[PATCH 2/6] cpufreq: schedutil: ignore the sugov kthread for frequencies selections

[Patrick Bellasi]

In system where multiple CPUs shares the same frequency domain a small
workload on a CPU can still be subject 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.

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
---
 kernel/sched/cpufreq_schedutil.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 084a98b..a3fe5e4 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -274,6 +274,8 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
 {
 	struct sugov_cpu *sg_cpu = container_of(hook, struct sugov_cpu, update_util);
 	struct sugov_policy *sg_policy = sg_cpu->sg_policy;
+	unsigned int cpu = smp_processor_id();
+	struct task_struct *curr = cpu_curr(cpu);
 	unsigned long util, max;
 	unsigned int next_f;
 
@@ -287,6 +289,10 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
 		goto done;
 	}
 
+	/* Skip updates generated by sugov kthreads */
+	if (curr == sg_policy->thread)
+		goto done;
+
 	sg_cpu->util = util;
 	sg_cpu->max = max;
 	sg_cpu->flags = flags;
-- 
2.7.4