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 <redacted>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rafael J. Wysocki <redacted>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-pm@vger.kernel.org

---
Changes from v2:
- rebased on v4.15-rc1
- moved at the end of the stack since considered more controversial
Changes from v1:
- move check before policy spinlock (JuriL)

Change-Id: I4d749458229b6496dd24a8c357be42cd35a739fd
---
 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 3eea8884e61b..a93ad5b0c40d 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c

@@ -270,6 +270,10 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
 	bool rt_mode;
 	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;
 

@@ -356,6 +360,10 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
 	unsigned int next_f;
 	bool rt_mode;
 
+	/* Skip updates generated by sugov kthreads */
+	if (unlikely(current == sg_policy->thread))
+		return;
+
 	raw_spin_lock(&sg_policy->update_lock);
 
 	sugov_get_util(&util, &max, sg_cpu->cpu);

-- 
2.14.1