[RFC PATCH 1/7] sched/pelt: Add a new function to approximate the future util_avg value

[Qais Yousef]

Given a util_avg value, the new function will return the future one
given a runtime delta.

This will be useful in later patches to help replace some magic margins
with more deterministic behavior.

Signed-off-by: Qais Yousef (Google) <qyousef@xxxxxxxxxxx>
---
 kernel/sched/pelt.c  | 22 +++++++++++++++++++++-
 kernel/sched/sched.h |  3 +++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
index 0f310768260c..50322005a0ae 100644
--- a/kernel/sched/pelt.c
+++ b/kernel/sched/pelt.c
@@ -466,4 +466,24 @@ int update_irq_load_avg(struct rq *rq, u64 running)
 
 	return ret;
 }
-#endif
+#endif /* CONFIG_HAVE_SCHED_AVG_IRQ */
+
+/*
+ * Approximate the new util_avg value assuming an entity has continued to run
+ * for @delta us.
+ */
+unsigned long approximate_util_avg(unsigned long util, u64 delta)
+{
+	struct sched_avg sa = {
+		.util_sum = util * PELT_MIN_DIVIDER,
+		.util_avg = util,
+	};
+
+	if (unlikely(!delta))
+		return util;
+
+	accumulate_sum(delta, &sa, 0, 0, 1);
+	___update_load_avg(&sa, 0);
+
+	return sa.util_avg;
+}
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 56eeb5b05b50..5f76b8a75a9f 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2997,6 +2997,9 @@ enum cpu_util_type {
 unsigned long effective_cpu_util(int cpu, unsigned long util_cfs,
 				 enum cpu_util_type type,
 				 struct task_struct *p);
+
+unsigned long approximate_util_avg(unsigned long util, u64 delta);
+
 /*
  * DVFS decision are made at discrete points. If CPU stays busy, the util will
  * continue to grow, which means it could need to run at a higher frequency
-- 
2.34.1