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SubjectRe: [RFC v3 0/5] Add capacity capping support to the CPU controller
Hi Rafael,

On Wed, Mar 15, 2017 at 6:04 PM, Rafael J. Wysocki <rafael@kernel.org> wrote:
> On Wed, Mar 15, 2017 at 1:59 PM, Patrick Bellasi
>>> 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.
>
> A question: Does "responsiveness" translate directly to "capacity" somehow?
>
> Moreover, how exactly is "responsiveness" defined?

Responsiveness is basically how quickly the UI is responding to user
interaction after doing its computation, application-logic and
rendering. Android apps have 2 important threads, the main thread (or
UI thread) which does all the work and computation for the app, and a
Render thread which does the rendering and submission of frames to
display pipeline for further composition and display.

We wish to bias towards performance than energy for this work since
this front facing to the user and we don't care about much about
energy for these tasks at this point, what's most critical is
completion as quickly as possible so the user experience doesn't
suffer from a performance issue that is noticeable.

One metric to define this is "Jank" where we drop frames and aren't
able to render on time. One of the reasons this can happen because the
main thread (UI thread) took longer than expected for some
computation. Whatever the interface - we'd just like to bias the
scheduling and frequency guidance to be more concerned with
performance and less with energy. And use this information for both
frequency selection and task placement. 'What we need' is also app
dependent since every app has its own main thread and is free to
compute whatever it needs. So Android can't estimate this - but we do
know that this app is user facing so in broad terms the interface is
used to say please don't sacrifice performance for these top-apps -
without accurately defining what these performance needs really are
because we don't know it.
For YouTube app for example, the complexity of the video decoding and
the frame rate are very variable depending on the encoding scheme and
the video being played. The flushing of the frames through the display
pipeline is also variable (frame rate depends on the video being
decoded), so this work is variable and we can't say for sure in
definitive terms how much capacity we need.

What we can do is with Patrick's work, we can take the worst case
based on measurements and specify say we need atleast this much
capacity regardless of what load-tracking thinks we need and then we
can scale frequency accordingly. This is the usecase for the minimum
capacity in his clamping patch. This is still not perfect in terms of
defining something accurately because - we don't even know how much we
need, but atleast in broad terms we have some way of telling the
governor to maintain atleast X capacity.

For the clamping of maximum capacity, there are usecases like
background tasks like Patrick said, but also usecases where we don't
want to run at max frequency even though load-tracking thinks that we
need to. For example, there are case where for foreground camera
tasks, where we want to provide sustainable performance without
entering thermal throttling, so the capping will help there.

>> 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.
>
> It looks like this could be addressed by adding a "this task should
> race to idle" flag too.

But he said 'kind-of' race-to-idle. Racing to idle all the time for
ex. at max frequency will be wasteful of energy so although we don't
care about energy much for top-apps, we do care a bit.

>
>> 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.
>
> Well, is the information on how much CPU capacity assign to those
> tasks really there in user space? What's the source of it if so?

I believe this is just a matter of tuning and modeling for what is
needed. For ex. to prevent thermal throttling as I mentioned and also
to ensure background activities aren't running at highest frequency
and consuming excessive energy (since racing to idle at higher
frequency is more expensive energy than running slower to idle since
we run at higher voltages at higher frequency and the slow of the
perf/W curve is steeper - p = c * V^2 * F. So the V component being
higher just drains more power quadratic-ally which is of no use to
background tasks - infact in some tests, we're just as happy with
setting them at much lower frequencies than what load-tracking thinks
is needed.

>>> 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].
>
> Thanks for the info, but my question was more about how it was related
> from the technical angle. IOW, there surely is some experience
> related to how user space can deal with energy problems and I would
> expect that experience to be an important factor in designing a kernel
> interface for that user space, so I wonder if any particular needs of
> the Android user space are addressed here.
>
> I'm not intimately familiar with Android, so I guess I would like to
> be educated somewhat on that. :-)

Hope this sheds some light into the Android side of things a bit.

Regards,
Joel

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