[PATCH v2 1/6] cpufreq: schedutil: ignore sugov kthreads

[Patrick Bellasi]

In system where multiple CPUs shares the same frequency domain a small
workload on a CPU can still be subject to frequency spikes, generated by
the activation of the sugov's kthread.

Since the sugov kthread is a special RT task, which goal is just that to
activate a frequency transition, it does not make sense for it to bias
the schedutil's frequency selection policy.

This patch exploits the information related to the current task to silently
ignore cpufreq_update_this_cpu() calls, coming from the RT scheduler, while
the sugov kthread is running.

Signed-off-by: Patrick Bellasi <patrick.bellasi@xxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
Cc: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-pm@xxxxxxxxxxxxxxx

---
Changes from v1:
- move check before policy spinlock (JuriL)
---
 kernel/sched/cpufreq_schedutil.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index c982dd0..eaba6d6 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -218,6 +218,10 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
 	unsigned int next_f;
 	bool busy;
 
+	/* Skip updates generated by sugov kthreads */
+	if (unlikely(current == sg_policy->thread))
+		return;
+
 	sugov_set_iowait_boost(sg_cpu, time, flags);
 	sg_cpu->last_update = time;
 
@@ -290,6 +294,10 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
 	unsigned long util, max;
 	unsigned int next_f;
 
+	/* Skip updates generated by sugov kthreads */
+	if (unlikely(current == sg_policy->thread))
+		return;
+
 	sugov_get_util(&util, &max);
 
 	raw_spin_lock(&sg_policy->update_lock);
-- 
2.7.4