+#ifdef CONFIG_SMP
+/*
+ * Approximate:
+ * val * y^n, where y^32 ~= 0.5 (~1 scheduling period)
+ */
+static __always_inline u64 decay_load(u64 val, u64 n)
+{
+ for (; n && val; n--) {
+ val *= 4008;
+ val >>= 12;
+ }
+
+ return val;
+}
+
+/*
+ * We can represent the historical contribution to runnable average as the
+ * coefficients of a geometric series. To do this we sub-divide our runnable
+ * history into segments of approximately 1ms (1024us); label the segment that
+ * occurred N-ms ago p_N, with p_0 corresponding to the current period, e.g.
+ *
+ * [<- 1024us ->|<- 1024us ->|<- 1024us ->| ...
+ * p0 p1 p2
+ * (now) (~1ms ago) (~2ms ago)
+ *
+ * Let u_i denote the fraction of p_i that the entity was runnable.
+ *
+ * We then designate the fractions u_i as our co-efficients, yielding the
+ * following representation of historical load:
+ * u_0 + u_1*y + u_2*y^2 + u_3*y^3 + ...
+ *
+ * We choose y based on the with of a reasonably scheduling period, fixing:
+ * y^32 = 0.5
+ *
+ * This means that the contribution to load ~32ms ago (u_32) will be weighted
+ * approximately half as much as the contribution to load within the last ms
+ * (u_0).
+ *
+ * When a period "rolls over" and we have new u_0`, multiplying the previous
+ * sum again by y is sufficient to update:
+ * load_avg = u_0` + y*(u_0 + u_1*y + u_2*y^2 + ... )
+ * = u_0 + u_1*y + u_2*y^2 + ... [re-labeling u_i --> u_{i+1}]
+ */
+static __always_inline int __update_entity_runnable_avg(u64 now,
+ struct sched_avg *sa,
+ int runnable)
+{
+ u64 delta;
+ int delta_w, decayed = 0;
+
+ delta = now - sa->last_runnable_update;
+ /*
+ * This should only happen when time goes backwards, which it
+ * unfortunately does during sched clock init when we swap over to TSC.
+ */
+ if ((s64)delta < 0) {
+ sa->last_runnable_update = now;
+ return 0;
+ }
+
+ /*
+ * Use 1024ns as the unit of measurement since it's a reasonable
+ * approximation of 1us and fast to compute.
+ */
+ delta >>= 10;
+ if (!delta)
+ return 0;
+ sa->last_runnable_update = now;
+
+ /* delta_w is the amount already accumulated against our next period */
+ delta_w = sa->runnable_avg_period % 1024;
+ if (delta + delta_w >= 1024) {
+ /* period roll-over */
+ decayed = 1;
+
+ /*
+ * Now that we know we're crossing a period boundary, figure
+ * out how much from delta we need to complete the current
+ * period and accrue it.
+ */
+ delta_w = 1024 - delta_w;
+ BUG_ON(delta_w > delta);
+ do {
+ if (runnable)
+ sa->runnable_avg_sum += delta_w;
+ sa->runnable_avg_period += delta_w;
+
+ /*
+ * Remainder of delta initiates a new period, roll over
+ * the previous.
+ */
+ sa->runnable_avg_sum =
+ decay_load(sa->runnable_avg_sum, 1);
+ sa->runnable_avg_period =
+ decay_load(sa->runnable_avg_period, 1);
+
+ delta -= delta_w;
+ /* New period is empty */
+ delta_w = 1024;
+ } while (delta >= 1024);
+ }
+
+ /* Remainder of delta accrued against u_0` */
+ if (runnable)
+ sa->runnable_avg_sum += delta;
+ sa->runnable_avg_period += delta;
+
+ return decayed;
+}
+
+/* Update a sched_entity's runnable average */
+static inline void update_entity_load_avg(struct sched_entity *se)
+{
+ __update_entity_runnable_avg(rq_of(cfs_rq_of(se))->clock_task, &se->avg,
+ se->on_rq);
+}
+#else
+static inline void update_entity_load_avg(struct sched_entity *se) {}
+#endif
+