[PATCH v2] sched: cpufreq: Rename map_util_perf to sugov_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 cpufreq_schedutil.c. This function relies on updating
util signal appropriately to give a headroom to grow. This is tied to
schedutil and scheduler and not something that can be shared with other
governors.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Signed-off-by: Qais Yousef <qyousef@layalina.io>
---

Changes in v2:

	1. Add Acked-by from Viresh and Raphael (Thanks!)
	2. Move the function to cpufreq_schedutil.c instead of sched.h
	3. Name space the function with sugov_ to indicate it is special to
	   this governor only and not generic.

 include/linux/sched/cpufreq.h    |  5 -----
 kernel/sched/cpufreq_schedutil.c | 20 +++++++++++++++++++-
 2 files changed, 19 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 eece6244f9d2..575df3599813 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -178,12 +178,30 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
 	return cpufreq_driver_resolve_freq(policy, freq);
 }
 
+/*
+ * 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 sugov_apply_dvfs_headroom(unsigned long util)
+{
+	return util + (util >> 2);
+}
+
 unsigned long sugov_effective_cpu_perf(int cpu, unsigned long actual,
 				 unsigned long min,
 				 unsigned long max)
 {
 	/* Add dvfs headroom to actual utilization */
-	actual = map_util_perf(actual);
+	actual = sugov_apply_dvfs_headroom(actual);
 	/* Actually we don't need to target the max performance */
 	if (actual < max)
 		max = actual;
-- 
2.34.1