Re: [RFC v3 0/5] Add capacity capping support to the CPU controller

[Patrick Bellasi]

On 15-Mar 12:41, Rafael J. Wysocki wrote:
> On Tuesday, February 28, 2017 02:38:37 PM Patrick Bellasi wrote:
> > Was: SchedTune: central, scheduler-driven, power-perfomance control
> > 
> > This series presents a possible alternative design for what has been presented
> > in the past as SchedTune. This redesign has been defined to address the main
> > concerns and comments collected in the LKML discussion [1] as well at the last
> > LPC [2].
> > The aim of this posting is to present a working prototype which implements
> > what has been discussed [2] with people like PeterZ, PaulT and TejunH.
> > 
> > The main differences with respect to the previous proposal [1] are:
> >  1. Task boosting/capping is now implemented as an extension on top of
> >     the existing CGroup CPU controller.
> >  2. The previous boosting strategy, based on the inflation of the CPU's
> >     utilization, has been now replaced by a more simple yet effective set
> >     of capacity constraints.
> > 
> > The proposed approach allows to constrain the minimum and maximum capacity
> > of a CPU depending on the set of tasks currently RUNNABLE on that CPU.
> > The set of active constraints are tracked by the core scheduler, thus they
> > apply across all the scheduling classes. The value of the constraints are
> > used to clamp the CPU utilization when the schedutil CPUFreq's governor
> > selects a frequency for that CPU.
> > 
> > This means that the new proposed approach allows to extend the concept of
> > tasks classification to frequencies selection, thus allowing informed
> > run-times (e.g. Android, ChromeOS, etc.) to efficiently implement different
> > optimization policies such as:
> >  a) Boosting of important tasks, by enforcing a minimum capacity in the
> >     CPUs where they are enqueued for execution.
> >  b) Capping of background tasks, by enforcing a maximum capacity.
> >  c) Containment of OPPs for RT tasks which cannot easily be switched to
> >     the usage of the DL class, but still don't need to run at the maximum
> >     frequency.
> 
> Do you have any practical examples of that, like for example what exactly
> Android is going to use this for?

In general, every "informed run-time" usually know quite a lot about
tasks requirements and how they impact the user experience.

In Android for example tasks are classified depending on their _current_
role. We can distinguish for example between:

- TOP_APP:    which are tasks currently affecting the UI, i.e. part of
              the app currently in foreground
- BACKGROUND: which are tasks not directly impacting the user
              experience

Given these information it could make sense to adopt different
service/optimization policy for different tasks.
For example, we can be interested in
giving maximum responsiveness to TOP_APP tasks while we still want to
be able to save as much energy as possible for the BACKGROUND tasks.

That's where the proposal in this series (partially) comes on hand.

What we propose is a "standard" interface to collect sensible
information from "informed run-times" which can be used to:

a) classify tasks according to the main optimization goals:
   performance boosting vs energy saving

b) support a more dynamic tuning of kernel side behaviors, mainly
   OPPs selection and tasks placement

Regarding this last point, this series specifically represents a
proposal for the integration with schedutil. The main usages we are
looking for in Android are:

a) Boosting the OPP selected for certain critical tasks, with the goal
   to speed-up their completion regardless of (potential) energy impacts.
   A kind-of "race-to-idle" policy for certain tasks.

b) Capping the OPP selection for certain non critical tasks, which is
   a major concerns especially for RT tasks in mobile context, but
   it also apply to FAIR tasks representing background activities.

> I gather that there is some experience with the current EAS implementation
> there, so I wonder how this work is related to that.

You right. We started developing a task boosting strategy a couple of
years ago. The first implementation we did is what is currently in use
by the EAS version in used on Pixel smartphones.

Since the beginning our attitude has always been "mainline first".
However, we found it extremely valuable to proof both interface's
design and feature's benefits on real devices. That's why we keep
backporting these bits on different Android kernels.

Google, which primary representatives are in CC, is also quite focused
on using mainline solutions for their current and future solutions.
That's why, after the release of the Pixel devices end of last year,
we refreshed and posted the proposal on LKML [1] and collected a first
run of valuable feedbacks at LCP [2].

This posting is an expression of the feedbacks collected so far and
the main goal for us are:
1) validate once more the soundness of a scheduler-driven run-time
   power-performance control which is based on information collected
   from informed run-time
2) get an agreement on whether the current interface can be considered
   sufficiently "mainline friendly" to have a chance to get merged
3) rework/refactor what is required if point 2 is not (yet) satisfied

It's worth to notice that these bits are completely independent from
EAS. OPP biasing (i.e. capping/boosting) is a feature which stand by
itself and it can be quite useful in many different scenarios where
EAS is not used at all. A simple example is making schedutil to behave
concurrently like the powersave governor for certain tasks and the
performance governor for other tasks.

As a final remark, this series is going to be a discussion topic in
the upcoming OSPM summit [3]. It would be nice if we can get there
with a sufficient knowledge of the main goals and the current status.
However, please let's keep discussing here about all the possible
concerns which can be raised about this proposal.

> Thanks,
> Rafael

Cheers Patrick

[1] https://lkml.org/lkml/2016/10/27/503
[2] https://lkml.org/lkml/2016/11/25/342
[3] http://retis.sssup.it/ospm-summit/

-- 
#include <best/regards.h>

Patrick Bellasi