Hi Juri,
On Wed, 15 Feb 2017 10:29:19 +0000
Juri Lelli <juri.le...@arm.com> wrote:
[...]
> > Ok, thanks; I think I can now see why this can result in a task
> > consuming more than the reserved utilisation. I still need some
> > time to convince me that "runtime / (deadline - t) > dl_runtime /
> > dl_deadline" is the correct check to use (in this case, shouldn't
> > we also change the admission test to use densities instead of
> > utilisations?)
>
> Right, this is what I was wondering as well, as dl_overflow()
> currently looks at the period. And I also have some recollection of
> this discussion happening already in the past, unfortunately it was
> not on the list.
>
> That discussion started with the following patch
[...]
> that we then dediced not to propose since (note that these are just my
> memories of the dicussion, so everything it's up for further
> discussion, also in light of the problem highlighted by Daniel)
>
> - SCHED_DEADLINE, as the documentation says, does AC using
> utilization
> - it is however true that a sufficient (but not necessary) test on
> UP for D_i != P_i cases is the one of my patch above
> - we have agreed in the past that the kernel should only check that
> we don't cause "overload" in the system (which is still the case if we
> consider utilizations), not "hard schedulability"
I remember a similar discussion; I think the decision about what to do
depends on what are the requirements: hard deadline guarantees (but in
this case global EDF is just a bad choice) or tardines no overload
guarantees?
My understanding was that the kernel guarantees that deadline tasks
will not starve non-deadline tasks, and that there is an upper bound
for the tardiness experienced by deadline tasks. If this understanding
is correct, then the current admission test is ok. But if I
misunderstood the purpose of the kernel admission test, then maybe your
patch is ok.
Then, it is important to keep the admission test consistent with the
checks performed in dl_entity_overflow() (but whatever we decide to do,
dl_entity_overflow() should be fixed).
Luca
> - also because on SMP systems "sum(WCET_i / min{D_i, P_i}) <= M"
> doesn't guarantee much more than the test base on P_i only (there
> not seem to be many/any papers around considering the D_i != P_i case
> on SMP actually)
> - basically the patch above would only matter for the UP/partitioned
> cases
>
> Thoughts?
>
> Thanks,
>
> - Juri
