Re: [PATCH v2 1/1] sched/fair: Check if prev_cpu has highest spare cap in feec()

[Vincent Guittot]

On Thu, 6 Oct 2022 at 10:11, Pierre Gondois <pierre.gondois@xxxxxxx> wrote:
>
> When evaluating the CPU candidates in the perf domain (pd) containing
> the previously used CPU (prev_cpu), find_energy_efficient_cpu()
> evaluates the energy of the pd:
> - without the task (base_energy)
> - with the task placed on prev_cpu (if the task fits)
> - with the task placed on the CPU with the highest spare capacity,
>   prev_cpu being excluded from this set
>
> If prev_cpu is already the CPU with the highest spare capacity,
> max_spare_cap_cpu will be the CPU with the second highest spare
> capacity.
>
> On an Arm64 Juno-r2, with a workload of 10 tasks at a 10% duty cycle,
> when prev_cpu and max_spare_cap_cpu are both valid candidates,
> prev_spare_cap > max_spare_cap at ~82%.
> Thus the energy of the pd when placing the task on max_spare_cap_cpu
> is computed with no possible positive outcome 82% most of the time.
>
> Do not consider max_spare_cap_cpu as a valid candidate if
> prev_spare_cap > max_spare_cap.
>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>

nit: the reviewed tag should be after the signed-off

> Signed-off-by: Pierre Gondois <pierre.gondois@xxxxxxx>

Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

> ---
>  kernel/sched/fair.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 914096c5b1ae..bcae7bdd5582 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6900,7 +6900,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>         for (; pd; pd = pd->next) {
>                 unsigned long cpu_cap, cpu_thermal_cap, util;
>                 unsigned long cur_delta, max_spare_cap = 0;
> -               bool compute_prev_delta = false;
> +               unsigned long prev_spare_cap = 0;
>                 int max_spare_cap_cpu = -1;
>                 unsigned long base_energy;
>
> @@ -6944,18 +6944,19 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>
>                         if (cpu == prev_cpu) {
>                                 /* Always use prev_cpu as a candidate. */
> -                               compute_prev_delta = true;
> +                               prev_spare_cap = cpu_cap;
>                         } else if (cpu_cap > max_spare_cap) {
>                                 /*
>                                  * Find the CPU with the maximum spare capacity
> -                                * in the performance domain.
> +                                * among the remaining CPUs in the performance
> +                                * domain.
>                                  */
>                                 max_spare_cap = cpu_cap;
>                                 max_spare_cap_cpu = cpu;
>                         }
>                 }
>
> -               if (max_spare_cap_cpu < 0 && !compute_prev_delta)
> +               if (max_spare_cap_cpu < 0 && prev_spare_cap == 0)
>                         continue;
>
>                 eenv_pd_busy_time(&eenv, cpus, p);
> @@ -6963,7 +6964,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>                 base_energy = compute_energy(&eenv, pd, cpus, p, -1);
>
>                 /* Evaluate the energy impact of using prev_cpu. */
> -               if (compute_prev_delta) {
> +               if (prev_spare_cap > 0) {
>                         prev_delta = compute_energy(&eenv, pd, cpus, p,
>                                                     prev_cpu);
>                         /* CPU utilization has changed */
> @@ -6974,7 +6975,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>                 }
>
>                 /* Evaluate the energy impact of using max_spare_cap_cpu. */
> -               if (max_spare_cap_cpu >= 0) {
> +               if (max_spare_cap_cpu >= 0 && max_spare_cap > prev_spare_cap) {
>                         cur_delta = compute_energy(&eenv, pd, cpus, p,
>                                                    max_spare_cap_cpu);
>                         /* CPU utilization has changed */
> --
> 2.25.1
>