linux-kernel - Re: [RFC PATCH v3 2/2] cpuidle: teo: Introduce util-awareness

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Message-ID: <CAJZ5v0idJTYR4+gooLnEV=kX_dDCgEdP1mgp81AEcFCcjeNoDw@mail.gmail.com>
Date:   Mon, 28 Nov 2022 15:54:44 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Kajetan Puchalski <kajetan.puchalski@....com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>, daniel.lezcano@...aro.org,
        lukasz.luba@....com, Dietmar.Eggemann@....com, dsmythies@...us.net,
        yu.chen.surf@...il.com, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v3 2/2] cpuidle: teo: Introduce util-awareness

On Mon, Nov 28, 2022 at 3:33 PM Kajetan Puchalski
<kajetan.puchalski@....com> wrote:
>
> On Fri, Nov 25, 2022 at 07:27:13PM +0100, Rafael J. Wysocki wrote:
> > > +/*
> > > + * The number of bits to shift the cpu's capacity by in order to determine
> > > + * the utilized threshold
> > > + */
> > > +#define UTIL_THRESHOLD_SHIFT 6
> >
> > Why is this particular number regarded as the best one?
>
> Based on my testing this number achieved the best balance of power and
> performance on average. It also makes sense from looking at the util
> plots. The resulting threshold is high enough to not be triggered by
> background noise and low enough to react quickly when activity starts
> ramping up.

It would be good to put some of this information (or maybe even all of
it) into the comment above the symbol definition.

> > > +static void teo_get_util(struct cpuidle_device *dev, struct teo_cpu *cpu_data)
> > > +{
> > > +       unsigned long util = sched_cpu_util(dev->cpu);
> > > +
> > > +       cpu_data->utilized = util > cpu_data->util_threshold;
> >
> > Why exactly do you need the local variable here?
>
> It's not necessarily needed, I can replace it with comparing the result
> of the call directly.
>
> > Then, if there's only one caller, maybe this could be folded into it?
>
> I do think it's nicer to have it separated in its own helper function, that
> way if anything more has to be done with the util it'll all be
> self-contained. Having only one caller shouldn't be a big issue, it's
> also the case for teo_middle_of_bin and teo_find_shallower_state in the
> current TEO implementation.

OK

> > > +               /* don't update metrics if the cpu was utilized during the last sleep */
> >
> > Why?
> >
> > The metrics are related to idle duration and cpu_data->utilized
> > indicates whether or not latency should be reduced.  These are
> > different things.
> >
> > Moreover, this is just one data point and there may not be any direct
> > connection between it and the decision made in this particular cycle.
>
> Agreed, v4 already has this part removed.
>
> > > +               if (!cpu_data->utilized)
> > > +                       teo_update(drv, dev);
> > >                 dev->last_state_idx = -1;
> > >         }
> > >
> > > @@ -323,6 +381,21 @@ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
> > >                         goto end;
> > >         }
> > >
> > > +       teo_get_util(dev, cpu_data);
> > > +       /* the cpu is being utilized and there's only 2 states to choose from */
> > > +       /* no need to consider metrics, choose the shallowest non-polling state and exit */
> >
> > A proper kernel-coding-style 2-line comment, please!
> >
> > Also I would say "utilized beyond the threshold" instead of just
> > "utilized" and "there are only 2 states" (plural).
>
> Both good points, I'll fix that.
>
> > > +       if (drv->state_count < 3 && cpu_data->utilized) {
> > > +               for (i = 0; i < drv->state_count; ++i) {
> > > +                       if (dev->states_usage[i].disable ||
> > > +                                       drv->states[i].flags & CPUIDLE_FLAG_POLLING)
> > > +                               continue;
> > > +                       break;
> >
> > This looks odd.  It would be more straightforward to do
> >
> > if (!dev->states_usage[i].disable && !(drv->states[i].flags &
> > CPUIDLE_FLAG_POLLING)) {
> >         idx = i;
> >         goto end;
> > }
> >
> > without the "break" and "continue".
>
> Fair enough, this works as well.
>
> > I've changed my mind with respect to adding the idx == 1 check to
> > this.  If the goal is to reduce latency for the "loaded" CPUs, this
> > applies to deeper idle states too.
>
> I see, this has no effect on arm devices one way or the other so I don't
> mind, it's completely up to you. In light of Doug's test results
> regarding this change, should I remove the check in v5?

Yes, please.