[PATCH v3 1/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE enter

[Patrick Bellasi]

Currently, sg_cpu's flags are set to the value defined by the last call
of the cpufreq_update_util(); for RT/DL classes this corresponds to the
SCHED_CPUFREQ_{RT/DL} flags always being set.

When multiple CPUs share the same frequency domain it might happen that
a CPU which executed an RT task, right before entering IDLE, has one of
the SCHED_CPUFREQ_RT_DL flags set, permanently, until it exits IDLE.

Although such an idle CPU is _going to be_ ignored by the
sugov_next_freq_shared():
  1. this kind of "useless RT requests" are ignored only if more then
     TICK_NSEC have elapsed since the last update
  2. we can still potentially trigger an already too late switch to
     MAX, which starts also a new throttling interval
  3. the internal state machine is not consistent with what the
     scheduler knows, i.e. the CPU is now actually idle

Thus, in sugov_next_freq_shared(), where utilisation and flags are
aggregated across all the CPUs of a frequency domain, it can turn out
that all the CPUs of that domain can run unnecessary at the maximum OPP
until another event happens in the idle CPU, which eventually clears the
SCHED_CPUFREQ_{RT/DL} flag, or the IDLE CPUs gets ignored after
TICK_NSEC [ns] since the CPU entering IDLE.

Such a behaviour can harm the energy efficiency of systems where RT
workloads are not so frequent and other CPUs in the same frequency
domain are running small utilisation workloads, which is a quite common
scenario in mobile embedded systems.

This patch proposes a solution which is aligned with the current
principle to update the flags each time a scheduling event happens. The
scheduling of the idle_task on a CPU is considered one of such
meaningful events.  That's why when the idle_task is selected for
execution we poke the schedutil policy to reset the flags for that CPU.

No frequency transitions are activated at that point, which is fair in
case the RT workload should come back in the future. However, this still
allows other CPUs in the same frequency domain to scale down the
frequency in case that should be possible.

Signed-off-by: Patrick Bellasi <patrick.bellasi@xxxxxxx>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@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 v2:
- use cpufreq_update_util() instead of cpufreq_update_this_cpu()
- rebased on v4.15-rc1

Changes from v1:
- added "unlikely()" around the statement (SteveR)

Change-Id: I1192ca9a3acb767cb3a745967a7a23a17e1af7b7
---
 include/linux/sched/cpufreq.h    | 1 +
 kernel/sched/cpufreq_schedutil.c | 7 +++++++
 kernel/sched/idle_task.c         | 4 ++++
 3 files changed, 12 insertions(+)

diff --git a/include/linux/sched/cpufreq.h b/include/linux/sched/cpufreq.h
index d1ad3d825561..bb5f778db023 100644
--- a/include/linux/sched/cpufreq.h
+++ b/include/linux/sched/cpufreq.h
@@ -11,6 +11,7 @@
 #define SCHED_CPUFREQ_RT	(1U << 0)
 #define SCHED_CPUFREQ_DL	(1U << 1)
 #define SCHED_CPUFREQ_IOWAIT	(1U << 2)
+#define SCHED_CPUFREQ_IDLE	(1U << 3)
 
 #define SCHED_CPUFREQ_RT_DL	(SCHED_CPUFREQ_RT | SCHED_CPUFREQ_DL)
 
diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 2f52ec0f1539..67339ccb5595 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -347,6 +347,12 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
 
 	sg_cpu->util = util;
 	sg_cpu->max = max;
+
+	/* CPU is entering IDLE, reset flags without triggering an update */
+	if (unlikely(flags & SCHED_CPUFREQ_IDLE)) {
+		sg_cpu->flags = 0;
+		goto done;
+	}
 	sg_cpu->flags = flags;
 
 	sugov_set_iowait_boost(sg_cpu, time, flags);
@@ -361,6 +367,7 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
 		sugov_update_commit(sg_policy, time, next_f);
 	}
 
+done:
 	raw_spin_unlock(&sg_policy->update_lock);
 }
 
diff --git a/kernel/sched/idle_task.c b/kernel/sched/idle_task.c
index d518664cce4f..6e8ae2aa7a13 100644
--- a/kernel/sched/idle_task.c
+++ b/kernel/sched/idle_task.c
@@ -30,6 +30,10 @@ pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
 	put_prev_task(rq, prev);
 	update_idle_core(rq);
 	schedstat_inc(rq->sched_goidle);
+
+	/* kick cpufreq (see the comment in kernel/sched/sched.h). */
+	cpufreq_update_util(rq, SCHED_CPUFREQ_IDLE);
+
 	return rq->idle;
 }
 
-- 
2.14.1