sched/core: Warn if ENERGY_AWARE is enabled but data is missing

If the EAS energy model is missing or incomplete, i.e. sd_scs is NULL, then
sched_group_energy will return -EINVAL on the assumption that it raced with a
CPU hotplug event. In that case, energy_diff will return 0 and the energy-aware
wake path will silently fail to trigger any migrations.

This case can be triggered by disabling CONFIG_SCHED_MC on existing platforms,
so that there are no sched_groups with the SD_SHARE_CAP_STATES flag, so that
sd_scs is NULL.

Add checks so that a warning is printed if EAS is ever enabled while the
necessary data is not present.

Change-Id: Id233a510b5ad8b7fcecac0b1d789e730bbfc7c4a
Signed-off-by: Brendan Jackman <brendan.jackman@arm.com>

Author: Brendan Jackman

kernel/sched/core.c

@@ -91,6 +91,8 @@
 #include <trace/events/sched.h>
 #include "walt.h"
 
+static bool have_sched_energy_data(void);
+
 DEFINE_MUTEX(sched_domains_mutex);
 DEFINE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
 
@@ -193,6 +195,10 @@ static int sched_feat_set(char *cmp)
 				sysctl_sched_features &= ~(1UL << i);
 				sched_feat_disable(i);
 			} else {
+				if (i == __SCHED_FEAT_ENERGY_AWARE)
+					WARN(!have_sched_energy_data(),
+					     "Missing sched energy data\n");
+
 				sysctl_sched_features |= (1UL << i);
 				sched_feat_enable(i);
 			}
@@ -6649,6 +6655,19 @@ static void init_sched_groups_capacity(int cpu, struct sched_domain *sd)
 	atomic_set(&sg->sgc->nr_busy_cpus, sg->group_weight);
 }
 
+static bool have_sched_energy_data(void)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		if (!rcu_dereference(per_cpu(sd_scs, cpu)) ||
+		    !rcu_dereference(per_cpu(sd_ea, cpu)))
+			return false;
+	}
+
+	return true;
+}
+
 /*
  * Check that the per-cpu provided sd energy data is consistent for all cpus
  * within the mask.
@@ -7461,6 +7480,9 @@ static int build_sched_domains(const struct cpumask *cpu_map,
 	}
 	rcu_read_unlock();
 
+	WARN(sched_feat(ENERGY_AWARE) && !have_sched_energy_data(),
+	     "Missing data for energy aware scheduling\n");
+
 	ret = 0;
 error:
 	__free_domain_allocs(&d, alloc_state, cpu_map);