cpufreq/schedutil: Only bind threads if needed

Remove the unconditional binding of sugov kthreads to the affected CPUs
if the cpufreq driver indicates that updates can happen from any CPU.
This allows userspace to set affinities to either save power (waking up
bigger CPUs on HMP can be expensive) or increasing performance (by
letting the utilized CPUs run without preemption of the sugov kthread).

Signed-off-by: Christian Loehle <[email protected]>
Acked-by: Viresh Kumar <[email protected]>
Acked-by: Vincent Guittot <[email protected]>
Acked-by: Rafael J. Wysocki <[email protected]>
Acked-by: Juri Lelli <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Rafael J. Wysocki <[email protected]>

Author: cloehle

kernel/sched/cpufreq_schedutil.c

@@ -666,7 +666,11 @@ static int sugov_kthread_create(struct sugov_policy *sg_policy)
 	}
 
 	sg_policy->thread = thread;
-	kthread_bind_mask(thread, policy->related_cpus);
+	if (policy->dvfs_possible_from_any_cpu)
+		set_cpus_allowed_ptr(thread, policy->related_cpus);
+	else
+		kthread_bind_mask(thread, policy->related_cpus);
+
 	init_irq_work(&sg_policy->irq_work, sugov_irq_work);
 	mutex_init(&sg_policy->work_lock);
 

kernel/sched/syscalls.c

@@ -1129,6 +1129,13 @@ int dl_task_check_affinity(struct task_struct *p, const struct cpumask *mask)
 	if (!task_has_dl_policy(p) || !dl_bandwidth_enabled())
 		return 0;
 
+	/*
+	 * The special/sugov task isn't part of regular bandwidth/admission
+	 * control so let userspace change affinities.
+	 */
+	if (dl_entity_is_special(&p->dl))
+		return 0;
+
 	/*
 	 * Since bandwidth control happens on root_domain basis,
 	 * if admission test is enabled, we only admit -deadline