Task scheduler behavior depends on frequency invariance (FI) support and
the resulting invariant load tracking signals. For example, in order to
make accurate predictions across CPUs for all performance states, Energy
Aware Scheduling (EAS) needs frequency-invariant load tracking signals
and therefore it has a direct dependency on FI. This dependency is known,
but EAS enablement is not yet conditioned on the presence of FI during
the built of the scheduling domain hierarchy.

Before this is done, the following must be considered: while
arch_scale_freq_invariant() will see changes in FI support and could
be used to condition the use of EAS, it could return different values
during system initialisation.

For arm64, such a scenario will happen for a system that does not support
cpufreq driven FI, but does support counter-driven FI. For such a system,
arch_scale_freq_invariant() will return false if called before counter
based FI initialisation, but change its status to true after it.
If EAS becomes explicitly dependent on FI this would affect the task
scheduler behavior which builds its scheduling domain hierarchy well
before the late counter-based FI init. During that process, EAS would be
disabled due to its dependency on FI.

Two points of future early calls to arch_scale_freq_invariant() which
would determine EAS enablement are:
 - (1) drivers/base/arch_topology.c:126 <<update_topology_flags_workfn>>
		rebuild_sched_domains();
       This will happen after CPU capacity initialisation.
 - (2) kernel/sched/cpufreq_schedutil.c:917 <<rebuild_sd_workfn>>
		rebuild_sched_domains_energy();
		-->rebuild_sched_domains();
       This will happen during sched_cpufreq_governor_change() for the
       schedutil cpufreq governor.

Therefore, before enforcing the presence of FI support for the use of EAS,
ensure the following: if there is a change in FI support status after
counter init, use the existing rebuild_sched_domains_energy() function to
trigger a rebuild of the scheduling and performance domains that in turn
will determine the enablement of EAS.
Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lkml.kernel.org/r/20201027180713.7642-3-ionela.voinescu@arm.com