RE: [Linux-hls-devel] Processes freezing up

[Luca A. <luc...@em...>]

Hi Paul,

just a quick mail (I am a little bit in hurry, right now); I'll send a
longer reply this evening.

[...] 
> I noted in one of John's HLS papers a scheduler hierarchy that looked
> like this:
> 
> ROOT---|
> |      |
> RES--JOIN
>        |
>       PS
> 
> Does this hierarchy give stronger guarantees to a RES task than the
> standard hierarchy in HLS? I assume the answer is "no" iff there are no
> rr1 tasks with priority >= 20?
Don't know about this (I'll need to re-read that paper)...

[...] 
> I'm trying to understand whether or not the standard hierarchy will
> suffice for our application, or whether we need to compose a (more)
> appropriate hierarchy. We want to guarantee our application meets its
> real time constraints (no missed deadlines). More below...
I think that the standard hierarchy should be enough... You eventually
will have to change some tasks' priorities.
If I remember well, the standard hierarchy is:

                    root
                     |
                     |
               -----rr1-----
               |     |     |
               |     |     |
              res1  rr2   ps1
             P=20   P=10  P=9  (rr2 is the default scheduler)

I assume you are not using the proportional share scheduler ps1. Anyway,
you have a lot of freedom in scheduling your tasks:
1) you can leave a task under the rr2 scheduler, changing its priority
to make it more or less important
2) you can move a task to the rr1 scheduler, giving it priority < 10, so
that it is scheduled in background respect to all the rr2 tasks
3) you can move a task to the rr1 scheduler, giving it priority 20 > P >
10, so that it is scheduled in foreground respect to all the rr2 tasks,
but in background respect to all the reserved tasks (res1)
3) you can move a task to the rr1 scheduler, giving it priority > 20, so
that it is scheduled in foreground respect to all the rr2 and res1 tasks

> I thought that all tasks were converted to HLS tasks (under rr2) when
> the scheduler is first loaded into the kernel?
Yes, this is correct: when the module is inserted all the tasks are
moved to the default scheduler (rr2), and when a task is created, it is
scheduled by the default scheduler.
But if you explicitly call sched_setscheduler() choosing the SCHED_RR,
SCHED_FIFO, or SCHED_OTHER policy, then the task returns to be a
"regular linux task", and is scheduled in background respect to all the
HLS tasks.

> When you say "background", do you mean rr2?
No, I was meaning that when a task decides to return to be a linux task
(by selecting the SCHED_RR, SCHED_FIFO, or SCHED_OTHER policy), it is
not scheduled anymore unless all the HLS tasks are idle.

> I assumed that the HLS rr2 scheduler is basically playing the role
> SCHED_OTHER did before HLS was loaded?
Yes, this is almost correct. The only problem is that if a task
explicitly selects SCHED_OTHER, it currently returns to be a non HLS
task. Setting SCHED_OTHER ---> default scheduler is a good idea, and I
will do it.

> The
> rr1 scheduler in the hierarchy is confusing me a little bit
> (apologies!).
Well, let's see if I remember well (John, please correct me...). If I am
not wrong, a root scheduler can have only a single scheduler as a child.
rr1 is the child of the root scheduler, and it is used to "schedule the
other schedulers" in a prioritized way. I hope this clarifies things a
little bit...

> > Yes... You can for example set "rt scheduler = rr1" (for the 
> > standard hierarchy), so that changing a task to SCHED_FIFO or 
> > SCHED_RR will really increase its priority.
> 
> What happens to res1 tasks in this case?
If you set task priorities < 20, it is not affected.

> Will the sched_setscheduler()
> call for a res1 task then fail if there are "demanding" SCHED_FIFO/RR
> tasks or vice versa?
No, we do not implement hierarchical guarantees. If some time-demanding
task is scheduled on rr1 with priority > 20, then the res1 tasks can
fail to get their reserved time even if sched_setscheduler() did not
fail.

				Luca
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