[PATCH] sched/fair: Fix frequency selection for non invariant case

[Vincent Guittot]

When frequency invariance is not enabled, get_capacity_ref_freq(policy)
returns the current frequency and the performance margin applied by
map_util_perf(), enabled the utilization to go above the maximum compute
capacity and to select a higher frequency than the current one.

The performance margin is now applied earlier in the path to take into
account some utilization clampings and we can't get an utilization higher
than the maximum compute capacity.

We must use a frequency above the current frequency to get a chance to
select a higher OPP when the current one becomes fully used. Apply
the same margin and returns a frequency 25% higher than the current one in
order to switch to the next OPP before we fully use the cpu at the current
one.

Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Closes: https://lore.kernel.org/lkml/CAHk-=wgWcYX2oXKtgvNN2LLDXP7kXkbo-xTfumEjmPbjSer2RQ@xxxxxxxxxxxxxx/
Reported-by: Wyes Karny <wkarny@xxxxxxxxx>
Closes: https://lore.kernel.org/lkml/20240114091240.xzdvqk75ifgfj5yx@wyes-pc/
Fixes: 9c0b4bb7f630 ("sched/cpufreq: Rework schedutil governor performance estimation")
Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Tested-by: Wyes Karny <wkarny@xxxxxxxxx>
---
 kernel/sched/cpufreq_schedutil.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 95c3c097083e..d12e95d30e2e 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -133,7 +133,11 @@ unsigned long get_capacity_ref_freq(struct cpufreq_policy *policy)
 	if (arch_scale_freq_invariant())
 		return policy->cpuinfo.max_freq;
 
-	return policy->cur;
+	/*
+	 * Apply a 25% margin so that we select a higher frequency than
+	 * the current one before the CPU is full busy
+	 */
+	return policy->cur + (policy->cur >> 2);
 }
 
 /**
-- 
2.34.1