Re: [PATCH 1/4] cpufreq: Add a cpufreq pressure feedback for the scheduler

[Lukasz Luba]

On 12/14/23 09:40, Rafael J. Wysocki wrote:
> On Thu, Dec 14, 2023 at 10:07 AM Lukasz Luba <lukasz.luba@xxxxxxx> wrote:
> > On 12/14/23 07:57, Vincent Guittot wrote:
> > > On Thu, 14 Dec 2023 at 06:43, Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
> > > > On 12-12-23, 15:27, Vincent Guittot wrote:
> > > > > @@ -2618,6 +2663,9 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy,
> > > > >         policy->max = __resolve_freq(policy, policy->max, CPUFREQ_RELATION_H);
> > > > >         trace_cpu_frequency_limits(policy);
> > > > >
> > > > > +     cpus = policy->related_cpus;
> > > > > +     cpufreq_update_pressure(cpus, policy->max);
> > > > > +
> > > > >         policy->cached_target_freq = UINT_MAX;
> > > >
> > > > One more question, why are you doing this from cpufreq_set_policy ? If
> > > > due to cpufreq cooling or from userspace, we end up limiting the
> > > > maximum possible frequency, will this routine always get called ?
> > >
> > > Yes, any update of a FREQ_QOS_MAX ends up calling cpufreq_set_policy()
> > > to update the policy->max
> >
> > Agree, cpufreq sysfs scaling_max_freq is also important to handle
> > in this new design. Currently we don't reflect that as reduced CPU
> > capacity in the scheduler. There was discussion when I proposed to feed
> > that CPU frequency reduction into thermal_pressure [1].
> >
> > The same applies for the DTPM which is missing currently the proper
> > impact to the CPU reduced capacity in the scheduler.
> >
> > IMHO any limit set into FREQ_QOS_MAX should be visible in this
> > new design of capacity reduction signaling.
> >
> > [1] https://lore.kernel.org/lkml/20220930094821.31665-2-lukasz.luba@xxxxxxx/
>
> Actually, freq_qos_read_value(&policy->constraints, FREQ_QOS_MAX) will
> return the requisite limit.

Yes, but we need to translate that information from freq domain
into capacity domain and plumb ii into scheduler as stolen CPU capacity.
Ideally, w/o any 'smoothing' but just instant value.
That's the hope of this patch set re-design.