless clsact, both the ones
> that
> >   have a skbedit action, and the ones that match on a skb priority in order to
> perform
> >   ingress policing. But this would be very brittle because the matching order
> would depend
> >   on the order in which the rules were introduced:
> >   rule 1: flower skb-priority 5 action police rate 34Mbps # note: matching on
> skb-priority doesn't exist (yet?)
> >   rule 2: flower vlan_prio 5 action skbedit priority 5
> >   In this case, traffic with VLAN priority 5 would not get rate-limited to
> 34Mbps.
> >
> > So this is one of the reasons why I preferred to defer the hard
> > questions and start with something simple (which for some reason still gets
> pushback).
> 
> You're jumping to classification while the configuration of the queues itself is
> still not defined. How does the user know how many queues there are to classify
> into?
> 
> Does this driver has descriptor rings for RX / completion? How does it decide
> which queue to pool at NAPI time?

DPAA2 has frame queues per each Rx traffic class and the decision from which queue
to pull frames from is made by the HW based on the queue priority within a channel
(there is one channel per each CPU).

If this should be modeled in software, then I assume there should be a NAPI instance
for each traffic class and the stack should know in which order to call the poll()
callbacks so that the priority is respected.

Ioana



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