sched/toplogy: Introduce the 'sched_energy_present' static key

In order to make sure Energy Aware Scheduling (EAS) will not impact
systems where no Energy Model is available, introduce a static key
guarding the access to EAS code. Since EAS is enabled on a
per-root-domain basis, the static key is enabled when at least one root
domain meets all conditions for EAS.

Signed-off-by: Quentin Perret <quentin.perret@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: adharmap@codeaurora.org
Cc: chris.redpath@arm.com
Cc: currojerez@riseup.net
Cc: dietmar.eggemann@arm.com
Cc: edubezval@gmail.com
Cc: gregkh@linuxfoundation.org
Cc: javi.merino@kernel.org
Cc: joel@joelfernandes.org
Cc: juri.lelli@redhat.com
Cc: morten.rasmussen@arm.com
Cc: patrick.bellasi@arm.com
Cc: pkondeti@codeaurora.org
Cc: rjw@rjwysocki.net
Cc: skannan@codeaurora.org
Cc: smuckle@google.com
Cc: srinivas.pandruvada@linux.intel.com
Cc: thara.gopinath@linaro.org
Cc: tkjos@google.com
Cc: valentin.schneider@arm.com
Cc: vincent.guittot@linaro.org
Cc: viresh.kumar@linaro.org
Link: https://lkml.kernel.org/r/20181203095628.11858-10-quentin.perret@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/sched.h

@@ -2296,3 +2296,7 @@ unsigned long scale_irq_capacity(unsigned long util, unsigned long irq, unsigned
 #else
 #define perf_domain_span(pd) NULL
 #endif
+
+#ifdef CONFIG_SMP
+extern struct static_key_false sched_energy_present;
+#endif

kernel/sched/topology.c

@@ -201,6 +201,7 @@ sd_parent_degenerate(struct sched_domain *sd, struct sched_domain *parent)
 	return 1;
 }
 
+DEFINE_STATIC_KEY_FALSE(sched_energy_present);
 #if defined(CONFIG_ENERGY_MODEL) && defined(CONFIG_CPU_FREQ_GOV_SCHEDUTIL)
 DEFINE_MUTEX(sched_energy_mutex);
 bool sched_energy_update;
@@ -273,6 +274,19 @@ static void destroy_perf_domain_rcu(struct rcu_head *rp)
 	free_pd(pd);
 }
 
+static void sched_energy_set(bool has_eas)
+{
+	if (!has_eas && static_branch_unlikely(&sched_energy_present)) {
+		if (sched_debug())
+			pr_info("%s: stopping EAS\n", __func__);
+		static_branch_disable_cpuslocked(&sched_energy_present);
+	} else if (has_eas && !static_branch_unlikely(&sched_energy_present)) {
+		if (sched_debug())
+			pr_info("%s: starting EAS\n", __func__);
+		static_branch_enable_cpuslocked(&sched_energy_present);
+	}
+}
+
 /*
  * EAS can be used on a root domain if it meets all the following conditions:
  *    1. an Energy Model (EM) is available;
@@ -299,7 +313,7 @@ static void destroy_perf_domain_rcu(struct rcu_head *rp)
 #define EM_MAX_COMPLEXITY 2048
 
 extern struct cpufreq_governor schedutil_gov;
-static void build_perf_domains(const struct cpumask *cpu_map)
+static bool build_perf_domains(const struct cpumask *cpu_map)
 {
 	int i, nr_pd = 0, nr_cs = 0, nr_cpus = cpumask_weight(cpu_map);
 	struct perf_domain *pd = NULL, *tmp;
@@ -365,7 +379,7 @@ static void build_perf_domains(const struct cpumask *cpu_map)
 	if (tmp)
 		call_rcu(&tmp->rcu, destroy_perf_domain_rcu);
 
-	return;
+	return !!pd;
 
 free:
 	free_pd(pd);
@@ -373,6 +387,8 @@ static void build_perf_domains(const struct cpumask *cpu_map)
 	rcu_assign_pointer(rd->pd, NULL);
 	if (tmp)
 		call_rcu(&tmp->rcu, destroy_perf_domain_rcu);
+
+	return false;
 }
 #else
 static void free_pd(struct perf_domain *pd) { }
@@ -2114,6 +2130,7 @@ static int dattrs_equal(struct sched_domain_attr *cur, int idx_cur,
 void partition_sched_domains(int ndoms_new, cpumask_var_t doms_new[],
 			     struct sched_domain_attr *dattr_new)
 {
+	bool __maybe_unused has_eas = false;
 	int i, j, n;
 	int new_topology;
 
@@ -2177,14 +2194,17 @@ void partition_sched_domains(int ndoms_new, cpumask_var_t doms_new[],
 	for (i = 0; i < ndoms_new; i++) {
 		for (j = 0; j < n && !sched_energy_update; j++) {
 			if (cpumask_equal(doms_new[i], doms_cur[j]) &&
-			    cpu_rq(cpumask_first(doms_cur[j]))->rd->pd)
+			    cpu_rq(cpumask_first(doms_cur[j]))->rd->pd) {
+				has_eas = true;
 				goto match3;
+			}
 		}
 		/* No match - add perf. domains for a new rd */
-		build_perf_domains(doms_new[i]);
+		has_eas |= build_perf_domains(doms_new[i]);
 match3:
 		;
 	}
+	sched_energy_set(has_eas);
 #endif
 
 	/* Remember the new sched domains: */