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Date: Thu, 2 Jun 2022 15:16:07 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Vincent Donnefort <vdonnefort@...gle.com>
Cc: peterz@...radead.org, mingo@...hat.com,
linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
morten.rasmussen@....com, chris.redpath@....com,
qperret@...gle.com, tao.zhou@...ux.dev, kernel-team@...roid.com
Subject: Re: [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@...gle.com> wrote:
>
> From: Dietmar Eggemann <dietmar.eggemann@....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@....com>
Reviewed-by: Vincent Guittot <vincent.guittot@...aro.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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