netdev - Re: [Intel-wired-lan] [next-queue RFC 0/4] ethtool: Add support for frame preemption

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Date:   Wed, 20 May 2020 14:42:25 -0700
From:   Andre Guedes <andre.guedes@...el.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Vinicius Costa Gomes <vinicius.gomes@...el.com>
Cc:     Jose.Abreu@...opsys.com, vladimir.oltean@....com, po.liu@....com,
        m-karicheri2@...com, intel-wired-lan@...ts.osuosl.org,
        netdev@...r.kernel.org, olteanv@...il.com,
        David Miller <davem@...emloft.net>
Subject: Re: [Intel-wired-lan] [next-queue RFC 0/4] ethtool: Add support for frame preemption

Hi,

Quoting Jakub Kicinski (2020-05-18 16:09:06)
> On Mon, 18 May 2020 16:05:08 -0700 Vinicius Costa Gomes wrote:
> > Jakub Kicinski <kuba@...nel.org> writes:
> > >> That was the (only?) strong argument in favor of having frame preemption
> > >> in the TC side when this was last discussed.
> > >> 
> > >> We can have a hybrid solution, we can move the express/preemptible per
> > >> queue map to mqprio/taprio/whatever. And have the more specific
> > >> configuration knobs, minimum fragment size, etc, in ethtool.
> > >> 
> > >> What do you think?  
> > >
> > > Does the standard specify minimum fragment size as a global MAC setting?  
> > 
> > Yes, it's a per-MAC setting, not per-queue. 
> 
> If standard defines it as per-MAC and we can reasonably expect vendors
> won't try to "add value" and make it per queue (unlikely here AFAIU),
> then for this part ethtool configuration seems okay to me.

Before we move forward with this hybrid approach, let's recap a few points that
we discussed in the previous thread and make sure it addresses them properly.

1) Frame Preemption (FP) can be enabled without EST, as described in IEEE
802.1Q. In this case, the user has to create a dummy EST schedule in taprio
just to be able to enable FP, which doesn't look natural.

2) Mpqrio already looks overloaded. Besides mapping traffic classes into
hardware queues, it also supports different modes and traffic shaping. Do we
want to add yet another setting to it?

Regards,

Andre