On Tue, Jan 15, 2019 at 10:15:02AM +0000, Patrick Bellasi wrote:

+static inline void
+uclamp_task_update_active(struct task_struct *p, unsigned int clamp_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);
+
+	/*
+	 * Setting the clamp bucket is serialized by task_rq_lock().
+	 * If the task is not yet RUNNABLE and its task_struct is not
+	 * affecting a valid clamp bucket, the next time it's enqueued,
+	 * it will already see the updated clamp bucket value.
+	 */
+	if (!p->uclamp[clamp_id].active)
+		goto done;
+
+	uclamp_cpu_dec_id(p, rq, clamp_id);
+	uclamp_cpu_inc_id(p, rq, clamp_id);
+
+done:
+	task_rq_unlock(rq, p, &rf);
+}

But.... __sched_setscheduler() actually does the whole dequeue + enqueue
thing already ?!? See where it does __setscheduler().