Date:	Wed, 16 Sep 2015 15:49:17 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Juri Lelli <juri.lelli@....com>
Cc:	Steve Muckle <steve.muckle@...aro.org>,
	Patrick Bellasi <Patrick.Bellasi@....com>,
	Ricky Liang <jcliang@...omium.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Jonathan Corbet <corbet@....net>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [RFC 08/14] sched/tune: add detailed documentation

On 16 September 2015 at 11:26, Juri Lelli <juri.lelli@....com> wrote:
>
> Hi Steve,
>
> thanks a lot for this interesting discussion.
>
> On 16/09/15 00:55, Steve Muckle wrote:
> > On 09/15/2015 08:00 AM, Patrick Bellasi wrote:
> >>> Agreed, though I also think those tunable values might also change for a
> >>> given set of tasks in different circumstances.
> >>
> >> Could you provide an example?
> >>
> >> In my view the per-task support should be exploited just for quite
> >> specialized tasks, which are usually not subject to many different
> >> phases during their execution.
> >
> > The surfaceflinger task in Android is a possible example. It can have
> > the same issue as the graphics controller task you mentioned - needing
> > to finish quickly so the overall display pipeline can meet its deadline,
> > but often not exerting enough CPU demand by itself to raise the
> > frequency high enough.
> >
>
> SurfaceFlinger timeliness requirements, and maybe AudioFlinger's and
> others' as well, might be better expressed by using other scheduling
> classes, IMHO. SCHED_DEADLINE, for example, has built-in explicit

I fully agree on this point that we must be sure to not create knob to
solve some latency/perf/power issue in a sched class whereas it can be
easily solved with a more appropriate sched class.
Surface flinger and sched_deadline is a good example for this kind of
"critical" task that can accept a limited amount of latency

Vincent

>
> deadlines awareness and might work better with this kind of activities.
> Not to mention that Android has already started using SCHED_FIFO for
> some of its time sensitive tasks. It seems to me that the long run goal
> should be to give the scheduler more information about what is going on
> and then use such information to do more informed decisions (scheduling,
> OPP selection, etc.).
>
> > Since mobile platforms are so power sensitive though, it won't be
> > possible to boost surfaceflinger all the time. Perhaps the
> > surfaceflinger boost could be managed by some sort of userspace daemon
> > monitoring the sort of usecase running and/or whether display deadlines
> > are being missed, and updating a schedtune boost cgroup.
> >
>
> I'd say you would like to "boost" just enough to meet a certain quality
> of service in the end.
>
> >> For example, in a graphics rendering pipeline usually we have a host
> > ...
> >> With SchedTune we would like to get a similar result to the one you
> >> describe using min_sample_time and above_hispeed_delay by linking
> >> somehow the "interpretation" of the PELT signal with the boost value.
> >>
> >> Right now we have in sched-DVFS an idle % headroom which is hardcoded
> >> to be ~20% of the current OPP capacity. When we cross that boundary
> >> that threshold with the CPU usage, we switch straight to the max OPP.
> >> If we could figure out a proper mechanism to link the boost signal to
> >> both the idle % headroom and the target OPP, I think we could achieve
> >> quite similar results than what you can get with the knobs offered by
> >> the interactive governor.
> >> The more you boost a task the bigger is the idle % headroom and
> >> the higher is the OPP you will jump.
> >
> > Let's say I have a system with one task (to set aside the per-task vs.
> > global policy issue temporarily) and I want to define a policy which
> >
> >  - quickly goes to 1.2GHz when the current frequency is less than
> >    that and demand exceeds capacity
> >
> >  - waits at least 40ms (or just "a longer time") before increasing the
> >    frequency if the current frequency is 1.2GHz or higher
> >
> > This is similar to (though a simplification of) what interactive is
> > often configured to do on mobile platforms. AFAIK it's a fairly common
> > strategy due to the power-perf curves and OPPs available on CPUs, and at
> > the same time striving to maintain decent UI responsiveness.
> >
>
> Not that this is already in place, but, once we'll have an energy model
> of the platform available to the scheduler (the EAS idea), shouldn't
> this kind of considerations be possible without any explicit
> configuration? I mean, it seems to me that you start reasoning about
> trade-offs after you obtained power-perf curves for your platform; but,
> once this data will be available to the scheduler, don't you think we
> could put a bit more intelligence there to make the same kind of
> decisions you would configure a governor to do?
>
> > Even with the proposed modification to link boost with idle % and target
> > OPP I don't think there'd currently be a way to express this policy,
> > which goes beyond the linear scaling of the magnitude of CPU demand
> > requested by a task, idle headroom or target OPP.
> >
> >>
> > ...
> >>> The hardcoded values in the
> >>> task load tracking algorithm seem concerning though from a tuning
> >>> standpoint.
> >>
> >> I agree, that's why we are thinking about the solution described
> >> before. Exploit the boost value to replace the hardcoded thresholds
> >> should allow to get more flexibility while being per-task defined.
> >> Hopefully, tuning per task can be more easy and effective than
> >> selection a single value fitting all needs.
> >>
> >>>
> >>>>> The interactive functionality would require additional knobs. I
> >>> ...
> >>>> However, regarding specifically the latency on OPP changes, there are
> >>>> a couple of extension we was thinking about:
> >>>> 1. link the SchedTune boost value with the % of idle headroom which
> >>>>    triggers an OPP increase
> >>>> 2. use the SchedTune boost value to defined the high frequency to jump
> >>>>    at when a CPU crosses the % of idle headroom
> >>>
> >>> Hmmm... This may be useful (only testing/profiling would tell) though it
> >>> may be nice to be able to tune these values.
> >>
> >> Again, in my view the tuning should be per task with a single knob.
> >> The value of the knob should than be properly mapped on other internal
> >> values to obtain a well defined behavior driven by information shared
> >> with the scheduler, i.e. a PELT signal.
> >>
> >>>> These are tunables which allows to parameterize the way the PELT
> >>>> signal for CPU usage is interpreted by the sched-DVFS governor.
> >>>>
> >>>> How such tunables should be exposed and tuned is to be discussed.
> >>>> Indeed, one of the main goals of the sched-DVFS and SchedTune
> >>>> specifically, is to simplify the tuning of a platform by exposing to
> >>>> userspace a reduced number of tunables, preferably just one.
> >>>
> >>> This last point (the desire for a single tunable) is perhaps at the root
> >>> of my main concern. There are users/vendors for whom the current
> >>> tunables are insufficient, resulting in their hacking the governors to
> >>> add more tunables or features in the policy.
> >>
> >> We should also consider that we are proposing not only a single
> >> tunable but also a completely different standpoint. Not more a "blind"
> >> system-wide view on the average system behaviors, but instead a more
> >> detailed view on tasks behaviors. A single tunable used to "tag" tasks
> >> maybe it's not such a limited solution in this design.
> >
> > I think the algorithm is still fairly blind. There still has to be a
> > heuristic for future CPU usage, it's now just per-task and in the
> > scheduler (PELT), whereas it used to be per-CPU and in the governor.
> >
> > This allows for good features like adjusting frequency right away on
> > task migration/creation/exit or per task boosting etc., but I think
> > policy will still be important. Tasks change their behavior all the
> > time, at least in the mobile usecases I've seen.
> >
> >>> Consolidating CPU frequency and idle management in the scheduler will
> >>> clean things up and probably make things more effective, but I don't
> >>> think it will remove the need for a highly configurable policy.
> >>
> >> This can be verified only by starting to use sched-DVFS + SchedTune on
> >> real/synthetic setup to verify which features are eventually missing,
> >> or specific use-cases not properly managed.
> >> If we are able to setup these experiments perhaps we will be able to
> >> identify a better design for a scheduler driver solution.
> >
> > Agree. I hope to be able to run some of these experiments to help.
> >
> >>> I'm curious about the drive for one tunable. Is that something there's
> > ...
> >> We have plenty of experience, collected on the past years, on CPUFreq
> >> governors and customer specific mods.
> >> Don't you think we can exploit that experience to reason around a
> >> fresh new design that allows to satisfy all requirements while
> >> providing possibly a simpler interface?
> >
> > Sure. I'm just communicating requirements I've seen :) .
> >
>
> And that's great! :-)
>
> >> I agree with you that all the current scenarios must be supported by
> >> the new proposal. We should probably start by listing them and come
> >> out with a set of test cases that allow to verify where we are wrt
> >> the state of the art.
> >
> > Sounds like a good plan to me... Perhaps we could discuss some mobile
> > usecases next week at Linaro Connect?
> >
>
> I'm up for it!
>
> Best,
>
> - Juri
>
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