Re: [PATCHv2 1/3] Revert: "cpuidle: teo: Introduce util-awareness"

[Qais Yousef <qyousef@layalina.io>]

On 06/11/24 12:24, Christian Loehle wrote:
> This reverts commit 9ce0f7c4bc64d820b02a1c53f7e8dba9539f942b.
> 
> 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: 9ce0f7c4bc64 ("cpuidle: teo: Introduce util-awareness")
> Reported-by: Qais Yousef <qyousef@layalina.io>
> Reported-by: Vincent Guittot <vincent.guittot@linaro.org>
> Signed-off-by: Christian Loehle <christian.loehle@arm.com>
> ---

Reviewed-by: Qais Yousef <qyousef@layalina.io>

Thanks!

--
Qais Yousef

>  drivers/cpuidle/governors/teo.c | 100 --------------------------------
>  1 file changed, 100 deletions(-)
> 
> diff --git a/drivers/cpuidle/governors/teo.c b/drivers/cpuidle/governors/teo.c
> index 7244f71c59c5..d8554c20cf10 100644
> --- a/drivers/cpuidle/governors/teo.c
> +++ b/drivers/cpuidle/governors/teo.c
> @@ -104,56 +104,16 @@
>   *      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 +148,6 @@ 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 +157,10 @@ 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 +327,6 @@ 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 +351,6 @@ 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 +474,6 @@ 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 +569,9 @@ 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;
> -- 
> 2.34.1
>