Re: Deadline scheduling (was: Re: Rearranging layout of code in the scheduler)

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2008-10-31 at 13:09 +0100, Henrik Austad wrote:

> Ah, ok, I thought introducing new syscalls was *really* frowned upon.

We prefer not to, but sometimes there just isn't any other option.
If we want to extend struct sched_param, we need 2 new syscalls.

> > Sure, but implementing an EDF class isn't really all that hard - esp if
> > you only want UP.
> > 
> > The real fun is in the PI stuff and schedulability tests on SMP.
> 
> As a start, that is the approach at least I would like to take. Once you have a 
> proven, functional EDF on a single core, you can extend that to handle several 
> cores, if you really want to.

Well, you're of course free to do so, but I don't think its a very
interesting thing to do.

> > > > > The main problem is that, especially to deal with SMP systems, we also
> > > > > need to investigate theoretical issues and find out what the best
> > > > > approach could be.
> > > 
> > > Yes, well, EDF is not optimal for SMP systems, only for single core. However, 
> > > you can do a pretty good attempt by assigning tasks to cores in a greedy 
> > > fashion (simply put the next task at the CPU with the lowest load).
> > > 
> > > As a further optimization, I guess you could do the whole sced-domain thing to 
> > > minimize the search space.
> > 
> > The problem with greedy binpacking heuristics is that your schedulablity
> > test are out the window, making the whole thing useless.
> 
> Well, not really. I mean, to be optimal, you should also consider WCET, but 
> then, that's really not interesting as IMHO that's the userspace-programmer's 
> responsibility. If the user wants to add tasks that sum up to 210% utilization, 
> it's really not much we can do anyway. You certainly wouldn't want the kernel to 
> stop accepting new jobs.
> 
> So, keep the kernel logic as simple as possible and move the job to the user. 
> By keeping the kernel logic simple - we make the job easier for the end-users. A 
> very complex EDF-scheduler will make the testing very difficult.
> 
> If, on the other hand, we *know* that the scheduler is not optimal, but that it 
> behaves in a predictable manner, the end users have a simpler task of finding 
> out why something bad happened.
> 
> Because, no matter *what* you do, and *how* you implement it, with *whatever* 
> features, there will be cases when things fall apart, and  having a simple, 
> predictable scheduler will be necessary to figure it out.

I agree that the scheduler should be simple, and even something like
PD^2 is relatively simple.

But I disagree that we should not do schedulability tests. Doing those,
and esp. enforcing tasks to their given limits increases the QoS for
others in the presence of faulty/malicious tasks.

Also, WCET is still the users responsibility.

If for each deadline task you specify a period, a deadline and a budget.
Then the WCET computation is reflected in the budget.

By enforcing the schedulability test and execution budget you raise the
quality of service, because even in the presence of a mis-behaving task
only that task will be impacted. The other tasks will still meet their
deadlines.

> > > No. You should have *either* FIFO/RR *or* EDF, not both at the same time. If 
> > > you absolutely require both, you should at least separate them on a per-core 
> > > basis. If you mix them, they need to be aware of the other in order to make 
> > > the right descision, and that is not good.
> > 
> > We _have_ to have both. Its that simple.
> 
> No, we do not. Or, at least not at the same time (see below)
> 
> > POSIX mandates we have SCHED_FIFO/RR, there is tons and tons of userspace that 
> > uses it, we cannot just replace it with a deadline scheduler.
> 
> I didn't mean to rip the whole fifo/rr out of the kernel, but adding a switch at 
> compile-time so that you could choose *either* normal, static RT *or* EDF. Then 
> we could, at least for the first few versions, have it depend on !SMP to avoid 
> the whole SMP-non-optimal-mess.

But _why_? why not leave FIFO/RR in? There is absolutely no downside to
keeping it around.

> > Thing is, you have to run hard tasks first, and scheduler weaker forms
> > in its slack time, otherwise you cannot guarantee anything.
> 
> Well, then you suddenly introduce priorities to the deadlines, and that is not 
> good. A hard task is not more important than a soft, but the effect of missing 
> the deadline is. If the schedule is infeasible, it really doesn't matter what 
> you do, as you will miss deadlines, and if you prioritize hard tasks, you will 
> end up starving firm and soft
> 
> Before you go on and tell me how wrong I am, note that I don't disagree with 
> you, I think choosing hrt before the others, is the best solution from an 
> implementation point of view.

This is, if you make the soft-deadline class aware of the hard-deadline
class's tasks and schedulability contraints, then you can keep the
soft-rt class schedulable too.

So srt is in no way less important, its just has less restrictions on
the schedule, therefore we can run it in the hrt slack/idle time.

And adding the schedulability test in the kernel avoids these starvation
issues, because you just cannot.

> > On UP - which is not interesting on a general purpose kernel that runs
> > on machines with up to 4096 CPUs.
> 
> But, and pardon my ignorance, will an EDF-scheduler be intersting for such a 
> large system? From what I've gathered, small systems are the ones that could 
> benefit from an EDF as you can analyze and predict behaviour, and then, since 
> EDF is optimal, tune the CPU-freq down and still know that things will work.
> 
> Some embedded people can probably provide a lot better input here than me, as 
> this is just a general idea I snapped up 'somewhere' (where somewhere is an 
> element of the set of all places I've been the last 6 months).

Not that large indeed, but people are interested in running RT workloads
on machines in the 32/64 scale.

And even the embedded folks are now staring quad core arm11 chips in the
face, wondering how to do things.

> > Then there are the pfair class of scheduling algorithms which can
> > theoretically yield up to 100% utilization on SMP systems.
> 
> Do you know about any pratical attempts on this, and what kind of result that 
> produced?

Fairly decent, http://www.cs.unc.edu/~anderson/papers/rtss08b.pdf

> > > Besides that, EDF is the simplest, most brain-dead scheduler you can imagine. 
> > > Basically you want to add the deadline to the tasks, put it in a sorted list 
> > > and pick the leftmost task every time untill it completes.
> > 
> > Sure, and all that is useless without schedulability tests.
> 
> Yes, but should the kernel do the schedulability test? Or should the ball be 
> passed on to userspace? To analyze the schedulability, you would need the worst 
> case execution time (WCET) of the process, and if the kernel/scheduler should 
> start trying to estimate that...
> 
> So, as a start, why not just 'ignore' WCET in the first versions, and that can 
> be added later on, if necessary.

Like said above, WCET is represented in the execution budget.

> A lot of good points, and I certainly see your side of it. However (and yes, I 
> have to argue a bit more ;)), I don't think an EDF-scheduler should contain a 
> lot of features.
> 
> If you want to use the EDF, why not give the user a list of consequenses like
> - Only a single core

There won't be a single core machine left soon ;-)

> - No other RT-scheduler, if other userspace program breaks, so be it, the user 
>   has been warned.

That's a no go, and I don't see why you would need that.

> - Best effort only

That's pretty useless imho. Best-effort and RT are a bit contradictory.

> - Provide handlers for a given set of signals that will be sent to any 
>   application missing a deadline

Yeah, the idea was to send SIGXCPU to tasks who exceed their budget (and
thus will miss their deadline).

> - no cpu-scaling
> - ... keep going, basically strip away every piece of dynamic behaviour and 
>   complex scheduling code

I'm thinking there's little useful left after all that ;-)