Re: [PATCH v3 3/4] Documentation/scheduler/sched-deadline.txt: improve and clarify AC bits

[Luca Abeni]

Hi,

On 09/02/2014 11:45 PM, Henrik Austad wrote:
[...]
> > + On multiprocessor systems with global EDF scheduling (non partitioned
> > + systems), a sufficient test for schedulability can not be based on the
> > + utilisations (it can be shown that task sets with utilisations slightly
> > + larger than 1 can miss deadlines regardless of the number of CPUs M).
> > + However, as previously stated, enforcing that the total utilisation is smaller
> > + than M is enough to guarantee that non real-time tasks are not starved and
> > + that the tardiness of real-time tasks has an upper bound.
>
> I'd _really_ appreciate a link to a paper where all of this is presented
> and proved!
Well, my original plan was to add the bibliography in the next round of patches...
Is this ok?

[...]
> > + As already stated in Section 3, a necessary condition to be respected to
> > + correctly schedule a set of real-time tasks is that the total utilisation
> > + is smaller than M. When talking about -deadline tasks, this requires to
> > + impose that the sum of the ratio between runtime and period for all tasks
> > + is smaller than M.
>
> "This requires to impose that .." uhm, what? Drop 'to impose'.
Ok. I'll send an updated patch to Juri in few days


> > [...] Notice that the ratio runtime/period is equivalent to
> > + the utilisation of a "traditional" real-time task, and is also often
> > + referred to as "bandwidth".
> > + The interface used to control the CPU bandwidth that can be allocated
> > + to -deadline tasks is similar to the one already used for -rt
> >    tasks with real-time group scheduling (a.k.a. RT-throttling - see
> >    Documentation/scheduler/sched-rt-group.txt), and is based on readable/
> >    writable control files located in procfs (for system wide settings).
> > @@ -232,8 +285,16 @@ CONTENTS
> >    950000. With rt_period equal to 1000000, by default, it means that -deadline
> >    tasks can use at most 95%, multiplied by the number of CPUs that compose the
> >    root_domain, for each root_domain.
> > -
> > - A -deadline task cannot fork.
> > + This means that non -deadline tasks will receive at least 5% of the CPU time,
> > + and that -deadline tasks will receive their runtime with a guaranteed
> > + worst-case delay respect to the "deadline" parameter. If "deadline" = "period"
> > + and the cpuset mechanism is used to implement partitioned scheduling (see
> > + Section 5), then this simple setting of the bandwidth management is able to
> > + deterministically guarantee that -deadline tasks will receive their runtime
> > + in a period.
>
> The whole 950000 / 1000000, is at least 50 *consecutive* ms given to non
> rt/dl tasks every second, or is this more finegrained now?
>
> If the 50ms can be given in a single go, then I don't think you can
> guarantee that deadline-tasks will receive their runtime in a period - a
> period can be <50ms, no?
Uhmm... Maybe there is something I am missing in how the SCHED_DEADLINE admission
control is implemented, but I do not know about any "50 consecutive ms to non dl
tasks" rule. I agree that if there is such a rule then deadline tasks are screwed.
Juri?


> > + Finally, notice that in order not to jeopardize this admission control a
> > + -deadline task cannot fork.
>
> s/this/the
> (there aren't any other admission controls in the kernel)
Ok; this will go in my updated patch



			Thanks,
				Luca
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