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    SubjectRe: [PATCH v9 5/7] sched/fair: Use the same cpumask per-PD throughout find_energy_efficient_cpu()
    On Mon, 23 May 2022 at 17:52, Vincent Donnefort <vdonnefort@google.com> wrote:
    >
    > From: Dietmar Eggemann <dietmar.eggemann@arm.com>
    >
    > 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@arm.com>

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

    >
    > 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
    >

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