linux-kernel - Re: [RFC PATCH 0/1] sched/pelt: Change PELT halflife at runtime

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Message-ID: <CAKfTPtDwDdpiQnUqi_OtER5EE0EN4ykDMqtwzHi3d7AyBd_aQA@mail.gmail.com>
Date:   Thu, 2 Mar 2023 09:00:26 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Qais Yousef <qyousef@...alina.io>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Kajetan Puchalski <kajetan.puchalski@....com>,
        Jian-Min Liu <jian-min.liu@...iatek.com>,
        Ingo Molnar <mingo@...nel.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Vincent Donnefort <vdonnefort@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Abhijeet Dharmapurikar <adharmap@...cinc.com>,
        Qais Yousef <qais.yousef@....com>,
        linux-kernel@...r.kernel.org,
        Jonathan JMChen <jonathan.jmchen@...iatek.com>
Subject: Re: [RFC PATCH 0/1] sched/pelt: Change PELT halflife at runtime

On Wed, 1 Mar 2023 at 18:25, Qais Yousef <qyousef@...alina.io> wrote:
>
> On 03/01/23 11:39, Vincent Guittot wrote:
> > On Thu, 23 Feb 2023 at 16:37, Qais Yousef <qyousef@...alina.io> wrote:
> > >
> > > On 02/09/23 17:16, Vincent Guittot wrote:
> > >
> > > > I don't see how util_est_faster can help this 1ms task here ? It's
> > > > most probably never be preempted during this 1ms. For such an Android
> > > > Graphics Pipeline short task, hasn't uclamp_min been designed for and
> > > > a better solution ?
> > >
> > > uclamp_min is being used in UI and helping there. But your mileage might vary
> > > with adoption still.
> > >
> > > The major motivation behind this is to help things like gaming as the original
> > > thread started. It can help UI and other use cases too. Android framework has
> > > a lot of context on the type of workload that can help it make a decision when
> > > this helps. And OEMs can have the chance to tune and apply based on the
> > > characteristics of their device.
> > >
> > > > IIUC how util_est_faster works, it removes the waiting time when
> > > > sharing cpu time with other tasks. So as long as there is no (runnable
> > > > but not running time), the result is the same as current util_est.
> > > > util_est_faster makes a difference only when the task alternates
> > > > between runnable and running slices.
> > > > Have you considered using runnable_avg metrics in the increase of cpu
> > > > freq ? This takes into the runnable slice and not only the running
> > > > time and increase faster than util_avg when tasks compete for the same
> > > > CPU
> > >
> > > Just to understand why we're heading into this direction now.
> > >
> > > AFAIU the desired outcome to have faster rampup time (and on HMP faster up
> > > migration) which both are tied to utilization signal.
> > >
> > > Wouldn't make the util response time faster help not just for rampup, but
> > > rampdown too?
> > >
> > > If we improve util response time, couldn't this mean we can remove util_est or
> > > am I missing something?
> >
> > not sure because you still have a ramping step whereas util_est
> > directly gives you the final tager
>
> I didn't get you. tager?

target

>
> >
> > >
> > > Currently we have util response which is tweaked by util_est and then that is
> > > tweaked further by schedutil with that 25% margin when maping util to
> > > frequency.
> >
> > the 25% is not related to the ramping time but to the fact that you
> > always need some margin to cover unexpected events and estimation
> > error
>
> At the moment we have
>
>         util_avg -> util_est -> (util_est_faster) -> util_map_freq -> schedutil filter ==> current frequency selection
>
> I think we have too many transformations before deciding the current
> frequencies. Which makes it hard to tweak the system response.

What is proposed here with runnable_avg is more to take a new input
when selecting a frequency: the level of contention on the cpu. But
this is not used to modify the utilization seen by the scheduler

>
> >
> > >
> > > I think if we can allow improving general util response time by tweaking PELT
> > > HALFLIFE we can potentially remove util_est and potentially that magic 25%
> > > margin too.
> > >
> > > Why the approach of further tweaking util_est is better?
> >
> > note that in this case it doesn't really tweak util_est but Dietmar
> > has taken into account runnable_avg to increase the freq in case of
> > contention
> >
> > Also IIUC Dietmar's results, the problem seems more linked to the
> > selection of a higher freq than increasing the utilization;
> > runnable_avg tests give similar perf results than shorter half life
> > and better power consumption.
>
> Does it ramp down faster too?

I don't think so.

To be honest, I'm not convinced that modifying the half time is the
right way to solve this. If it was only a matter of half life not
being suitable for a system, the halk life would be set once at boot
and people would not ask to modify it at run time.

>
>
> Thanks
>
> --
> Qais Yousef
>
> >
> > >
> > > Recently phoronix reported that schedutil behavior is suboptimal and I wonder
> > > if the response time is contributing to that
> > >
> > >         https://www.phoronix.com/review/schedutil-quirky-2023
> > >
> > >
> > > Cheers
> > >
> > > --
> > > Qais Yousef