[PATCH 2/2] powercap: idle_inject: Create per-CPU threads on registration

@@ -10,11 +10,9 @@
  * It relies on the smpboot kthreads feature providing common code for CPU
  * hotplug and thread [un]parking.
  *
- * All of the kthreads used for idle injection are created at init time.
- *
- * Next, the users of the idle injection framework provide a cpumask via
- * its register function. The kthreads will be synchronized with respect to
- * this cpumask.
+ * Users of the idle injection framework provide a cpumask via its register
+ * function. The kthreads are created on demand for CPUs in this mask and
+ * will be synchronized with respect to it.
  *
  * The idle + run duration is specified via separate helpers and that allows
  * idle injection to be started.

@@ -40,6 +38,7 @@
 #include <linux/cpu.h>
 #include <linux/hrtimer.h>
 #include <linux/kthread.h>
+#include <linux/mutex.h>
 #include <linux/sched.h>
 #include <linux/slab.h>
 #include <linux/smpboot.h>

@@ -60,6 +59,7 @@ struct idle_inject_thread {
 /**
  * struct idle_inject_device - idle injection data
  * @timer: idle injection period timer
+ * @threads: per-device CPU hotplug thread descriptor
  * @idle_duration_us: duration of CPU idle time to inject
  * @run_duration_us: duration of CPU run time to allow
  * @latency_us: max allowed latency

@@ -82,6 +82,7 @@ struct idle_inject_thread {
  */
 struct idle_inject_device {
 	struct hrtimer timer;
+	struct smp_hotplug_thread threads;
 	unsigned int idle_duration_us;
 	unsigned int run_duration_us;
 	unsigned int latency_us;

@@ -92,6 +93,9 @@ struct idle_inject_device {
 static DEFINE_PER_CPU(struct idle_inject_thread, idle_inject_thread);
 static DEFINE_PER_CPU(struct idle_inject_device *, idle_inject_device);
 
+/* Serialize CPU ownership changes and the associated thread lifecycle. */
+static DEFINE_MUTEX(idle_inject_lock);
+
 /**
  * idle_inject_wakeup - Wake up idle injection threads
  * @ii_dev: target idle injection device

@@ -279,7 +283,8 @@ void idle_inject_stop(struct idle_inject_device *ii_dev)
 		iit = per_cpu_ptr(&idle_inject_thread, cpu);
 		iit->should_run = 0;
 
-		wait_task_inactive(iit->tsk, TASK_ANY);
+		if (iit->tsk)
+			wait_task_inactive(iit->tsk, TASK_ANY);
 	}
 
 	cpu_hotplug_enable();

@@ -319,12 +324,12 @@ static int idle_inject_should_run(unsigned int cpu)
  * idle
  *
  * This function creates an idle injection control device structure for the
- * given set of CPUs and initializes the timer associated with it. This
+ * given set of CPUs, registers their kthreads and initializes the timer. This
  * function also allows to register update()callback.
  * It does not start any injection cycles.
  *
- * Return: NULL if memory allocation fails, idle injection control device
- * pointer on success.
+ * Return: NULL if allocation or thread registration fails, or a CPU is already
+ * registered; idle injection control device pointer on success.
  */
 
 struct idle_inject_device *idle_inject_register_full(struct cpumask *cpumask,

@@ -342,6 +347,8 @@ struct idle_inject_device *idle_inject_register_full(struct cpumask *cpumask,
 	ii_dev->latency_us = UINT_MAX;
 	ii_dev->update = update;
 
+	mutex_lock(&idle_inject_lock);
+
 	for_each_cpu(cpu, to_cpumask(ii_dev->cpumask)) {
 
 		if (per_cpu(idle_inject_device, cpu)) {

@@ -352,6 +359,20 @@ struct idle_inject_device *idle_inject_register_full(struct cpumask *cpumask,
 		per_cpu(idle_inject_device, cpu) = ii_dev;
 	}
 
+	ii_dev->threads = (struct smp_hotplug_thread) {
+		.store = &idle_inject_thread.tsk,
+		.setup = idle_inject_setup,
+		.thread_fn = idle_inject_fn,
+		.thread_comm = "idle_inject/%u",
+		.thread_should_run = idle_inject_should_run,
+		.cpumask = to_cpumask(ii_dev->cpumask),
+	};
+
+	if (smpboot_register_percpu_thread(&ii_dev->threads))
+		goto out_rollback;
+
+	mutex_unlock(&idle_inject_lock);
+
 	return ii_dev;
 
 out_rollback:

@@ -361,6 +382,8 @@ struct idle_inject_device *idle_inject_register_full(struct cpumask *cpumask,
 		per_cpu(idle_inject_device, cpu_rb) = NULL;
 	}
 
+	mutex_unlock(&idle_inject_lock);
+
 	kfree(ii_dev);
 
 	return NULL;

@@ -372,11 +395,11 @@ EXPORT_SYMBOL_NS_GPL(idle_inject_register_full, "IDLE_INJECT");
  * @cpumask: CPUs to be affected by idle injection
  *
  * This function creates an idle injection control device structure for the
- * given set of CPUs and initializes the timer associated with it.  It does not
- * start any injection cycles.
+ * given set of CPUs, registers their kthreads and initializes the timer. It
+ * does not start any injection cycles.
  *
- * Return: NULL if memory allocation fails, idle injection control device
- * pointer on success.
+ * Return: NULL if allocation or thread registration fails, or a CPU is already
+ * registered; idle injection control device pointer on success.
  */
 struct idle_inject_device *idle_inject_register(struct cpumask *cpumask)
 {

@@ -398,23 +421,15 @@ void idle_inject_unregister(struct idle_inject_device *ii_dev)
 
 	idle_inject_stop(ii_dev);
 
+	mutex_lock(&idle_inject_lock);
+
+	smpboot_unregister_percpu_thread(&ii_dev->threads);
+
 	for_each_cpu(cpu, to_cpumask(ii_dev->cpumask))
 		per_cpu(idle_inject_device, cpu) = NULL;
 
+	mutex_unlock(&idle_inject_lock);
+
 	kfree(ii_dev);
 }
 EXPORT_SYMBOL_NS_GPL(idle_inject_unregister, "IDLE_INJECT");
-
-static struct smp_hotplug_thread idle_inject_threads = {
-	.store = &idle_inject_thread.tsk,
-	.setup = idle_inject_setup,
-	.thread_fn = idle_inject_fn,
-	.thread_comm = "idle_inject/%u",
-	.thread_should_run = idle_inject_should_run,
-};
-
-static int __init idle_inject_init(void)
-{
-	return smpboot_register_percpu_thread(&idle_inject_threads);
-}
-early_initcall(idle_inject_init);