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

[Thomas Gleixner <tglx@linutronix.de>]

On Thu, 22 Mar 2018, Jesus Sanchez-Palencia wrote:
> Our plan was to work directly with the Qbv-like scheduling (per-port) just after
> the cbs qdisc (Qav), but the feedback here and offline was that there were use
> cases for a more simplistic launchtime approach (per-queue) as well. We've
> decided to invest on it first (and postpone the 'taprio' qdisc until there was
> NIC available with HW support for it, basically).

I missed that discussion due to other urgent stuff on my plate. Just
skimmed through it. More below.

> You are right, and we agree, that using tbs for a per-port schedule of any sort
> will require a SW scheduler to be developed on top of it, but we've never said
> the contrary either. Our vision has always been that these are separate
> mechanisms with different use-cases, so we do see the value for the kernel to
> provide both.
> 
> In other words, tbs is not the final solution for Qbv, and we agree that a 'TAS'
> qdisc is still necessary. And due to the wide range of applications and hw being
> used for those out there, we need both specially given that one does not block
> the other.

So what's the plan for this? Having TAS as a separate entity or TAS feeding
into the proposed 'basic' time transmission thing?

The general objection I have with the current approach is that it creates
the playground for all flavours of misdesigned user space implementations
and just replaces the home brewn and ugly user mode network adapter
drivers.

But that's not helping the cause at all. There is enough crappy stuff out
there already and I rather see a proper designed slice management which can
be utilized and improved by all involved parties.

All variants which utilize the basic time driven packet transmission are
based on periodic explicit plan scheduling with (local) network wide time
slice assignment.

It does not matter whether you feed VLAN traffic into a time slice, where
the VLAN itself does not even have to know about it, or if you have aware
applications feeding packets to a designated timeslot. The basic principle
of this is always the same.

So coming back to last years discussion. It totally went into the wrong
direction because it turned from an approach (the patches) which came from
the big picture to an single use case and application centric view. That's
just wrong and I regret that I didn't have the time to pay attention back
then.

You always need to look at the big picture first and design from there, not
the other way round. There will always be the argument:

    But my application is special and needs X

It's easy to fall for that. From a long experience I know that none of
these claims ever held. These arguments are made because the people making
them have either never looked at the big picture or are simply refusing to
do so because it would cause them work.

If you start from the use case and application centric view and ignore the
big picture then you end up in a gazillion of extra magic features over
time which could have been completely avoided if you had put your foot down
and made everyone to agree on a proper and versatile design in the first
place.

The more low level access you hand out in the beginning the less commonly
used, improved and maintained infrastrucure you will get in the end. That
has happened before in other areas and it will happen here as well. You
create a user space ABI which you cant get rid off and before you come out
with the proper interface after that a large number of involved parties
have gone off and implemented on top of the low level ABI and they will
never look back.

In the (not so) long run this will create a lot more issues than it
solves. A simple example is that you cannot run two applications which
easily could share the network in parallel without major surgery because
both require to be the management authority.

I've not yet seen a convincing argument why this low level stuff with all
of its weird flavours is superiour over something which reflects the basic
operating principle of TSN.

Thanks,

	tglx