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Revert: "cpuidle: teo: Introduce util-awareness"

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This reverts commit 9ce0f7c4bc.

Util-awareness was reported to be too aggressive in selecting shallower
states. Additionally a single threshold was found to not be suitable
for reasoning about sleep length as, for all practical purposes,
almost arbitrary sleep lengths are still possible for any load value.

Fixes: 9ce0f7c4bc ("cpuidle: teo: Introduce util-awareness")
Link: https://patch.msgid.link/20240628095955.34096-2-christian.loehle@arm.com
Reported-by: Qais Yousef <qyousef@layalina.io>
Reported-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Qais Yousef <qyousef@layalina.io>
Tested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Christian Loehle <christian.loehle@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
This commit is contained in:
Christian Loehle 2024-06-28 10:59:53 +01:00 committed by Rafael J. Wysocki
parent 63e6b02f57
commit 0a2998fa48
1 changed files with 0 additions and 105 deletions
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105
drivers/cpuidle/governors/teo.c
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@ -2,13 +2,8 @@
/*
* 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>
*/
/**
@ -104,56 +99,16 @@
* 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 closest shallower idle state will
* be selected instead, as long as 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>
#include "gov.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.
@ -188,7 +143,6 @@ struct teo_bin {
* @next_recent_idx: Index of the next @recent_idx entry to update.
* @recent_idx: Indices of bins corresponding to recent "intercepts".
* @tick_hits: Number of "hits" after TICK_NSEC.
* @util_threshold: Threshold above which the CPU is considered utilized
*/
struct teo_cpu {
s64 time_span_ns;
@ -198,28 +152,10 @@ struct teo_cpu {
int next_recent_idx;
int recent_idx[NR_RECENT];
unsigned int tick_hits;
unsigned long util_threshold;
};
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.
@ -386,7 +322,6 @@ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
int constraint_idx = 0;
int idx0 = 0, idx = -1;
bool alt_intercepts, alt_recent;
bool cpu_utilized;
s64 duration_ns;
int i;
@ -411,32 +346,6 @@ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
if (!dev->states_usage[0].disable)
idx = 0;
cpu_utilized = teo_cpu_is_utilized(dev->cpu, cpu_data);
/*
* If the CPU is being utilized over the threshold and there are only 2
* states to choose from, the metrics need not be considered, so choose
* the shallowest non-polling state and exit.
*/
if (drv->state_count < 3 && cpu_utilized) {
/*
* If state 0 is enabled and it is not a polling one, select it
* right away unless the scheduler tick has been stopped, in
* which case care needs to be taken to leave the CPU in a deep
* enough state in case it is not woken up any time soon after
* all. If state 1 is disabled, though, state 0 must be used
* anyway.
*/
if ((!idx && !(drv->states[0].flags & CPUIDLE_FLAG_POLLING) &&
teo_state_ok(0, drv)) || dev->states_usage[1].disable) {
idx = 0;
goto out_tick;
}
/* Assume that state 1 is not a polling one and use it. */
idx = 1;
duration_ns = drv->states[1].target_residency_ns;
goto end;
}
/* Compute the sums of metrics for early wakeup pattern detection. */
for (i = 1; i < drv->state_count; i++) {
struct teo_bin *prev_bin = &cpu_data->state_bins[i-1];
@ -560,18 +469,6 @@ 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, unless the scheduler tick has
* been stopped already and the shallower state's target residency is
* not sufficiently large.
*/
if (cpu_utilized) {
i = teo_find_shallower_state(drv, dev, idx, KTIME_MAX, true);
if (teo_state_ok(i, drv))
idx = i;
}
/*
* Skip the timers check if state 0 is the current candidate one,
* because an immediate non-timer wakeup is expected in that case.
@ -667,11 +564,9 @@ 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;