Re: SCHED_DEADLINE with CPU affinity

[stanner]

Am 13.01.2020 10:22 schrieb Juri Lelli:
> Hi,
>
> Sorry for the delay in repling (Xmas + catching-up w/ emails).

No worries

> > I fear I have not understood quite well yet why this
> > "workaround" leads to (presumably) the same results as set_affinity
> > would. From what I have read, I understand it as follows: For
> > sched_dead, admission control tries to guarantee that the requested
> > policy can be executed. To do so, it analyzes the current workload
> > situation, taking especially the number of cores into account.
> >
> > Now, with a pre-configured set, the kernel knows which tasks will run
> > on which core, therefore it's able to judge wether a process can be
> > deadline scheduled or not. But when using the default way, you could
> > start your processes as SCHED_OTHER, set SCHED_DEADLINE as policy and
> > later many of them could suddenly call set_affinity, desiring to run 
> > on
> > the same core, therefore provoking collisions.
>
> But setting affinity would still have to pass admission control, and
> should fail in the case you are describing (IIUC).
>
> https://elixir.bootlin.com/linux/latest/source/kernel/sched/core.c#L5433

Well, no, that's not what I meant.
I understand that the kernel currently rejects the combination of 
set_affinity and
sched_setattr.
My question, basically is: Why does it work with exclusive cpu-sets?

As I wrote above, I assume that the difference is that the kernel knows 
which
programs will run on which core beforehand and therefore can check the
rules of admission control, whereas without exclusive cpu_sets it could 
happen
any time that certain (other) deadline applications decide to switch 
cores manually,
causing collisions with a deadline task already running on this core.

You originally wrote that this solution is "currently" required; that's 
why assume that
in theory the admission control check could also be done dynamically 
when
sched_setattr or set_affinity are called (after each other, without 
exclusive cpu sets).

Have I been clear enough now? Basically I want to know why 
cpusets+sched_deadline
works whereas set_affinity+sched_deadline is rejected, although both 
seem to lead
to the same result.

P.