[RFC PATCH 2/7] sched/pelt: Add a new function to approximate runtime to reach given util

[Qais Yousef]

It is basically the ramp-up time from 0 to a given value. Will be used
later to implement new tunable to control response time  for schedutil.

Signed-off-by: Qais Yousef (Google) <qyousef@xxxxxxxxxxx>
---
 kernel/sched/pelt.c  | 21 +++++++++++++++++++++
 kernel/sched/sched.h |  1 +
 2 files changed, 22 insertions(+)

diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
index 50322005a0ae..f673b9ab92dc 100644
--- a/kernel/sched/pelt.c
+++ b/kernel/sched/pelt.c
@@ -487,3 +487,24 @@ unsigned long approximate_util_avg(unsigned long util, u64 delta)
 
 	return sa.util_avg;
 }
+
+/*
+ * Approximate the required amount of runtime in ms required to reach @util.
+ */
+u64 approximate_runtime(unsigned long util)
+{
+	struct sched_avg sa = {};
+	u64 delta = 1024; // period = 1024 = ~1ms
+	u64 runtime = 0;
+
+	if (unlikely(!util))
+		return runtime;
+
+	while (sa.util_avg < util) {
+		accumulate_sum(delta, &sa, 0, 0, 1);
+		___update_load_avg(&sa, 0);
+		runtime++;
+	}
+
+	return runtime;
+}
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 5f76b8a75a9f..2b889ad399de 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2999,6 +2999,7 @@ unsigned long effective_cpu_util(int cpu, unsigned long util_cfs,
 				 struct task_struct *p);
 
 unsigned long approximate_util_avg(unsigned long util, u64 delta);
+u64 approximate_runtime(unsigned long util);
 
 /*
  * DVFS decision are made at discrete points. If CPU stays busy, the util will
-- 
2.34.1