RE: [EXT] Re: [net-next] net: dsa: felix: disable always guard band bit for TAS config

[Xiaoliang Yang]

Hi Michael,

On 2021-05-06 21:25, Michael Walle <michael@xxxxxxxx> wrote:
> Am 2021-05-04 23:33, schrieb Vladimir Oltean:
> > [ trimmed the CC list, as this is most likely spam for most people ]
> >
> > On Tue, May 04, 2021 at 10:23:11PM +0200, Michael Walle wrote:
> >> Am 2021-05-04 21:17, schrieb Vladimir Oltean:
> >> > On Tue, May 04, 2021 at 09:08:00PM +0200, Michael Walle wrote:
> >> > > > > > > As explained in another mail in this thread, all queues
> >> > > > > > > are marked as scheduled. So this is actually a no-op,
> >> > > > > > > correct? It doesn't matter if it set or not set for now. Dunno why
> we even care for this bit then.
> >> > > > > >
> >> > > > > > It matters because ALWAYS_GUARD_BAND_SCH_Q reduces the
> >> > > > > > available throughput when set.
> >> > > > >
> >> > > > > Ahh, I see now. All queues are "scheduled" but the guard band
> >> > > > > only applies for "non-scheduled" -> "scheduled" transitions.
> >> > > > > So the guard band is never applied, right? Is that really
> >> > > > > what we want?
> >> > > >
> >> > > > Xiaoliang explained that yes, this is what we want. If the end
> >> > > > user wants a guard band they can explicitly add a "sched-entry
> >> > > > 00" in the tc-taprio config.
> >> > >
> >> > > You're disabling the guard band, then. I figured, but isn't that
> >> > > suprising for the user? Who else implements taprio? Do they do it
> >> > > in the same way? I mean this behavior is passed right to the
> >> > > userspace and have a direct impact on how it is configured. Of
> >> > > course a user can add it manually, but I'm not sure that is what
> >> > > we want here. At least it needs to be documented somewhere. Or
> >> > > maybe it should be a switchable option.
> >> > >
> >> > > Consider the following:
> >> > > sched-entry S 01 25000
> >> > > sched-entry S fe 175000
> >> > > basetime 0
> >> > >
> >> > > Doesn't guarantee, that queue 0 is available at the beginning of
> >> > > the cycle, in the worst case it takes up to <begin of cycle> +
> >> > > ~12.5us until the frame makes it through (given gigabit and 1518b
> >> > > frames).
> >> > >
> >> > > Btw. there are also other implementations which don't need a
> >> > > guard band (because they are store-and-forward and cound the
> >> > > remaining bytes). So yes, using a guard band and scheduling is
> >> > > degrading the performance.
> >> >
> >> > What is surprising for the user, and I mentioned this already in
> >> > another thread on this patch, is that the Felix switch overruns the
> >> > time gate (a packet taking 2 us to transmit will start transmission
> >> > even if there is only 1 us left of its time slot, delaying the
> >> > packets from the next time slot by 1 us). I guess that this is why
> >> > the ALWAYS_GUARD_BAND_SCH_Q bit exists, as a way to avoid these
> >> > overruns, but it is a bit of a poor tool for that job. Anyway,
> >> > right now we disable it and live with the overruns.
> >>
> >> We are talking about the same thing here. Why is that a poor tool?
> >
> > It is a poor tool because it revolves around the idea of "scheduled
> > queues" and "non-scheduled queues".
> >
> > Consider the following tc-taprio schedule:
> >
> >       sched-entry S 81 2000 # TC 7 and 0 open, all others closed
> >       sched-entry S 82 2000 # TC 7 and 1 open, all others closed
> >       sched-entry S 84 2000 # TC 7 and 2 open, all others closed
> >       sched-entry S 88 2000 # TC 7 and 3 open, all others closed
> >       sched-entry S 90 2000 # TC 7 and 4 open, all others closed
> >       sched-entry S a0 2000 # TC 7 and 5 open, all others closed
> >       sched-entry S c0 2000 # TC 7 and 6 open, all others closed
> >
> > Otherwise said, traffic class 7 should be able to send any time it
> > wishes.
> 
> What is the use case behind that? TC7 (with the highest priority) may always
> take precedence of the other TCs, thus what is the point of having a dedicated
> window for the others.
> 
> Anyway, I've tried it and there are no hiccups. I've meassured the delta
> between the start of successive packets and they are always ~12370ns for a
> 1518b frame. TC7 is open all the time, which makes sense. It only happens if
> you actually close the gate, eg. you have a sched-entry where a TC7 bit is not
> set. In this case, I can see a difference between ALWAYS_GUARD_BAND_SCH_Q
> set and not set. If it is set, there is up to a ~12.5us delay added (of course it
> depends on when the former frame was scheduled).
> 
> It seems that also needs to be 1->0 transition.
> 
> You've already mentioned that the switch violates the Qbv standard.
> What makes you think so? IMHO before that patch, it wasn't violated.
> Now it likely is (still have to confirm that). How can this be reasonable?
> 
> If you have a look at the initial commit message, it is about making it possible
> to have a smaller gate window, but that is not possible because of the current
> guard band of ~12.5us. It seems to be a shortcut for not having the MAXSDU
> (and thus the length of the guard band) configurable. Yes (static) guard bands
> will have a performance impact, also described in [1]. You are trading the
> correctness of the TAS for performance. And it is the sole purpose of Qbv to
> have a determisitc way (in terms of timing) of sending the frames.
> 
> And telling the user, hey, we know we violate the Qbv standard, please insert
> the guard bands yourself if you really need them is not a real solution. As
> already mentioned, (1) it is not documented anywhere, (2) can't be shared
> among other switches (unless they do the same workaround) and (3) what am
> I supposed to do for TSN compliance testing. Modifying the schedule that is
> about to be checked (and thus given by the compliance suite)?
>
I disable the always guard band bit because each gate control list needs to reserve a guard band slot, which affects performance. The user does not need to set a guard band for each queue transmission. For example, "sched-entry S 01 2000 sched-entry S fe 98000". Queue 0 is protected traffic and has smaller frames, so there is no need to reserve a guard band during the open time of queue 0. The user can add the following guard band before protected traffic: "sched-entry S 00 25000 sched-entry S 01 2000 sched-entry S fe 73000"

I checked other devices such as ENETC and it can calculate how long the frame transmission will take and determine whether to transmit before the gate is closed. The VSC9959 device does not have this hardware function. If we implement guard bands on each queue, we need to reserve a 12.5us guard band for each GCL, even if it only needs to send a small packet. This confuses customers.

actually, I'm not sure if this will violate the Qbv standard. I looked up the Qbv standard spec, and found it only introduce the guard band before protected window (Annex Q (informative)Traffic scheduling). It allows to design the schedule to accommodate the intended pattern of transmission without overrunning the next gate-close event for the traffic classes concerned.

Thanks,
Xiaoliang