[PATCH] sched: cpufreq: Rename map_util_perf to apply_dvfs_headroom

[Qais Yousef <qyousef@layalina.io>]

We are providing headroom for the utilization to grow until the next
decision point to pick the next frequency. Give the function a better
name and give it some documentation. It is not really mapping anything.

Also move it to sched.h. This function relies on updating util signal
appropriately to give a headroom to grow. This is more of a scheduler
functionality than cpufreq. Move it to sched.h where all the other util
handling code belongs.

Signed-off-by: Qais Yousef <qyousef@layalina.io>
---
 include/linux/sched/cpufreq.h    |  5 -----
 kernel/sched/cpufreq_schedutil.c |  2 +-
 kernel/sched/sched.h             | 17 +++++++++++++++++
 3 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/include/linux/sched/cpufreq.h b/include/linux/sched/cpufreq.h
index bdd31ab93bc5..d01755d3142f 100644
--- a/include/linux/sched/cpufreq.h
+++ b/include/linux/sched/cpufreq.h
@@ -28,11 +28,6 @@ static inline unsigned long map_util_freq(unsigned long util,
 {
 	return freq * util / cap;
 }
-
-static inline unsigned long map_util_perf(unsigned long util)
-{
-	return util + (util >> 2);
-}
 #endif /* CONFIG_CPU_FREQ */
 
 #endif /* _LINUX_SCHED_CPUFREQ_H */
diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 95c3c097083e..abbd1ddb0359 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -179,7 +179,7 @@ unsigned long sugov_effective_cpu_perf(int cpu, unsigned long actual,
 				 unsigned long max)
 {
 	/* Add dvfs headroom to actual utilization */
-	actual = map_util_perf(actual);
+	actual = apply_dvfs_headroom(actual);
 	/* Actually we don't need to target the max performance */
 	if (actual < max)
 		max = actual;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index e58a54bda77d..0da3425200b1 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -3002,6 +3002,23 @@ unsigned long sugov_effective_cpu_perf(int cpu, unsigned long actual,
 				 unsigned long min,
 				 unsigned long max);
 
+/*
+ * 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
+ * before the next decision point was reached. IOW, we can't follow the util as
+ * it grows immediately, but there's a delay before we issue a request to go to
+ * higher frequency. The headroom caters for this delay so the system continues
+ * to run at adequate performance point.
+ *
+ * This function provides enough headroom to provide adequate performance
+ * assuming the CPU continues to be busy.
+ *
+ * At the moment it is a constant multiplication with 1.25.
+ */
+static inline unsigned long apply_dvfs_headroom(unsigned long util)
+{
+	return util + (util >> 2);
+}
 
 /*
  * Verify the fitness of task @p to run on @cpu taking into account the
-- 
2.34.1