Re: [PATCH v2 9/9] sched/fair: Consider capacity inversion in util_fits_cpu()

[Valentin Schneider <vschneid@redhat.com>]

On 04/08/22 15:36, Qais Yousef wrote:
> We do consider thermal pressure in util_fits_cpu() for uclamp_min only.
> With the exception of the biggest cores which by definition are the max
> performance point of the system and all tasks by definition should fit.
>
> Even under thermal pressure, the capacity of the biggest CPU is the
> highest in the system and should still fit every task. Except when it
> reaches capacity inversion point, then this is no longer true.
>
> We can handle this by using the inverted capacity as capacity_orig in
> util_fits_cpu(). Which not only addresses the problem above, but also
> ensure uclamp_max now considers the inverted capacity. Force fitting
> a task when a CPU is in this adverse state will contribute to making the
> thermal throttling last longer.
>
> Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> ---
>  kernel/sched/fair.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index cb32dc9a057f..77ae343e32a3 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4293,12 +4293,16 @@ static inline int util_fits_cpu(unsigned long util,
>        * For uclamp_max, we can tolerate a drop in performance level as the
>        * goal is to cap the task. So it's okay if it's getting less.
>        *
> -	 * In case of capacity inversion, which is not handled yet, we should
> -	 * honour the inverted capacity for both uclamp_min and uclamp_max all
> -	 * the time.
> +	 * In case of capacity inversion we should honour the inverted capacity
> +	 * for both uclamp_min and uclamp_max all the time.
>        */
> -	capacity_orig = capacity_orig_of(cpu);
> -	capacity_orig_thermal = capacity_orig - arch_scale_thermal_pressure(cpu);
> +	capacity_orig = cpu_in_capacity_inversion(cpu);
> +	if (capacity_orig) {
> +		capacity_orig_thermal = capacity_orig;
> +	} else {
> +		capacity_orig = capacity_orig_of(cpu);
> +		capacity_orig_thermal = capacity_orig - arch_scale_thermal_pressure(cpu);
> +	}
>

IIUC the rq->cpu_capacity_inverted computation in update_cpu_capacity() can be
summarised as:

- If there is a PD with equal cap_orig, but higher effective (orig - thermal)
  capacity
  OR
  there is a PD with pd_cap_orig > cpu_effective_cap:
  rq->cpu_capacity_inverted = capacity_orig - thermal_load_avg(rq)

- Else:
  rq->cpu_capacity_inverted = 0

Then, the code above uses either rq->cpu_capacity_inverted if it is
non-zero, otherwise:

  capacity_orig - arch_scale_thermal_pressure(cpu);

Why use average thermal pressure in one case, and use instantaneous
thermal pressure in the other?

Can't we get rid of rq->cpu_capacity_inverted and replace this whole thing
with an unconditional

  capacity_orig_thermal = capacity_orig_of(cpu) - thermal_load_avg(cpu_rq(cpu));

?

>       /*
>        * We want to force a task to fit a cpu as implied by uclamp_max.
> --
> 2.25.1