[RFC PATCH v5 2/2] cpuidle: teo: Introduce util-awareness

[Kajetan Puchalski]

Modern interactive systems, such as recent Android phones, tend to have power
efficient shallow idle states. Selecting deeper idle states on a device while a
latency-sensitive workload is running can adversely impact performance due to
increased latency. Additionally, if the CPU wakes up from a deeper sleep before
its target residency as is often the case, it results in a waste of energy on
top of that.

At the moment, all the available idle governors operate mainly based on their
own past correctness metrics along with timer events without taking into account
any scheduling information. Especially on interactive systems, this results in
them frequently selecting a deeper idle state and then waking up before its
target residency is hit, thus leading to increased wakeup latency and lower
performance with no power saving. For 'menu' while web browsing on Android for
instance, those types of wakeups ('too deep') account for over 24% of all
wakeups.

At the same time, on some platforms C0 can be power efficient enough to warrant
wanting to prefer it over C1. This is because the power usage of the two states
can be so close that sufficient amounts of too deep C1 sleeps can completely
offset the C1 power saving to the point where it would've been more power
efficient to just use C0 instead.

Sleeps that happened in C0 while they could have used C1 ('too shallow') only
save less power than they otherwise could have. Too deep sleeps, on the other
hand, harm performance and nullify the potential power saving from using C1 in
the first place. While taking this into account, it is clear that on balance it
is preferable for an idle governor to have more too shallow sleeps instead of
more too deep sleeps on those kinds of platforms.

This patch specifically tunes TEO to minimise too deep sleeps and minimise
latency to achieve better performance. To this end, before selecting the next
idle state it uses the avg_util signal of a CPU's runqueue in order to determine
to what extent the CPU is being utilized. This util value is then compared to a
threshold defined as a percentage of the cpu's capacity (capacity >> 6 ie. ~1.5%
in the current implementation). If the util is above the threshold, the
idle state selected by TEO metrics will be reduced by 1, thus selecting a
shallower state. If the util is below the threshold, the governor defaults to
the TEO metrics mechanism to try to select the deepest available idle state
based on the closest timer event and its own correctness.

The main goal of this is to reduce latency and increase performance for some
workloads. Under some workloads it will result in an increase in power usage
(Geekbench 5) while for other workloads it will also result in a decrease in
power usage compared to TEO (PCMark Web, Jankbench, Speedometer).

It can provide drastically decreased latency and performance benefits in certain
types of workloads that are sensitive to latency.

Signed-off-by: Kajetan Puchalski <kajetan.puchalski@xxxxxxx>
---
 drivers/cpuidle/governors/teo.c | 85 ++++++++++++++++++++++++++++++++-
 1 file changed, 84 insertions(+), 1 deletion(-)

diff --git a/drivers/cpuidle/governors/teo.c b/drivers/cpuidle/governors/teo.c
index e2864474a98d..2f85dde301d3 100644
--- a/drivers/cpuidle/governors/teo.c
+++ b/drivers/cpuidle/governors/teo.c
@@ -2,8 +2,13 @@
 /*
  * Timer events oriented CPU idle governor
  *
+ * TEO governor:
  * Copyright (C) 2018 - 2021 Intel Corporation
  * Author: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
+ *
+ * Util-awareness mechanism:
+ * Copyright (C) 2022 Arm Ltd.
+ * Author: Kajetan Puchalski <kajetan.puchalski@xxxxxxx>
  */
 
 /**
@@ -99,14 +104,55 @@
  *      select the given idle state instead of the candidate one.
  *
  * 3. By default, select the candidate state.
+ *
+ * Util-awareness mechanism:
+ *
+ * The idea behind the util-awareness extension is that there are two distinct
+ * scenarios for the CPU which should result in two different approaches to idle
+ * state selection - utilized and not utilized.
+ *
+ * In this case, 'utilized' means that the average runqueue util of the CPU is
+ * above a certain threshold.
+ *
+ * When the CPU is utilized while going into idle, more likely than not it will
+ * be woken up to do more work soon and so a shallower idle state should be
+ * selected to minimise latency and maximise performance. When the CPU is not
+ * being utilized, the usual metrics-based approach to selecting the deepest
+ * available idle state should be preferred to take advantage of the power
+ * saving.
+ *
+ * In order to achieve this, the governor uses a utilization threshold.
+ * The threshold is computed per-cpu as a percentage of the CPU's capacity
+ * by bit shifting the capacity value. Based on testing, the shift of 6 (~1.56%)
+ * seems to be getting the best results.
+ *
+ * Before selecting the next idle state, the governor compares the current CPU
+ * util to the precomputed util threshold. If it's below, it defaults to the
+ * TEO metrics mechanism. If it's above, the idle state will be reduced to C0
+ * as long as C0 is not a polling state.
  */
 
 #include <linux/cpuidle.h>
 #include <linux/jiffies.h>
 #include <linux/kernel.h>
+#include <linux/sched.h>
 #include <linux/sched/clock.h>
+#include <linux/sched/topology.h>
 #include <linux/tick.h>
 
+/*
+ * The number of bits to shift the cpu's capacity by in order to determine
+ * the utilized threshold.
+ *
+ * 6 was chosen based on testing as the number that achieved the best balance
+ * of power and performance on average.
+ *
+ * The resulting threshold is high enough to not be triggered by background
+ * noise and low enough to react quickly when activity starts to ramp up.
+ */
+#define UTIL_THRESHOLD_SHIFT 6
+
+
 /*
  * The PULSE value is added to metrics when they grow and the DECAY_SHIFT value
  * is used for decreasing metrics on a regular basis.
@@ -137,9 +183,11 @@ struct teo_bin {
  * @time_span_ns: Time between idle state selection and post-wakeup update.
  * @sleep_length_ns: Time till the closest timer event (at the selection time).
  * @state_bins: Idle state data bins for this CPU.
- * @total: Grand total of the "intercepts" and "hits" mertics for all bins.
+ * @total: Grand total of the "intercepts" and "hits" metrics for all bins.
  * @next_recent_idx: Index of the next @recent_idx entry to update.
  * @recent_idx: Indices of bins corresponding to recent "intercepts".
+ * @util_threshold: Threshold above which the CPU is considered utilized
+ * @utilized: Whether the last sleep on the CPU happened while utilized
  */
 struct teo_cpu {
 	s64 time_span_ns;
@@ -148,10 +196,22 @@ struct teo_cpu {
 	unsigned int total;
 	int next_recent_idx;
 	int recent_idx[NR_RECENT];
+	unsigned long util_threshold;
+	bool utilized;
 };
 
 static DEFINE_PER_CPU(struct teo_cpu, teo_cpus);
 
+/**
+ * teo_get_util - Update the CPU utilized status
+ * @dev: Target CPU
+ * @cpu_data: Governor CPU data for the target CPU
+ */
+static void teo_get_util(struct cpuidle_device *dev, struct teo_cpu *cpu_data)
+{
+	cpu_data->utilized = sched_cpu_util(dev->cpu) > cpu_data->util_threshold;
+}
+
 /**
  * teo_update - Update CPU metrics after wakeup.
  * @drv: cpuidle driver containing state data.
@@ -323,6 +383,20 @@ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 			goto end;
 	}
 
+	teo_get_util(dev, cpu_data);
+	/*
+	 * The cpu is being utilized over the threshold there are only 2 states to choose from.
+	 * No need to consider metrics, choose the shallowest non-polling state and exit.
+	 */
+	if (drv->state_count < 3 && cpu_data->utilized) {
+		for (i = 0; i < drv->state_count; ++i) {
+			if (!dev->states_usage[i].disable && !(drv->states[i].flags & CPUIDLE_FLAG_POLLING)) {
+				idx = i;
+				goto end;
+			}
+		}
+	}
+
 	/*
 	 * Find the deepest idle state whose target residency does not exceed
 	 * the current sleep length and the deepest idle state not deeper than
@@ -454,6 +528,13 @@ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 	if (idx > constraint_idx)
 		idx = constraint_idx;
 
+	/*
+	 * If the CPU is being utilized over the threshold,
+	 * choose a shallower non-polling state to improve latency
+	 */
+	if (cpu_data->utilized)
+		idx = teo_find_shallower_state(drv, dev, idx, duration_ns, true);
+
 end:
 	/*
 	 * Don't stop the tick if the selected state is a polling one or if the
@@ -510,9 +591,11 @@ static int teo_enable_device(struct cpuidle_driver *drv,
 			     struct cpuidle_device *dev)
 {
 	struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);
+	unsigned long max_capacity = arch_scale_cpu_capacity(dev->cpu);
 	int i;
 
 	memset(cpu_data, 0, sizeof(*cpu_data));
+	cpu_data->util_threshold = max_capacity >> UTIL_THRESHOLD_SHIFT;
 
 	for (i = 0; i < NR_RECENT; i++)
 		cpu_data->recent_idx[i] = -1;
-- 
2.37.1