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

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Message-ID: <29959a1a-fc45-6870-fa11-311866b51aa0@ti.com>
Date:   Tue, 19 May 2020 12:34:30 -0400
From:   Murali Karicheri <m-karicheri2@...com>
To:     Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        Jakub Kicinski <kuba@...nel.org>
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>, <Jose.Abreu@...opsys.com>
Subject: Re: [next-queue RFC 0/4] ethtool: Add support for frame preemption



On 5/18/20 6:06 PM, Vinicius Costa Gomes wrote:
> Hi,
> 
> Jakub Kicinski <kuba@...nel.org> writes:
>>
>> 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.
> 
> 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.

Isn't this a pure h/w feature? FPE is implemented at L2 and involves
fragments that are only seen by h/w and never at Linux network core
unlike IP fragments and is transparent to network stack. However it
enhances priority handling at h/w to the next level by pre-empting 
existing lower priority traffic to give way to express queue traffic
and improve latency. So everything happens in h/w. So ethtool makes
perfect sense here as it is a queue configuration. I agree with Vinicius
and Vladmir to support this in ethtool instead of TC.

Murali
> 
> What do you think?
> 
> 
> Cheers,
> 

-- 
Murali Karicheri
Texas Instruments