cpufreq: Make cpufreq_update_pressure() fall back to cpuinfo.max_freq

If arch_scale_freq_ref() is not defined for a given arch (like x86, for
example), cpufreq_update_pressure() will always set cpufreq_pressure to
zero for all CPUs in the system, which is generally problematic on
systems with asymmetric capacity [1].

However, in the absence of arch_scale_freq_ref(), it is reasonable
to assume that cpuinfo.max_freq is the maximum sustainable frequency
for the given cpufreq policy.  Moreover, there are cases in which
arch_scale_freq_ref() would need to be defined to return essentially
the cpuinfo.max_freq value anyway (for example, intel_pstate on
hybrid platforms).

For the above reasons, update cpufreq_update_pressure() to fall back to
using cpuinfo.max_freq as the reference frequency if zero is returned by
arch_scale_freq_ref().

Fixes: 75d659317b ("cpufreq: Add a cpufreq pressure feedback for the scheduler")
Link: https://lore.kernel.org/lkml/CAKfTPtBuRLfYNnR4w--cFZYZy-R8gaPEgVwCcaMmbCcJ2H-muQ@mail.gmail.com/ [1]
Signed-off-by: Rafael J. Wysocki <[email protected]>
Acked-by: Viresh Kumar <[email protected]>
Reviewed-by: Zhongqiu Han <[email protected]>
Tested-by: Ricardo Neri <[email protected]> # cluster scheduling
Acked-by: Vincent Guittot <[email protected]>
Link: https://patch.msgid.link/[email protected]

drivers/cpufreq/cpufreq.c

@@ -2586,6 +2586,9 @@ static void cpufreq_update_pressure(struct cpufreq_policy *policy)
 
 	cpu = cpumask_first(policy->related_cpus);
 	max_freq = arch_scale_freq_ref(cpu);
+	if (!max_freq)
+		max_freq = policy->cpuinfo.max_freq;
+
 	capped_freq = policy->max;
 
 	/*