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

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Date:   Mon, 18 May 2020 13:56:13 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Vinicius Costa Gomes <vinicius.gomes@...el.com>
Cc:     David Miller <davem@...emloft.net>, olteanv@...il.com,
        intel-wired-lan@...ts.osuosl.org, jeffrey.t.kirsher@...el.com,
        netdev@...r.kernel.org, vladimir.oltean@....com, po.liu@....com,
        m-karicheri2@...com, Jose.Abreu@...opsys.com
Subject: Re: [next-queue RFC 0/4] ethtool: Add support for frame preemption

On Mon, 18 May 2020 12:05:04 -0700 Vinicius Costa Gomes wrote:
> David Miller <davem@...emloft.net> writes:
> > From: Vladimir Oltean <olteanv@...il.com>
> > Date: Sun, 17 May 2020 00:03:39 +0300
> >  
> >> As to why this doesn't go to tc but to ethtool: why would it go to tc?  
> >
> > Maybe you can't %100 duplicate the on-the-wire special format and
> > whatever, but the queueing behavior ABSOLUTELY you can emulate in
> > software.  
> 
> Just saying what Vladimir said in different words: the queueing behavior
> is already implemented in software, by mqprio or taprio, for example.
> 
> That is to say, if we add frame preemption support to those qdiscs all
> they will do is pass the information to the driver, and that's it. They
> won't be able to use that information at all.
> 
> The mental model I have for this feature is that is more similar to the
> segmentation offloads, energy efficient ethernet or auto-negotiation
> than it is to a traffic shaper like CBS.

Please take a look at the example from the cover letter:

$ ethtool $ sudo ./ethtool --show-frame-preemption
enp3s0 Frame preemption settings for enp3s0:
	support: supported
	active: active
	supported queues: 0xf
	supported queues: 0xe
	minimum fragment size: 68

Reading this I have no idea what 0xe is. I have to go and query TC API
to see what priorities and queues that will be. Which IMHO is a strong
argument that this information belongs there in the first place.