Re: [PATCH 2/2] sched/cpufreq: Update schedutil's DVFS request to reach the boost frequencies

[Vincent Guittot]

On Thu, 6 Aug 2026 at 11:02, Christian Loehle <christian.loehle@xxxxxxx> wrote:
>
> On 8/6/26 05:42, Sibi Sankar wrote:
> > capacity_freq_ref, exposed to schedutil via get_capacity_ref_freq(),
> > was introduced by commit 9942cb22ea45 ("sched/topology: Add a new
> > arch_scale_freq_ref() method") as a fixed anchor that does not move at
> > runtime. However, schedutil uses that same fixed anchor as the reference
> > plugged into map_util_freq() which saturates exactly at capacity_freq_ref.
> >
> > As a result, a system with cpufreq boost enabled effectively never runs at
> > boost frequencies under schedutil-governed load. Fix this by plugging in
> > policy-max into the map_util_freq equation, so that the DVFS requests
> > translates to the actual cpufreq driver ceiling.
> >
> > Signed-off-by: Sibi Sankar <sibi.sankar@xxxxxxxxxxxxxxxx>
> > ---
> >  kernel/sched/cpufreq_schedutil.c | 13 ++++++++++---
> >  1 file changed, 10 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index a1782755efcc..dcefbeaa0702 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -195,10 +195,17 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
> >                                 unsigned long util, unsigned long max)
> >  {
> >       struct cpufreq_policy *policy = sg_policy->policy;
> > -     unsigned int freq;
> > +     unsigned int freq, ref;
> >
> > -     freq = get_capacity_ref_freq(policy);
> > -     freq = map_util_freq(util, freq, max);
> > +     ref = get_capacity_ref_freq(policy);
> > +
> > +     /*
> > +      * That fixed anchor governs how utilization is interpreted, but
> > +      * the DVFS request is free to target the current policy ceiling.
> > +      * Using ref alone would saturate the util->freq map at ref so
> > +      * use policy->max to reach boost frequencies.
>
> I'm not sure those two statements are compatible with the implementation below?
> util / max is expressed in the capacity scale established using ref, multiplying
> that ratio by policy->max changes the interpretation of every util value.
>
>
> > +      */
> > +     freq = map_util_freq(util, max(ref, READ_ONCE(policy->max)), max);
>
> Isn't the underlying problem that schedutil can't handle requests above capacity 1024?
>
> effective_cpu_util() caps util at max, and sugov_effective_cpu_perf() applies
> the 25% headroom before clipping the result back to that same value.
> Therefore the input here cannot exceed 1024, and mapping it against ref
> can never request a frequency above ref.
> Using policy->max makes boost reachable, but also stretches the complete frequency range.
> For example, with ref = 4454400, policy->max = 4723200, and effective util 640, the request
> changes from 2784000 to 2952000.
>
> Making 'boost frequencies' truly compatible with schedutil (or PELT/CAS for that matter)
> is a discussion that is yet to be had?
> See also
> https://lore.kernel.org/lkml/20250626093018.106265-1-dietmar.eggemann@xxxxxxx/

cpufreq_pressure and freq_qos_update_request could handle the case
where boost freq is used as the ref freq when setting capacity but
disabled. I can't remember if everything was already in place when we
discussed this last time

>