[RFCv4,4/6] sched/core: sync task_group's with CPU's clamp groups

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ba31bb4e14c7..e4ce25dbad6f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -754,6 +754,12 @@  static void set_load_weight(struct task_struct *p)
 #ifdef CONFIG_UTIL_CLAMP
 /**
  * uclamp_mutex: serialize updates of TG's utilization clamp values
+ *
+ * A task groups's utilization clamp value update is usually triggered from a
+ * user-space process (slow-path) but it requires a synchronization with the
+ * scheduler's (fast-path) enqueue/dequeue operations.
+ * While the fast-path synchronization is protected by RQs spinlock, this
+ * mutex ensure that we sequentially serve user-space requests.
  */
 static DEFINE_MUTEX(uclamp_mutex);
 
@@ -1022,6 +1028,52 @@  static inline void uclamp_cpu_put(struct task_struct *p, int cpu, int clamp_id)
 		uclamp_cpu_update(cpu, clamp_id);
 }
 
+/**
+ * uclamp_task_update_active: update the clamp group of a RUNNABLE task
+ * @p: the task which clamp groups must be updated
+ * @clamp_id: the clamp index to consider
+ * @group_id: the clamp group to update
+ *
+ * Each time the clamp value of a task group is changed, the old and new clamp
+ * groups have to be updated for each CPU containing a RUNNABLE task belonging
+ * to this tasks group. Sleeping tasks are not updated since they will be
+ * enqueued with the proper clamp group index at their next activation.
+ */
+static inline void
+uclamp_task_update_active(struct task_struct *p, int clamp_id, int group_id)
+{
+	struct rq_flags rf;
+	struct rq *rq;
+
+	/*
+	 * Lock the task and the CPU where the task is (or was) queued.
+	 *
+	 * We might lock the (previous) RQ of a !RUNNABLE task, but that's the
+	 * price to pay to safely serialize util_{min,max} updates with
+	 * enqueues, dequeues and migration operations.
+	 * This is the same locking schema used by __set_cpus_allowed_ptr().
+	 */
+	rq = task_rq_lock(p, &rf);
+
+	/*
+	 * The setting of the clamp group is serialized by task_rq_lock().
+	 * Thus, if the task's task_struct is not referencing a valid group
+	 * index, then that task is not yet RUNNABLE or it's going to be
+	 * enqueued with the proper clamp group value.
+	 */
+	if (!uclamp_task_active(p))
+		goto done;
+
+	/* Release p's currently referenced clamp group */
+	uclamp_cpu_put(p, task_cpu(p), clamp_id);
+
+	/* Get p's new clamp group */
+	uclamp_cpu_get(p, task_cpu(p), clamp_id);
+
+done:
+	task_rq_unlock(rq, p, &rf);
+}
+
 /**
  * uclamp_group_put: decrease the reference count for a clamp group
  * @clamp_id: the clamp index which was affected by a task group
@@ -1070,6 +1122,8 @@  static inline int uclamp_group_get(struct cgroup_subsys_state *css,
 	struct uclamp_map *uc_map = &uclamp_maps[clamp_id][0];
 	int prev_group_id = uc_tg->group_id;
 	int next_group_id = UCLAMP_NONE;
+	struct css_task_iter it;
+	struct task_struct *p;
 	unsigned long flags;
 
 	/* Lookup for a usable utilization clamp group */
@@ -1091,6 +1145,18 @@  static inline int uclamp_group_get(struct cgroup_subsys_state *css,
 	uc_map[next_group_id].tg_count += 1;
 	raw_spin_unlock_irqrestore(&uc_map[next_group_id].tg_lock, flags);
 
+	/* Newly created TG don't have tasks assigned */
+	if (!css)
+		goto release;
+
+	/* Update clamp groups for RUNNABLE tasks in this TG */
+	css_task_iter_start(css, &it);
+	while ((p = css_task_iter_next(&it)))
+		uclamp_task_update_active(p, clamp_id, next_group_id);
+	css_task_iter_end(&it);
+
+release:
+
 	/* Release the previous clamp group */
 	uclamp_group_put(clamp_id, prev_group_id);
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index b0f17c19c0f6..164a8ac152b3 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -462,6 +462,27 @@  static inline bool uclamp_task_affects(struct task_struct *p, int clamp_id)
 	return (task_group_id != UCLAMP_NONE);
 }
 
+/**
+ * uclamp_task_active: check if a task is currently clamping a CPU
+ * @p: the task to check
+ *
+ * A task affects the utilization clamp of a CPU if it references a valid
+ * clamp group index for at least one clamp index.
+ *
+ * Return: true if p is currently clamping the utilization of its CPU.
+ */
+static inline bool uclamp_task_active(struct task_struct *p)
+{
+	int clamp_id;
+
+	for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) {
+		if (uclamp_task_affects(p, clamp_id))
+			return true;
+	}
+
+	return false;
+}
+
 /**
  * uclamp_group_active: check if a clamp group is active on a CPU
  * @uc_cpu: the array of clamp groups for a CPU