Re: [PATCH v4 2/3] sched/fair: Take thermal pressure into account while estimating energy

[Lukasz Luba]

On 6/16/21 6:24 PM, Dietmar Eggemann wrote:
> On 15/06/2021 18:09, Lukasz Luba wrote:
> > On 6/15/21 4:31 PM, Dietmar Eggemann wrote:
> > > On 14/06/2021 21:11, Lukasz Luba wrote:
>
> [...]
>
> > > It's important to highlight that this will only fix this issue between
> > > schedutil and EAS when it's due to `thermal pressure` (today only via
> > > CPU cooling). There are other places which could restrict policy->max
> > > via freq_qos_update_request() and EAS will be unaware of it.
> >
> > True, but for this I have some other plans.
>
> As long as people are aware of the fact that this was developed to be
> beneficial for `EAS - IPA` integration, I'm fine with this.

Good. I had in mind that I will have to do some re-work on this
thermal pressure code in the cpufreq cooling, to satisfy our roadmap
goals...

> [...]
>
> > > IMHO, this means that this is catered for the IPA governor then. I'm not
> > > sure if this would be beneficial when another thermal governor is used?
> >
> > Yes, it will be, the cpufreq_set_cur_state() is called by
> > thermal exported function:
> > thermal_cdev_update()
> >    __thermal_cdev_update()
> >      thermal_cdev_set_cur_state()
> >        cdev->ops->set_cur_state(cdev, target)
> >
> > So it can be called not only by IPA. All governors call it, because
> > that's the default mechanism.
>
> True, but I'm still not convinced that it is useful outside `EAS - IPA`.

It is. So in mainline thermal there is another governor: fair_share [1],
which uses 'weights' to split the cooling effort across cooling devices
in the thermal zone. That governor would manage CPUs and GPU and
set throttling like IPA.

> > > The mechanical side of the code would allow for such benefits, I just
> > > don't know if their CPU cooling device + thermal zone setups would cater
> > > for this?
> >
> > Yes, it's possible. Even for custom vendor governors (modified clones
> > of IPA)
>
> Let's stick to mainline here ;-) It's complicated enough ...

I agree, so there isn't only IPA in mainline.

> [...]
>
> > > Maybe shorter?
> > >
> > >           struct cpumask *pd_mask = perf_domain_span(pd);
> > > -       unsigned long cpu_cap =
> > > arch_scale_cpu_capacity(cpumask_first(pd_mask));
> > > +       int cpu = cpumask_first(pd_mask);
> > > +       unsigned long cpu_cap = arch_scale_cpu_capacity(cpu);
> > > +       unsigned long _cpu_cap = cpu_cap -
> > > arch_scale_thermal_pressure(cpu);
> > >           unsigned long max_util = 0, sum_util = 0;
> > > -       unsigned long _cpu_cap = cpu_cap;
> > > -       int cpu;
> > > -
> > > -       _cpu_cap -= arch_scale_thermal_pressure(cpumask_first(pd_mask));
> >
> > Could be, but still, the definitions should be sorted from longest on
> > top, to shortest at the bottom. I wanted to avoid modifying too many
> > lines with this simple patch.
>
> Only if there are no dependencies, but here we have already `cpu_cap ->
> pd_mask`. OK, not a big deal.

True, those dependencies are tricky to sort them properly, so I coded
it this way.

[snip]

> > I see what you mean, but this might cause some issues in the design
> > (per-cpu scmi cpu perf control). Let's use this EM pointer gently ;)
>
> OK, with the requirement that clients see the EM as ro:
>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>

Thank you Dietmar for the review!

Regards,
Lukasz

[1] 
https://elixir.bootlin.com/linux/v5.13-rc6/source/drivers/thermal/gov_fair_share.c#L111