Re: [PATCH v5 00/10] track CPU utilization

[luca abeni]

Hi,

On Wed, 6 Jun 2018 14:20:46 +0100
Quentin Perret <quentin.perret@xxxxxxx> wrote:
[...]
> > However, IMHO, these are corner cases and in the average case it is
> > better to rely on running_bw and reduce the CPU frequency
> > accordingly.  
> 
> My point was that accepting to go at a lower frequency than required
> by this_bw is fundamentally unsafe. If you're at a low frequency when
> a DL task starts, there are real situations where you won't be able
> to increase the frequency immediately, which can eventually lead to
> missing deadlines. Now, if this risk is known, has been discussed,
> and is accepted, that's fair enough. I'm just too late for the
> discussion :-)

Well, our conclusion was that this issue can be addressed when
designing the scheduling parameters:
- If we do not consider frequency scaling, a task can respect its
  deadlines if the SCHED_DEADLINE runtime is larger than the task's
  execution time and the SCHED_DEADLINE period is smaller than the
  task's period (and if some kind of "global" admission test is
  respected)
- Considering frequency scaling (and 0-time frequency switches), the
  SCHED_DEADLINE runtime must be larger than the task execution time at
  the highest frequency
- If the frequency switch time is larger than 0, then the
  SCHED_DEADLINE runtime must be larger than the task execution time
  (at the highest frequency) plus the frequency switch time

If this third condition is respected, I think that deadline misses can
be avoided even if running_bw is used (and the CPU takes a considerable
time to switch frequency). Of course, this requires an over-allocation
of runtime (and the global admission test has more probabilities to
fail)...



			Luca