[PATCH 2/3] cpufreq: schedutil: Fix selection algorithm while reducing frequency

[Viresh Kumar]

While reducing frequency if there are no frequencies available between
"current" and "next" calculated frequency, then the core will never
select the "next" frequency.

For example, consider the possible range of frequencies as 900 MHz, 1
GHz, 1.1 GHz, and 1.2 GHz. If the current frequency is 1.1 GHz and the
next frequency (based on current utilization) is 1 GHz, then the
schedutil governor will try to set the average of these as the next
frequency (i.e. 1.05 GHz).

Because we always try to find the lowest frequency greater than equal to
the target frequency, cpufreq_driver_resolve_freq() will end up
returning 1.1 GHz only. And we will not be able to reduce the frequency
eventually. The worst hit is the policy->min frequency as that will
never get selected after the frequency is increased once.

This affects all the drivers that provide ->target() or ->target_index()
callbacks.

Though for cpufreq drivers, like intel_pstate, which provide ->target()
but not ->resolve_freq() (i.e.  cpufreq_driver_resolve_freq() simply
returns the next frequency), sg_policy->next_freq gets updated with the
average frequency. And so we will finally select the min frequency when
the next_freq is 1 more than the min frequency as the average then will
be equal to the min frequency. But that will also take lots of
iterations of the schedutil update callbacks to happen.

Fix that by not using the average value for the next_freq in such cases.

Note that this still doesn't fix the drivers which provide ->target()
but don't provide ->resolve_freq() (e.g. intel_pstate) and such drivers
need to be updated to provide the ->resolve_freq() callbacks as well in
order to fix this.

Fixes: 39b64aa1c007 ("cpufreq: schedutil: Reduce frequencies slower")
Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
---
 kernel/sched/cpufreq_schedutil.c | 33 ++++++++++++++++++++++++++++-----
 1 file changed, 28 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 1852bd73d903..30e6a62d227c 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -117,6 +117,17 @@ static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
 	}
 }
 
+static unsigned int resolve_freq(struct sugov_policy *sg_policy,
+				 unsigned int freq)
+{
+	if (freq == sg_policy->cached_raw_freq &&
+	    sg_policy->next_freq != UINT_MAX)
+		return sg_policy->next_freq;
+
+	sg_policy->cached_raw_freq = freq;
+	return cpufreq_driver_resolve_freq(sg_policy->policy, freq);
+}
+
 /**
  * get_next_freq - Compute a new frequency for a given cpufreq policy.
  * @sg_policy: schedutil policy object to compute the new frequency for.
@@ -145,6 +156,7 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
 	struct cpufreq_policy *policy = sg_policy->policy;
 	unsigned int freq = arch_scale_freq_invariant() ?
 				policy->cpuinfo.max_freq : policy->cur;
+	unsigned int target, original = 0;
 
 	freq = (freq + (freq >> 2)) * util / max;
 
@@ -156,13 +168,24 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
 	if (freq < policy->min)
 		freq = policy->min;
 
-	if (sg_policy->next_freq > freq)
+	if (sg_policy->next_freq > freq) {
+		original = freq;
 		freq = (sg_policy->next_freq + freq) >> 1;
+	}
 
-	if (freq == sg_policy->cached_raw_freq && sg_policy->next_freq != UINT_MAX)
-		return sg_policy->next_freq;
-	sg_policy->cached_raw_freq = freq;
-	return cpufreq_driver_resolve_freq(policy, freq);
+	target = resolve_freq(sg_policy, freq);
+
+	/*
+	 * While reducing frequency if there are no frequencies available
+	 * between "original" and "next_freq", resolve_freq() will return
+	 * next_freq because we always try to find the lowest frequency greater
+	 * than equal to the "freq". Fix that by going directly to the
+	 * "original" frequency in that case.
+	 */
+	if (unlikely(original && target == sg_policy->next_freq))
+		target = resolve_freq(sg_policy, original);
+
+	return target;
 }
 
 static void sugov_get_util(unsigned long *util, unsigned long *max)
-- 
2.13.0.70.g6367777092d9