Date:   Thu, 2 Jun 2022 15:58:23 +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,
        Vincent Donnefort <vincent.donnefort@....com>
Subject: Re: [PATCH v9 6/7] sched/fair: Remove task_util from effective
 utilization in feec()

On Mon, 23 May 2022 at 17:52, Vincent Donnefort <vdonnefort@...gle.com> wrote:
>
> From: Vincent Donnefort <vincent.donnefort@....com>
>
> The energy estimation in find_energy_efficient_cpu() (feec()) relies on
> the computation of the effective utilization for each CPU of a perf domain
> (PD). This effective utilization is then used as an estimation of the busy
> time for this pd. The function effective_cpu_util() which gives this value,
> scales the utilization relative to IRQ pressure on the CPU to take into
> account that the IRQ time is hidden from the task clock. The IRQ scaling is
> as follow:
>
>    effective_cpu_util = irq + (cpu_cap - irq)/cpu_cap * util
>
> Where util is the sum of CFS/RT/DL utilization, cpu_cap the capacity of
> the CPU and irq the IRQ avg time.
>
> If now we take as an example a task placement which doesn't raise the OPP
> on the candidate CPU, we can write the energy delta as:
>
>   delta = OPPcost/cpu_cap * (effective_cpu_util(cpu_util + task_util) -
>                              effective_cpu_util(cpu_util))
>         = OPPcost/cpu_cap * (cpu_cap - irq)/cpu_cap * task_util
>
> We end-up with an energy delta depending on the IRQ avg time, which is a
> problem: first the time spent on IRQs by a CPU has no effect on the
> additional energy that would be consumed by a task. Second, we don't want
> to favour a CPU with a higher IRQ avg time value.
>
> Nonetheless, we need to take the IRQ avg time into account. If a task
> placement raises the PD's frequency, it will increase the energy cost for
> the entire time where the CPU is busy. A solution is to only use
> effective_cpu_util() with the CPU contribution part. The task contribution
> is added separately and scaled according to prev_cpu's IRQ time.
>
> No change for the FREQUENCY_UTIL component of the energy estimation. We
> still want to get the actual frequency that would be selected after the
> task placement.
>
> Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
> Signed-off-by: Vincent Donnefort <vdonnefort@...gle.com>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 57074f27c0d2..5586b6848858 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6693,61 +6693,96 @@ static unsigned long cpu_util_without(int cpu, struct task_struct *p)
>  }
>
>  /*
> - * compute_energy(): Estimates the energy that @pd would consume if @p was
> - * migrated to @dst_cpu. compute_energy() predicts what will be the utilization
> - * landscape of @pd's CPUs after the task migration, and uses the Energy Model
> - * to compute what would be the energy if we decided to actually migrate that
> - * task.
> + * energy_env - Utilization landscape for energy estimation.
> + * @task_busy_time: Utilization contribution by the task for which we test the
> + *                  placement. Given by eenv_task_busy_time().
> + * @pd_busy_time:   Utilization of the whole perf domain without the task
> + *                  contribution. Given by eenv_pd_busy_time().
> + * @cpu_cap:        Maximum CPU capacity for the perf domain.
> + * @pd_cap:         Entire perf domain capacity. (pd->nr_cpus * cpu_cap).
> + */
> +struct energy_env {
> +       unsigned long task_busy_time;
> +       unsigned long pd_busy_time;
> +       unsigned long cpu_cap;
> +       unsigned long pd_cap;
> +};
> +
> +/*
> + * Compute the task busy time for compute_energy(). This time cannot be
> + * injected directly into effective_cpu_util() because of the IRQ scaling.
> + * The latter only makes sense with the most recent CPUs where the task has
> + * run.
> + */
> +static inline void eenv_task_busy_time(struct energy_env *eenv,
> +                                      struct task_struct *p, int prev_cpu)
> +{
> +       unsigned long busy_time, max_cap = arch_scale_cpu_capacity(prev_cpu);
> +       unsigned long irq = cpu_util_irq(cpu_rq(prev_cpu));
> +
> +       if (unlikely(irq >= max_cap))
> +               busy_time = max_cap;
> +       else
> +               busy_time = scale_irq_capacity(task_util_est(p), irq, max_cap);
> +
> +       eenv->task_busy_time = busy_time;
> +}
> +
> +/*
> + * Compute the perf_domain (PD) busy time for compute_energy(). Based on the
> + * utilization for each @pd_cpus, it however doesn't take into account
> + * clamping since the ratio (utilization / cpu_capacity) is already enough to
> + * scale the EM reported power consumption at the (eventually clamped)
> + * cpu_capacity.
> + *
> + * The contribution of the task @p for which we want to estimate the
> + * energy cost is removed (by cpu_util_next()) and must be calculated
> + * separately (see eenv_task_busy_time). This ensures:
> + *
> + *   - A stable PD utilization, no matter which CPU of that PD we want to place
> + *     the task on.
> + *
> + *   - A fair comparison between CPUs as the task contribution (task_util())
> + *     will always be the same no matter which CPU utilization we rely on
> + *     (util_avg or util_est).
> + *
> + * Set @eenv busy time for the PD that spans @pd_cpus. This busy time can't
> + * exceed @eenv->pd_cap.
>   */
> -static long
> -compute_energy(struct task_struct *p, int dst_cpu, struct cpumask *cpus,
> -              struct perf_domain *pd)
> +static inline void eenv_pd_busy_time(struct energy_env *eenv,
> +                                    struct cpumask *pd_cpus,
> +                                    struct task_struct *p)
>  {
> -       unsigned long max_util = 0, sum_util = 0, cpu_cap;
> +       unsigned long busy_time = 0;
>         int cpu;
>
> -       cpu_cap = arch_scale_cpu_capacity(cpumask_first(cpus));
> -       cpu_cap -= arch_scale_thermal_pressure(cpumask_first(cpus));
> +       for_each_cpu(cpu, pd_cpus) {
> +               unsigned long util = cpu_util_next(cpu, p, -1);
>
> -       /*
> -        * The capacity state of CPUs of the current rd can be driven by CPUs
> -        * of another rd if they belong to the same pd. So, account for the
> -        * utilization of these CPUs too by masking pd with cpu_online_mask
> -        * instead of the rd span.
> -        *
> -        * 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(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;
> +               busy_time += effective_cpu_util(cpu, util, ENERGY_UTIL, NULL);
> +       }
>
> -               /*
> -                * When @p is placed on @cpu:
> -                *
> -                * util_running = max(cpu_util, cpu_util_est) +
> -                *                max(task_util, _task_util_est)
> -                *
> -                * while cpu_util_next is: max(cpu_util + task_util,
> -                *                             cpu_util_est + _task_util_est)
> -                */
> -               if (cpu == dst_cpu) {
> -                       tsk = p;
> -                       util_running =
> -                               cpu_util_next(cpu, p, -1) + task_util_est(p);
> -               }
> +       eenv->pd_busy_time = min(eenv->pd_cap, busy_time);
> +}
>
> -               /*
> -                * Busy time computation: utilization clamping is not
> -                * required since the ratio (sum_util / cpu_capacity)
> -                * is already enough to scale the EM reported power
> -                * consumption at the (eventually clamped) cpu_capacity.
> -                */
> -               cpu_util = effective_cpu_util(cpu, util_running, ENERGY_UTIL,
> -                                             NULL);
> +/*
> + * Compute the maximum utilization for compute_energy() when the task @p
> + * is placed on the cpu @dst_cpu.
> + *
> + * Returns the maximum utilization among @eenv->cpus. This utilization can't
> + * exceed @eenv->cpu_cap.
> + */
> +static inline unsigned long
> +eenv_pd_max_util(struct energy_env *eenv, struct cpumask *pd_cpus,
> +                struct task_struct *p, int dst_cpu)
> +{
> +       unsigned long max_util = 0;
> +       int cpu;
>
> -               sum_util += min(cpu_util, cpu_cap);
> +       for_each_cpu(cpu, pd_cpus) {
> +               struct task_struct *tsk = (cpu == dst_cpu) ? p : NULL;
> +               unsigned long util = cpu_util_next(cpu, p, dst_cpu);
> +               unsigned long cpu_util;
>
>                 /*
>                  * Performance domain frequency: utilization clamping
> @@ -6756,12 +6791,29 @@ compute_energy(struct task_struct *p, int dst_cpu, struct cpumask *cpus,
>                  * NOTE: in case RT tasks are running, by default the
>                  * FREQUENCY_UTIL's utilization can be max OPP.
>                  */
> -               cpu_util = effective_cpu_util(cpu, util_freq, FREQUENCY_UTIL,
> -                                             tsk);
> -               max_util = max(max_util, min(cpu_util, cpu_cap));
> +               cpu_util = effective_cpu_util(cpu, util, FREQUENCY_UTIL, tsk);
> +               max_util = max(max_util, cpu_util);
>         }
>
> -       return em_cpu_energy(pd->em_pd, max_util, sum_util, cpu_cap);
> +       return min(max_util, eenv->cpu_cap);
> +}
> +
> +/*
> + * compute_energy(): Use the Energy Model to estimate the energy that @pd would
> + * consume for a given utilization landscape @eenv. If @dst_cpu < 0 the task

I find this comment a bit confusing because compute_energy() adds the
task contribution if dst_cpu >= 0 but doesn't remove it. The fact that
eenv->pd_busy_time has been previously computed without the
contribution of the task, is outside the scope of this this function
whereas the comment suggest that the remove will happen in
compute_energy()

> + * contribution is removed from the energy estimation.
> + */
> +static inline unsigned long
> +compute_energy(struct energy_env *eenv, struct perf_domain *pd,
> +              struct cpumask *pd_cpus, struct task_struct *p, int dst_cpu)
> +{
> +       unsigned long max_util = eenv_pd_max_util(eenv, pd_cpus, p, dst_cpu);
> +       unsigned long busy_time = eenv->pd_busy_time;
> +
> +       if (dst_cpu >= 0)
> +               busy_time = min(eenv->pd_cap, busy_time + eenv->task_busy_time);
> +
> +       return em_cpu_energy(pd->em_pd, max_util, busy_time, eenv->cpu_cap);
>  }
>
>  /*
> @@ -6807,11 +6859,12 @@ 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;
> -       unsigned long cpu_cap, util, base_energy = 0;
> +       struct root_domain *rd = this_rq()->rd;
> +       unsigned long base_energy = 0;
>         struct sched_domain *sd;
>         struct perf_domain *pd;
> +       struct energy_env eenv;
>
>         rcu_read_lock();
>         pd = rcu_dereference(rd->pd);
> @@ -6834,22 +6887,36 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>         if (!task_util_est(p))
>                 goto unlock;
>
> +       eenv_task_busy_time(&eenv, p, prev_cpu);
> +
>         for (; pd; pd = pd->next) {
> -               unsigned long cur_delta, spare_cap, max_spare_cap = 0;
> +               unsigned long cpu_cap, cpu_thermal_cap, util;
> +               unsigned long cur_delta, max_spare_cap = 0;
>                 bool compute_prev_delta = false;
>                 unsigned long base_energy_pd;
>                 int max_spare_cap_cpu = -1;
>
>                 cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask);
>
> -               for_each_cpu_and(cpu, cpus, sched_domain_span(sd)) {
> +               /* Account thermal pressure for the energy estimation */
> +               cpu = cpumask_first(cpus);
> +               cpu_thermal_cap = arch_scale_cpu_capacity(cpu);
> +               cpu_thermal_cap -= arch_scale_thermal_pressure(cpu);
> +
> +               eenv.cpu_cap = cpu_thermal_cap;
> +               eenv.pd_cap = 0;
> +
> +               for_each_cpu(cpu, cpus) {
> +                       eenv.pd_cap += cpu_thermal_cap;
> +
> +                       if (!cpumask_test_cpu(cpu, sched_domain_span(sd)))
> +                               continue;
> +
>                         if (!cpumask_test_cpu(cpu, p->cpus_ptr))
>                                 continue;
>
>                         util = cpu_util_next(cpu, p, cpu);
>                         cpu_cap = capacity_of(cpu);
> -                       spare_cap = cpu_cap;
> -                       lsub_positive(&spare_cap, util);
>
>                         /*
>                          * Skip CPUs that cannot satisfy the capacity request.
> @@ -6862,15 +6929,17 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>                         if (!fits_capacity(util, cpu_cap))
>                                 continue;
>
> +                       lsub_positive(&cpu_cap, util);
> +
>                         if (cpu == prev_cpu) {
>                                 /* Always use prev_cpu as a candidate. */
>                                 compute_prev_delta = true;
> -                       } else if (spare_cap > max_spare_cap) {
> +                       } else if (cpu_cap > max_spare_cap) {
>                                 /*
>                                  * Find the CPU with the maximum spare capacity
>                                  * in the performance domain.
>                                  */
> -                               max_spare_cap = spare_cap;
> +                               max_spare_cap = cpu_cap;
>                                 max_spare_cap_cpu = cpu;
>                         }
>                 }
> @@ -6878,13 +6947,15 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>                 if (max_spare_cap_cpu < 0 && !compute_prev_delta)
>                         continue;
>
> +               eenv_pd_busy_time(&eenv, cpus, p);
>                 /* Compute the 'base' energy of the pd, without @p */
> -               base_energy_pd = compute_energy(p, -1, cpus, pd);
> +               base_energy_pd = compute_energy(&eenv, pd, cpus, p, -1);
>                 base_energy += base_energy_pd;
>
>                 /* Evaluate the energy impact of using prev_cpu. */
>                 if (compute_prev_delta) {
> -                       prev_delta = compute_energy(p, prev_cpu, cpus, pd);
> +                       prev_delta = compute_energy(&eenv, pd, cpus, p,
> +                                                   prev_cpu);
>                         if (prev_delta < base_energy_pd)

side question:
-base_energy_pd is the energy for the perf domain without task p
-prev_delta is the energy for the same perf domain if task p is put on dst_cpu

How can prev_delta be lower than base_energy ?

if dst_cpu doesn't belong to the perf domain, prev_delta should be
equal to base_energy_pd
if dst_cpu belongs to the perf domain, the compute_energy should be
higher because the busy_time will be higher

>                                 goto unlock;
>                         prev_delta -= base_energy_pd;
> @@ -6893,8 +6964,8 @@ 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, cpus,
> -                                                  pd);
> +                       cur_delta = compute_energy(&eenv, pd, cpus, p,
> +                                                  max_spare_cap_cpu);
>                         if (cur_delta < base_energy_pd)

same question as above

>                                 goto unlock;
>                         cur_delta -= base_energy_pd;
> --
> 2.36.1.124.g0e6072fb45-goog
>

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