Re: [PATCH 0/7] sched/deadline: fix cpusets bandwidth accounting

[Luca Abeni <luca.abeni@santannapisa.it>]

On Fri, 25 Aug 2017 08:02:43 +0200
luca abeni <luca.abeni@santannapisa.it> wrote:
[...]
> > The above demonstrate that even if we have two CPUsets new task belong
> > to the "default" CPUset and as such can use all the available CPUs.  
> 
> I still have a doubt (probably showing all my ignorance about
> CPUsets :)... In this situation, we have 3 CPUsets: "default",
> set1, and set2... Is everyone of these CPUsets associated to a
> root domain (so, we have 3 root domains)? Or only set1 and set2 are
> associated to a root domain?

Ok, after reading (and hopefully understanding better :) the code, I
think this question was kind of silly... There are only 2 root domains,
corresponding to set1 and set2 (right?).

[...]

> > So above we'd run the acceptance test on root
> > domain A and B before promoting the task.  Of course we'd also have to
> > add the utilisation of that task to both root domain.  Although simple
> > it goes at the core of the DL scheduler and touches pretty much every
> > aspect of it, something I'm reluctant to embark on.  
> 
> I see... So, the "default" CPUset does not have any root domain
> associated to it? If it had, we could just subtract the maximum
> utilizations of set1 and set2 to it when creating the root domains of
> set1 and set2.
...
So, this idea of mine had no sense.

I think the correct solution is what you implemented in your patchset
(if I understand it correctly).

If we want to have task spanning multiple root domains, many more
changes in the code are needed... I am wondering if it would make more
sense to track utilizations per runqueue (instead of per root domain):
- when a task tries to become SCHED_DEADLINE, we count how many CPUs are
  in its affinity mask. Let's call "n" this number
- then, we sum u / n (where "u" is the task's utilization) to the
  utilization of every runqueue that is in its affinity mask, and we
  check if all the sums are below the schedulability bound

For tasks spanning one single root domain, this should be equivalent to
the current admission test. Moreover, this check should ensure that no
root domain can be ever overloaded (even if tasks span multiple
domains).
But I do not know the locking implications for this idea... I suspect
it will not scale :(



				Luca