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

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Date:   Wed, 11 Apr 2018 16:38:44 -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 <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,

On 04/11/2018 01:16 PM, Thomas Gleixner wrote:
>>>> Putting it all together, we end up with:
>>>>
>>>> 1) a new txtime aware qdisc, tbs, to be used per queue. Its cli will look like:
>>>> $ tc qdisc add (...) tbs clockid CLOCK_REALTIME delta 150000 offload sorting
>>>
>>> Why CLOCK_REALTIME? The only interesting time in a TSN network is
>>> CLOCK_TAI, really.
>>
>> REALTIME was just an example here to show that the qdisc has to be configured
>> with a clockid parameter. Are you suggesting that instead both of the new qdiscs
>> (i.e. tbs and taprio) should always be using CLOCK_TAI implicitly?
> 
> I think so. It's _the_ network time on which everything is based on.

Yes, but more on this below.

> 
>>>> 2) a new cmsg-interface for setting a per-packet timestamp that will be used
>>>> either as a txtime or as deadline by tbs (and further the NIC driver for the
>>>> offlaod case): SCM_TXTIME.
>>>>
>>>> 3) a new socket option: SO_TXTIME. It will be used to enable the feature for a
>>>> socket, and will have as parameters a clockid and a txtime mode (deadline or
>>>> explicit), that defines the semantics of the timestamp set on packets using
>>>> SCM_TXTIME.
>>>>
>>>> 4) a new #define DYNAMIC_CLOCKID 15 added to include/uapi/linux/time.h .
>>>
>>> Can you remind me why we would need that?
>>
>> So there is a "clockid" that can be used for the full hw offload modes. On this
>> case, the txtimes are in reference to the NIC's PTP clock, and, as discussed, we
>> can't just use a clockid that was computed from the fd pointing to /dev/ptpX .
> 
> And the NICs PTP clock is CLOCK_TAI, so there should be no reason to have
> yet another clock, right?

Just breaking this down a bit, yes, TAI is the network time base, and the NICs
PTP clock use that because PTP is (commonly) based on TAI. After the PHCs have
been synchronized over the network (e.g. with ptp4l), my understanding is that
if applications want to use the clockid_t CLOCK_TAI as a network clock reference
it's required that something (i.e. phc2sys) is synchronizing the PHCs and the
system clock, and also that something calls adjtime to apply the TAI vs UTC
offset to CLOCK_TAI.

If we are fine with those 'dependencies', then I agree there is no need for
another clock.

I was thinking about the full offload use-cases, thus when no scheduling is
happening inside the qdiscs. Applications could just read the time from the PHC
clocks directly without having to rely on any of the above. On this case,
userspace would use DYNAMIC_CLOCK just to flag that this is the case, but I must
admit it's not clear to me how common of a use-case that is, or even if it makes
sense.

Thanks,
Jesus

> 
> Thanks,
> 
> 	tglx
>