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

[Vladimir Oltean]

On Thu, May 06, 2021 at 04:41:51PM +0200, Michael Walle wrote:
> Am 2021-05-06 15:50, schrieb Vladimir Oltean:
> > On Thu, May 06, 2021 at 03:25:07PM +0200, Michael Walle 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.
> > 
> > Worst case latency is obviously better for an intermittent stream (not
> > more than one packet in flight at a time) in TC7 than it is for any
> > stream in TC6-TC0. But intermittent streams in TC6-TC0 also have their
> > own worst case guarantees (assuming that 2000 ns is enough to fit one
> > TC 7 frame and one frame from the TC6-TC0 range).
> 
> Oh and I missed that, TC0-TC6 probably won't work because that gate is
> too narrow (12.5us guard band) unless of course you set MAXSDU to a
> smaller value. Which would IMHO be the correct thing to do here.

I'm not sure that this is exactly true. I know that I tested once, and
the switch is happy to send a packet as long as the time window is 33 ns
or larger (Idon't . Can't remember if the ALWAYS_GUARD_BAND_SCH_Q bit was set or
not, and I'm traveling right now so I don't have an LS1028A board handy
to re-test.