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

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Date:   Thu, 19 Apr 2018 12:03:39 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jesus Sanchez-Palencia <jesus.sanchez-palencia@...el.com>
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

On Wed, 11 Apr 2018, Jesus Sanchez-Palencia wrote:
> On 04/11/2018 01:16 PM, Thomas Gleixner wrote:
> >> 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.

I don't think it makes a lot of sense because the only use case for that is
a full user space scheduler which routes _ALL_ traffic. I don't think
that's something which we want to proliferate.

So I'd rather start off with the CLOCK_TAI assumption and if the need
really arises we can discuss that separately. So you can take a clockid
into account when designing the ABI, but have it CLOCK_TAI only for the
start.

Thanks,

	tglx