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

From: Michael Walle
Date: Fri May 07 2021 - 03:35:24 EST

Hi Xiaoliang,

Am 2021-05-07 09:16, schrieb Xiaoliang Yang:
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"

Again, you're passing the handling of the guard band to the user,
which is an implementation detail for this switch (unless there is
a new switch for it on the qdisc IMHO). And (1), (2) and (3) from
above is still valid.

Consider the entry
sched-entry S 01 2000
sched-entry S 02 20000

A user assumes that TC0 is open for 2us. But with your change
it is bascially open for 2us + 12.5us. And even worse, it is not
deterministic. A frame in the subsequent queue (ie TC1) can be
scheduled anywhere beteeen 0us and 12.5us after opening the gate,
depending on wether there is still a frame transmitting on TC0.

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.

How about getting it right and working on how we can set the MAXSDU
per queue and thus making the guard band smaller?

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.

Vladimir already quoted "IEEE 802.1Q-2018 clause 8.6.8.4". I didn't
check it, though.

A static guard band is one of the options you have to fulfill that.
Granted, it is not that efficient, but it is how the switch handles
it.

-michael