Re: [PATCH v5 10/14] sched/cpufreq: Refactor the utilization aggregation method

[Quentin Perret <quentin.perret@arm.com>]

On Wednesday 01 Aug 2018 at 10:35:32 (+0200), Rafael J. Wysocki wrote:
> On Wed, Aug 1, 2018 at 10:23 AM, Quentin Perret <quentin.perret@arm.com> wrote:
> > On Wednesday 01 Aug 2018 at 09:32:49 (+0200), Rafael J. Wysocki wrote:
> >> On Tue, Jul 31, 2018 at 9:31 PM,  <skannan@codeaurora.org> wrote:
> >> >> On Monday 30 Jul 2018 at 12:35:27 (-0700), skannan@codeaurora.org wrote:
> >> >>> If it's going to be a different aggregation from what's done for
> >> >>> frequency
> >> >>> guidance, I don't see the point of having this inside schedutil. Why not
> >> >>> keep it inside the scheduler files?
> >> >>
> >> >> This code basically results from a discussion we had with Peter on v4.
> >> >> Keeping everything centralized can make sense from a maintenance
> >> >> perspective, I think. That makes it easy to see the impact of any change
> >> >> to utilization signals for both EAS and schedutil.
> >> >
> >> > In that case, I'd argue it makes more sense to keep the code centralized in
> >> > the scheduler. The scheduler can let schedutil know about the utilization
> >> > after it aggregates them. There's no need for a cpufreq governor to know
> >> > that there are scheduling classes or how many there are. And the scheduler
> >> > can then choose to aggregate one way for task packing and another way for
> >> > frequency guidance.
> >>
> >> Also the aggregate utilization may be used by cpuidle governors in
> >> principle to decide how deep they can go with idle state selection.
> >
> > The only issue I see with this right now is that some of the things done
> > in this function are policy decisions which really belong to the governor,
> > I think.
> 
> Well, the scheduler makes policy decisions too, in quite a few places. :-)

That is true ... ;-) But not so much about frequency selection yet I guess

> The really important consideration here is whether or not there may be
> multiple governors making different policy decisions in that respect.
> If not, then where exactly the single policy decision is made doesn't
> particularly matter IMO.

I think some users of the aggregated utilization signal do want to make
slightly different decisions (I'm thinking about the RT-go-to-max thing
again which makes perfect sense in sugov, but could possibly hurt EAS).

So the "hard" part of this work is to figure out what really is a
governor-specific policy decision, and what is common between all users.
I put "hard" between quotes because I only see the case of RT as truly
sugov-specific for now.

If we also want a special case for DL, Peter's enum should work OK, and
enable to add more special cases for new users (cpuidle ?) if needed.
But maybe that is something for later ?

Thanks,
Quentin