Re: [PATCH v6 2/2] cpuidle: teo: Introduce util-awareness

Rafael J. Wysocki
2023-01-05T17:19:22Z

On Thu, Jan 5, 2023 at 4:34 PM Vincent Guittot
<vincent.guittot@linaro.org> wrote:
>
> On Thu, 5 Jan 2023 at 16:07, Rafael J. Wysocki <rafael@kernel.org> wrote:
> >
> > On Thu, Jan 5, 2023 at 3:52 PM Kajetan Puchalski
> > <kajetan.puchalski@arm.com> wrote:
> > >
> > > 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, none of the available idle governors take any scheduling
> > > information into account. They also tend to overestimate the idle
> > > duration quite often, which causes them to select excessively deep idle
> > > states, 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 idle state 0 can be power efficient
> > > enough to warrant wanting to prefer it over idle state 1. This is because
> > > the power usage of the two states can be so close that sufficient amounts
> > > of too deep state 1 sleeps can completely offset the state 1 power saving to the
> > > point where it would've been more power efficient to just use state 0 instead.
> > > This is of course for systems where state 0 is not a polling state, such as
> > > arm-based devices.
> > >
> > > Sleeps that happened in state 0 while they could have used state 1 ('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 state 1 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 prefer shallower idle states in
> > > order to reduce wakeup latency and 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.
> > >
> > > Example test results:
> > >
> > > 1. GB5 (better score, latency & more power usage)
> > >
> > > | metric | menu | teo | teo-util-aware |
> > > | ------------------------------------- | -------------- | ----------------- | ----------------- |
> > > | gmean score | 2826.5 (0.0%) | 2764.8 (-2.18%) | 2865 (1.36%) |
> > > | gmean power usage [mW] | 2551.4 (0.0%) | 2606.8 (2.17%) | 2722.3 (6.7%) |
> > > | gmean too deep % | 14.99% | 9.65% | 4.02% |
> > > | gmean too shallow % | 2.5% | 5.96% | 14.59% |
> > > | gmean task wakeup latency (asynctask) | 78.16μs (0.0%) | 61.60μs (-21.19%) | 54.45μs (-30.34%) |
> > >
> > > 2. Jankbench (better score, latency & less power usage)
> > >
> > > | metric | menu | teo | teo-util-aware |
> > > | ------------------------------------- | -------------- | ----------------- | ----------------- |
> > > | gmean frame duration | 13.9 (0.0%) | 14.7 (6.0%) | 12.6 (-9.0%) |
> > > | gmean jank percentage | 1.5 (0.0%) | 2.1 (36.99%) | 1.3 (-17.37%) |
> > > | gmean power usage [mW] | 144.6 (0.0%) | 136.9 (-5.27%) | 121.3 (-16.08%) |
> > > | gmean too deep % | 26.00% | 11.00% | 2.54% |
> > > | gmean too shallow % | 4.74% | 11.89% | 21.93% |
> > > | gmean wakeup latency (RenderThread) | 139.5μs (0.0%) | 116.5μs (-16.49%) | 91.11μs (-34.7%) |
> > > | gmean wakeup latency (surfaceflinger) | 124.0μs (0.0%) | 151.9μs (22.47%) | 87.65μs (-29.33%) |
> > >
> > > Signed-off-by: Kajetan Puchalski <kajetan.puchalski@arm.com>
> >
> > This looks good enough for me.
> >
> > There are still a couple of things I would change in it, but I may as
> > well do that when applying it, so never mind.
> >
> > The most important question for now is what the scheduler people think
> > about calling sched_cpu_util() from a CPU idle governor. Peter,
> > Vincent?
>
> I don't see a problem with using sched_cpu_util() outside the
> scheduler as it's already used in thermal and dtpm to get cpu
> utilization.

OK, thanks!

> >
> > > ---
> > > drivers/cpuidle/governors/teo.c | 92 ++++++++++++++++++++++++++++++++-
> > > 1 file changed, 91 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/cpuidle/governors/teo.c b/drivers/cpuidle/governors/teo.c
> > > index e2864474a98d..2a2be4f45b70 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@intel.com>
> > > + *
> > > + * Util-awareness mechanism:
> > > + * Copyright (C) 2022 Arm Ltd.
> > > + * Author: Kajetan Puchalski <kajetan.puchalski@arm.com>
> > > */
> > >
> > > /**
> > > @@ -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 +196,29 @@ 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_cpu_is_utilized - Check if the CPU's util is above the threshold
> > > + * @cpu: Target CPU
> > > + * @cpu_data: Governor CPU data for the target CPU
> > > + */
> > > +#ifdef CONFIG_SMP
> > > +static bool teo_cpu_is_utilized(int cpu, struct teo_cpu *cpu_data)
> > > +{
> > > + return sched_cpu_util(cpu) > cpu_data->util_threshold;
> > > +}
> > > +#else
> > > +static bool teo_cpu_is_utilized(int cpu, struct teo_cpu *cpu_data)
> > > +{
> > > + return false;
> > > +}
> > > +#endif
> > > +
> > > /**
> > > * teo_update - Update CPU metrics after wakeup.
> > > * @drv: cpuidle driver containing state data.
> > > @@ -323,6 +390,20 @@ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
> > > goto end;
> > > }
> > >
> > > + cpu_data->utilized = teo_cpu_is_utilized(dev->cpu, 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 +535,13 @@ 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 +598,11 @@ 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
> > >