linux-kernel - Re: [PATCH v4] sched/fair: unlink misfit task from cpu overutilized

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Date:   Thu, 26 Jan 2023 12:42:01 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...nel.org,
        peterz@...radead.org, qyousef@...alina.io, rafael@...nel.org,
        viresh.kumar@...aro.org, vschneid@...hat.com,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     lukasz.luba@....com, wvw@...gle.com, xuewen.yan94@...il.com,
        han.lin@...iatek.com, Jonathan.JMChen@...iatek.com
Subject: Re: [PATCH v4] sched/fair: unlink misfit task from cpu overutilized

On 19/01/2023 17:42, Vincent Guittot wrote:
> By taking into account uclamp_min, the 1:1 relation between task misfit
> and cpu overutilized is no more true as a task with a small util_avg may
> not fit a high capacity cpu because of uclamp_min constraint.
> 
> Add a new state in util_fits_cpu() to reflect the case that task would fit
> a CPU except for the uclamp_min hint which is a performance requirement.
> 
> Use -1 to reflect that a CPU doesn't fit only because of uclamp_min so we
> can use this new value to take additional action to select the best CPU
> that doesn't match uclamp_min hint.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
> 
> Change since v3:
> - Keep current condition for uclamp_max_fits in util_fits_cpu()
> - Update some comments

We had already this discussion whether this patch can also remove
Capacity Inversion (CapInv).

After studying the code again, I'm not so sure anymore.

This patch:

(1) adds a dedicated return value (-1) to util_fits_cpu() when:

`util fits 80% capacity_of() && util < uclamp_min && uclamp_min >
capacity_orig_thermal (region c)`

(2) Enhancements to the CPU selection in sic() and feec() to cater for
this new return value.

IMHO this doesn't make the intention of CapInv in util_fits_cpu()
obsolete, which is using:

in CapInv:

  capacity_orig         = capacity_orig_of() - thermal_load_avg
  capacity_orig_thermal = capacity_orig_of() - thermal_load_avg

not in CapInv:

  capacity_orig         = capacity_orig_of()
  capacity_orig_thermal = capacity_orig_of() - th_pressure

Maybe I still miss a bit of the story?

v3 hints to removing the bits in the next version:

https://lkml.kernel.org/r/20230115001906.v7uq4ddodrbvye7d@airbuntu

>  kernel/sched/fair.c | 105 ++++++++++++++++++++++++++++++++++----------
>  1 file changed, 82 insertions(+), 23 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d4db72f8f84e..54e14da53274 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4561,8 +4561,8 @@ static inline int util_fits_cpu(unsigned long util,
>  	 * handle the case uclamp_min > uclamp_max.
>  	 */
>  	uclamp_min = min(uclamp_min, uclamp_max);
> -	if (util < uclamp_min && capacity_orig != SCHED_CAPACITY_SCALE)
> -		fits = fits && (uclamp_min <= capacity_orig_thermal);
> +	if (fits && (util < uclamp_min) && (uclamp_min > capacity_orig_thermal))
> +		return -1;

Or does the definition 'return -1 if util fits but uclamp doesn't' make
the distinction between capacity_orig and capacity_orig_thermal obsolete
and so CapInv?

[...]

>  static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
> @@ -6138,6 +6142,7 @@ static inline bool cpu_overutilized(int cpu)
>  	unsigned long rq_util_min = uclamp_rq_get(cpu_rq(cpu), UCLAMP_MIN);
>  	unsigned long rq_util_max = uclamp_rq_get(cpu_rq(cpu), UCLAMP_MAX);
>  
> +	/* Return true only if the utilization doesn't fits CPU's capacity */

small typo: s/doesn't fits/doesn't fit

[...]

> @@ -6946,12 +6952,28 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
>  
>  		if (!available_idle_cpu(cpu) && !sched_idle_cpu(cpu))
>  			continue;
> -		if (util_fits_cpu(task_util, util_min, util_max, cpu))
> +
> +		fits = util_fits_cpu(task_util, util_min, util_max, cpu);
> +
> +		/* This CPU fits with all requirements */
> +		if (fits > 0)
>  			return cpu;
> +		/*
> +		 * Only the min performance hint (i.e. uclamp_min) doesn't fit.
> +		 * Look for the CPU with best capacity.
> +		 */
> +		else if (fits < 0)
> +			cpu_cap = capacity_orig_of(cpu) - thermal_load_avg(cpu_rq(cpu));

Still don't grasp why we use thermal_load_avg() here? Looks to me that
this would only match the CapInv case in util_fits_cpu().

[...]