Re: [PATCH v9 5/7] sched/fair: Use the same cpumask per-PD throughout find_energy_efficient_cpu()

[Vincent Guittot]

On Mon, 23 May 2022 at 17:52, Vincent Donnefort <vdonnefort@xxxxxxxxxx> wrote:
>
> From: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
>
> The Perf Domain (PD) cpumask (struct em_perf_domain.cpus) stays
> invariant after Energy Model creation, i.e. it is not updated after
> CPU hotplug operations.
>
> That's why the PD mask is used in conjunction with the cpu_online_mask
> (or Sched Domain cpumask). Thereby the cpu_online_mask is fetched
> multiple times (in compute_energy()) during a run-queue selection
> for a task.
>
> cpu_online_mask may change during this time which can lead to wrong
> energy calculations.
>
> To be able to avoid this, use the select_rq_mask per-cpu cpumask to
> create a cpumask out of PD cpumask and cpu_online_mask and pass it
> through the function calls of the EAS run-queue selection path.
>
> The PD cpumask for max_spare_cap_cpu/compute_prev_delta selection
> (find_energy_efficient_cpu()) is now ANDed not only with the SD mask
> but also with the cpu_online_mask. This is fine since this cpumask
> has to be in syc with the one used for energy computation
> (compute_energy()).
> An exclusive cpuset setup with at least one asymmetric CPU capacity
> island (hence the additional AND with the SD cpumask) is the obvious
> exception here.
>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>

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

>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2d7bba2f1da2..57074f27c0d2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6700,14 +6700,14 @@ static unsigned long cpu_util_without(int cpu, struct task_struct *p)
>   * task.
>   */
>  static long
> -compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
> +compute_energy(struct task_struct *p, int dst_cpu, struct cpumask *cpus,
> +              struct perf_domain *pd)
>  {
> -       struct cpumask *pd_mask = perf_domain_span(pd);
>         unsigned long max_util = 0, sum_util = 0, cpu_cap;
>         int cpu;
>
> -       cpu_cap = arch_scale_cpu_capacity(cpumask_first(pd_mask));
> -       cpu_cap -= arch_scale_thermal_pressure(cpumask_first(pd_mask));
> +       cpu_cap = arch_scale_cpu_capacity(cpumask_first(cpus));
> +       cpu_cap -= arch_scale_thermal_pressure(cpumask_first(cpus));
>
>         /*
>          * The capacity state of CPUs of the current rd can be driven by CPUs
> @@ -6718,7 +6718,7 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
>          * If an entire pd is outside of the current rd, it will not appear in
>          * its pd list and will not be accounted by compute_energy().
>          */
> -       for_each_cpu_and(cpu, pd_mask, cpu_online_mask) {
> +       for_each_cpu(cpu, cpus) {
>                 unsigned long util_freq = cpu_util_next(cpu, p, dst_cpu);
>                 unsigned long cpu_util, util_running = util_freq;
>                 struct task_struct *tsk = NULL;
> @@ -6805,6 +6805,7 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
>   */
>  static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>  {
> +       struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_rq_mask);
>         unsigned long prev_delta = ULONG_MAX, best_delta = ULONG_MAX;
>         struct root_domain *rd = cpu_rq(smp_processor_id())->rd;
>         int cpu, best_energy_cpu = prev_cpu, target = -1;
> @@ -6839,7 +6840,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>                 unsigned long base_energy_pd;
>                 int max_spare_cap_cpu = -1;
>
> -               for_each_cpu_and(cpu, perf_domain_span(pd), sched_domain_span(sd)) {
> +               cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask);
> +
> +               for_each_cpu_and(cpu, cpus, sched_domain_span(sd)) {
>                         if (!cpumask_test_cpu(cpu, p->cpus_ptr))
>                                 continue;
>
> @@ -6876,12 +6879,12 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>                         continue;
>
>                 /* Compute the 'base' energy of the pd, without @p */
> -               base_energy_pd = compute_energy(p, -1, pd);
> +               base_energy_pd = compute_energy(p, -1, cpus, pd);
>                 base_energy += base_energy_pd;
>
>                 /* Evaluate the energy impact of using prev_cpu. */
>                 if (compute_prev_delta) {
> -                       prev_delta = compute_energy(p, prev_cpu, pd);
> +                       prev_delta = compute_energy(p, prev_cpu, cpus, pd);
>                         if (prev_delta < base_energy_pd)
>                                 goto unlock;
>                         prev_delta -= base_energy_pd;
> @@ -6890,7 +6893,8 @@ 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) {
> -                       cur_delta = compute_energy(p, max_spare_cap_cpu, pd);
> +                       cur_delta = compute_energy(p, max_spare_cap_cpu, cpus,
> +                                                  pd);
>                         if (cur_delta < base_energy_pd)
>                                 goto unlock;
>                         cur_delta -= base_energy_pd;
> --
> 2.36.1.124.g0e6072fb45-goog
>