netdev - Re: [RFC v3 net-next 13/18] net/sched: Introduce the TBS Qdisc

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Date:   Thu, 22 Mar 2018 16:26:12 -0700
From:   Jesus Sanchez-Palencia <jesus.sanchez-palencia@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     netdev@...r.kernel.org, jhs@...atatu.com, xiyou.wangcong@...il.com,
        jiri@...nulli.us, vinicius.gomes@...el.com,
        richardcochran@...il.com, anna-maria@...utronix.de,
        henrik@...tad.us, john.stultz@...aro.org, levi.pearson@...man.com,
        edumazet@...gle.com, willemb@...gle.com, mlichvar@...hat.com
Subject: Re: [RFC v3 net-next 13/18] net/sched: Introduce the TBS Qdisc

Hi Thomas,


On 03/22/2018 03:11 PM, Thomas Gleixner wrote:

(...)

>> Having the sorting always enabled requires that a valid static clockid is passed
>> to the qdisc. For the hw offload mode, that means that the PHC and one of the
>> system clocks must be synchronized since hrtimers do not support dynamic clocks.
>> Not all systems do that or want to, and given that we do not want to perform
>> crosstimestamping between the packets' clock reference and the qdisc's one, the
>> only solution for these systems would be using the raw hw offload mode.
> 
> There are two variants of hardware offload:
> 
> 1) Full hardware offload
> 
>    That bypasses the queue completely. You just stick the thing into the
>    scatter gather buffers. Except when there is no room anymore, then you
>    have to queue, but it does not make any difference if you queue in FIFO
>    or in time order. The packets go out in time order anyway.


Illustrating your variants with the current qdisc's setup arguments.

The above is:
- sorting off
- offload on

(I call it a 'raw' fifo as a reference to the usage of qdisc_enqueue_tail() and
qdisc_dequeue_head(), basically.)


> 
> 2) Single packet hardware offload
> 
>    What you do here is to schedule a hrtimer a bit earlier than the first
>    packet tx time and when it fires stick the packet into the hardware and
>    rearm the timer for the next one.


The above is:
- sorting on
- offload on

right?


So, are you just opposing to the case where sorting off + offload off is used?
(i.e. the scheduled FIFO case)



> 
>    The whole point of TSN with hardware support is that you have:
> 
>        - Global network time
> 
>        and
> 
>        - Frequency adjustment of the system time base
> 
>     PTP is TAI based and the kernel exposes clock TAI directly through
>     hrtimers. You don't need dynamic clocks for that.
> 
>     You can even use clock MONOTONIC as it basically is just
> 
>        TAI - offset>
> If the network card uses anything else than TAI or a time stamp with a
> strict correlation to TAI for actual TX scheduling then the whole thing is
> broken to begin with.


Sure, I agree.

Thanks,
Jesus

> 
> Thanks,
> 
> 	tglx
>