[PATCH 2/3] sched/topology: condition EAS enablement on FIE support

[Ionela Voinescu]

In order to make accurate predictions across CPUs and for all performance
states, Energy Aware Scheduling (EAS) needs frequency-invariant load
tracking signals.

EAS task placement aims to minimize energy consumption, and does so in
part by limiting the search space to only CPUs with the highest spare
capacity (CPU capacity - CPU utilization) in their performance domain.
Those candidates are the placement choices that will keep frequency at
its lowest possible and therefore save the most energy.

But without frequency invariance, a CPU's utilization is relative to the
CPU's current performance level, and not relative to its maximum
performance level, which determines its capacity. As a result, it will
fail to correctly indicate any potential spare capacity obtained by an
increase in a CPU's performance level. Therefore, a non-invariant
utilization signal would render the EAS task placement logic invalid.

Now that we properly report support for the Frequency Invariance Engine
(FIE) through arch_scale_freq_invariant() for arm and arm64 systems, we
can assert it is the case when initializing EAS. Warn and bail out
otherwise.

Signed-off-by: Ionela Voinescu <ionela.voinescu@xxxxxxx>
Suggested-by: Quentin Perret <qperret@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/topology.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 4073f693e2b5..348d563c2210 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -328,6 +328,7 @@ static void sched_energy_set(bool has_eas)
  *    3. no SMT is detected.
  *    4. the EM complexity is low enough to keep scheduling overheads low;
  *    5. schedutil is driving the frequency of all CPUs of the rd;
+ *    6. frequency invariance support is present;
  *
  * The complexity of the Energy Model is defined as:
  *
@@ -376,6 +377,12 @@ static bool build_perf_domains(const struct cpumask *cpu_map)
 		goto free;
 	}
 
+	if (!arch_scale_freq_invariant()) {
+		pr_warn("rd %*pbl: Disabling EAS: frequency-invariant load tracking not supported",
+			cpumask_pr_args(cpu_map));
+		goto free;
+	}
+
 	for_each_cpu(i, cpu_map) {
 		/* Skip already covered CPUs. */
 		if (find_pd(pd, i))
-- 
2.17.1