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

[Christian Loehle <christian.loehle@arm.com>]

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@oss.qualcomm.com>
> ---
>  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@arm.com/