[PATCH v2] sched: cpufreq: Rename map_util_perf to sugov_apply_dvfs_headroom

From: Qais Yousef
Date: Tue Feb 20 2024 - 13:08:40 EST


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@xxxxxxxxxx>
Acked-by: Rafael J. Wysocki <rafael@xxxxxxxxxx>
Signed-off-by: Qais Yousef <qyousef@xxxxxxxxxxx>
---

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