Re: [PATCH] sched/deadline: Add sched_dl documentation

[Luca Abeni]

Hi Steven,

On Mon, 27 Jan 2014 10:35:56 -0500
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
[...]
> > + to be executed first.  Thanks to this feature, also tasks that do
> > not
> > + strictly comply with the "traditional" real-time task model (see
> > Section 3)
> > + can effectively use the new policy.
> > +
> > + In more details, the CBS algorithm assigns scheduling deadlines to
> > + tasks in the following way:
> > +
> > +  - Each SCHED_DEADLINE task is characterised by the "runtime",
> > +    "deadline", and "period" parameters;
> > +
> > +  - The state of the task is described by a "scheduling deadline",
> > and
> > +    a "current runtime". These two parameters are initially set to
> > 0; +
> > +  - When a SCHED_DEADLINE task wakes up (becomes ready for
> > execution),
> > +    the scheduler checks if
> > +
> > +                    current runtime                runtime
> > +         ---------------------------------- > ----------------
> > +         scheduling deadline - current time         period
> 
> Just a nit on formatting above. I was confused at first because the
> above looks more like a movement in time (the > being an arrow), then
> after reading the below, I realized that the above is actually an
> equation. This can be fixed by adding more spaces between the
> fractions and the greater than sign:
> 
>                  current runtime                      runtime
>       ----------------------------------    >    ----------------
>       scheduling deadline - current time               period
Ok; I was not sure about how to add an equation in the document, and I
did the first thing I could think about. I'll fix it.

> > +
> > +    then, if the scheduling deadline is smaller than the current
> > time, or
> > +    this condition is verified, the scheduling deadline and the
> > +    current budget are re-initialised as
> > +
> > +         scheduling deadline = current time + deadline
> > +         current runtime = runtime
> > +
> > +    otherwise, the scheduling deadline and the current runtime are
> > +    left unchanged;
> 
> I've been trying to wrap my head around this a bit. I started writing
> an email about this when I first examined the patches and never sent
> it out :-p
> 
> Lets take a case where deadline == period. It seems that the above
> would be true any time there was any delay to starting the task or the
> task was interrupted by another SCHED_DEADLINE task.
Not sure about this... Notice that above only applies when a task wakes
up (moving from a "sleeping" state to a "ready to run" state). Not when
an already ready task is scheduled.
Or did I misunderstand your comment?


> For example, lets say we have two SD tasks. One that has 50ms runtime
> and a 100ms period. The other has a 1ms runtime and a 10ms period.
> 
> The above two should work perfectly fine together. The 10ms period
> task will constantly schedule in on the 100ms task.
> 
> When the 100ms task runs, it could easily be delayed by 1ms due to the
> 10ms task. Then lets look at the above equation
See above: the check for the 100ms task is only performed when the task
unblocks (at the beginning of its period), not when it's scheduled
(after the 10ms taks).

This check is designed to take care of the following situation:
- consider a task with runtime 10ms and period equal to deadline equal
  to 100ms
- assume the task wakes up at time t, and is assigned "remaining
  runtime" 10ms and "scheduling deadline" t+100ms
- assume the task blocks after executing for 2ms. The "remaining
  runtime" is now 8ms
- can the task use "remaining runtime" 8ms and "scheduling deadline"
  t+100ms when it wakes up again?
  Answer: if it wakes up before t+20ms, it can. Otherwise, it cannot,
  because it would consume a fraction of CPU time larger than 10%,
  causing missed deadlines in other tasks.

[...]
> > + SCHED_DEADLINE can be used to schedule real-time tasks
> > guaranteeing that
> > + the jobs' deadlines of a task are respected. In order to do this,
> > a task
> > + must be scheduled by setting:
> > +
> > +  - runtime >= WCET
> > +  - deadline = D
> > +  - period <= P
> > +
> > + IOW, if runtime >= WCET and if period is >= P, then the
> > scheduling deadlines
> > + and the absolute deadlines (d_j) coincide, so a proper admission
> > control
> > + allows to respect the jobs' absolute deadlines for this task
> > (this is what is
> > + called "hard schedulability property" and is an extension of
> > Lemma 1 of [2]).
> 
> I wonder if we should state the obvious (which is never obvious). That
> is the user must also have the following.
> 
>   runtime < deadline <= period
> 
> Although it is fine for deadline = period, runtime should be less than
> deadline, otherwise the task will take over the system.
I think if "runtime < deadline <= period" is not respected, then the
admission control will fail... But yes, repeating it here can be
useful. If needed I'll add it to the document.


			Thanks,
				Luca
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