On Tue, Jan 15, 2019 at 10:15:00AM +0000, Patrick Bellasi wrote:

+static inline unsigned int uclamp_bucket_value(unsigned int clamp_value)
+{
+#define UCLAMP_BUCKET_DELTA (SCHED_CAPACITY_SCALE / CONFIG_UCLAMP_BUCKETS_COUNT)
+#define UCLAMP_BUCKET_UPPER (UCLAMP_BUCKET_DELTA * CONFIG_UCLAMP_BUCKETS_COUNT)
+
+	if (clamp_value >= UCLAMP_BUCKET_UPPER)
+		return SCHED_CAPACITY_SCALE;
+
+	return UCLAMP_BUCKET_DELTA * (clamp_value / UCLAMP_BUCKET_DELTA);
+}

+static void uclamp_bucket_inc(struct uclamp_se *uc_se, unsigned int clamp_id,
+			      unsigned int clamp_value)
+{
+	union uclamp_map *uc_maps = &uclamp_maps[clamp_id][0];
+	unsigned int prev_bucket_id = uc_se->bucket_id;
+	union uclamp_map uc_map_old, uc_map_new;
+	unsigned int free_bucket_id;
+	unsigned int bucket_value;
+	unsigned int bucket_id;
+
+	bucket_value = uclamp_bucket_value(clamp_value);

Aahh!!

So why don't you do:

	bucket_id = clamp_value / UCLAMP_BUCKET_DELTA;
	bucket_value = bucket_id * UCLAMP_BUCKET_DELTA;

+	do {
+		/* Find the bucket_id of an already mapped clamp bucket... */
+		free_bucket_id = UCLAMP_BUCKETS;
+		for (bucket_id = 0; bucket_id < UCLAMP_BUCKETS; ++bucket_id) {
+			uc_map_old.data = atomic_long_read(&uc_maps[bucket_id].adata);
+			if (free_bucket_id == UCLAMP_BUCKETS && !uc_map_old.se_count)
+				free_bucket_id = bucket_id;
+			if (uc_map_old.value == bucket_value)
+				break;
+		}
+
+		/* ... or allocate a new clamp bucket */
+		if (bucket_id >= UCLAMP_BUCKETS) {
+			/*
+			 * A valid clamp bucket must always be available.
+			 * If we cannot find one: refcounting is broken and we
+			 * warn once. The sched_entity will be tracked in the
+			 * fast-path using its previous clamp bucket, or not
+			 * tracked at all if not yet mapped (i.e. it's new).
+			 */
+			if (unlikely(free_bucket_id == UCLAMP_BUCKETS)) {
+				SCHED_WARN_ON(free_bucket_id == UCLAMP_BUCKETS);
+				return;
+			}
+			bucket_id = free_bucket_id;
+			uc_map_old.data = atomic_long_read(&uc_maps[bucket_id].adata);
+		}

And then skip all this?

+
+		uc_map_new.se_count = uc_map_old.se_count + 1;
+		uc_map_new.value = bucket_value;
+
+	} while (!atomic_long_try_cmpxchg(&uc_maps[bucket_id].adata,
+					  &uc_map_old.data, uc_map_new.data));
+
+	uc_se->value = clamp_value;
+	uc_se->bucket_id = bucket_id;
+
+	if (uc_se->mapped)
+		uclamp_bucket_dec(clamp_id, prev_bucket_id);
+
+	/*
+	 * Task's sched_entity are refcounted in the fast-path only when they
+	 * have got a valid clamp_bucket assigned.
+	 */
+	uc_se->mapped = true;
+}