linux-kernel - [PATCH v3 6/6] cpufreq: schedutil: ignore sugov kthreads

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Message-Id: <20171130114723.29210-7-patrick.bellasi@arm.com>
Date:   Thu, 30 Nov 2017 11:47:23 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...roid.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: [PATCH v3 6/6] cpufreq: schedutil: ignore sugov kthreads

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@....com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
Cc: Viresh Kumar <viresh.kumar@...aro.org>
Cc: linux-kernel@...r.kernel.org
Cc: linux-pm@...r.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