Re: [PATCH v3 1/2] sched/fair: Refactor CPU utilization functions

[Vincent Guittot <vincent.guittot@linaro.org>]

On Mon, 15 May 2023 at 13:57, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
>
> There is a lot of code duplication in cpu_util_next() & cpu_util_cfs().
>
> Remove this by allowing cpu_util_next() to be called with p = NULL.
> Rename cpu_util_next() to cpu_util() since the '_next' suffix is no
> longer necessary to distinct cpu utilization related functions.
> Implement cpu_util_cfs(cpu) as cpu_util(cpu, p = NULL, -1).
>
> This will allow to code future related cpu util changes only in one
> place, namely in cpu_util().
>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

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

> ---
>  kernel/sched/fair.c  | 63 ++++++++++++++++++++++++++++++++++----------
>  kernel/sched/sched.h | 47 +--------------------------------
>  2 files changed, 50 insertions(+), 60 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 3f8135d7c89d..9874e28d5e38 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7145,11 +7145,41 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>         return target;
>  }
>
> -/*
> - * Predicts what cpu_util(@cpu) would return if @p was removed from @cpu
> - * (@dst_cpu = -1) or migrated to @dst_cpu.
> - */
> -static unsigned long cpu_util_next(int cpu, struct task_struct *p, int dst_cpu)
> +/**
> + * cpu_util() - Estimates the amount of CPU capacity used by CFS tasks.
> + * @cpu: the CPU to get the utilization for
> + * @p: task for which the CPU utilization should be predicted or NULL
> + * @dst_cpu: CPU @p migrates to, -1 if @p moves from @cpu or @p == NULL
> + *
> + * The unit of the return value must be the same as the one of CPU capacity
> + * so that CPU utilization can be compared with CPU capacity.
> + *
> + * CPU utilization is the sum of running time of runnable tasks plus the
> + * recent utilization of currently non-runnable tasks on that CPU.
> + * It represents the amount of CPU capacity currently used by CFS tasks in
> + * the range [0..max CPU capacity] with max CPU capacity being the CPU
> + * capacity at f_max.
> + *
> + * The estimated CPU utilization is defined as the maximum between CPU
> + * utilization and sum of the estimated utilization of the currently
> + * runnable tasks on that CPU. It preserves a utilization "snapshot" of
> + * previously-executed tasks, which helps better deduce how busy a CPU will
> + * be when a long-sleeping task wakes up. The contribution to CPU utilization
> + * of such a task would be significantly decayed at this point of time.
> + *
> + * CPU utilization can be higher than the current CPU capacity
> + * (f_curr/f_max * max CPU capacity) or even the max CPU capacity because
> + * of rounding errors as well as task migrations or wakeups of new tasks.
> + * CPU utilization has to be capped to fit into the [0..max CPU capacity]
> + * range. Otherwise a group of CPUs (CPU0 util = 121% + CPU1 util = 80%)
> + * could be seen as over-utilized even though CPU1 has 20% of spare CPU
> + * capacity. CPU utilization is allowed to overshoot current CPU capacity
> + * though since this is useful for predicting the CPU capacity required
> + * after task migrations (scheduler-driven DVFS).
> + *
> + * Return: (Estimated) utilization for the specified CPU.
> + */
> +static unsigned long cpu_util(int cpu, struct task_struct *p, int dst_cpu)
>  {
>         struct cfs_rq *cfs_rq = &cpu_rq(cpu)->cfs;
>         unsigned long util = READ_ONCE(cfs_rq->avg.util_avg);
> @@ -7160,9 +7190,9 @@ static unsigned long cpu_util_next(int cpu, struct task_struct *p, int dst_cpu)
>          * contribution. In all the other cases @cpu is not impacted by the
>          * migration so its util_avg is already correct.
>          */
> -       if (task_cpu(p) == cpu && dst_cpu != cpu)
> +       if (p && task_cpu(p) == cpu && dst_cpu != cpu)
>                 lsub_positive(&util, task_util(p));
> -       else if (task_cpu(p) != cpu && dst_cpu == cpu)
> +       else if (p && task_cpu(p) != cpu && dst_cpu == cpu)
>                 util += task_util(p);
>
>         if (sched_feat(UTIL_EST)) {
> @@ -7198,7 +7228,7 @@ static unsigned long cpu_util_next(int cpu, struct task_struct *p, int dst_cpu)
>                  */
>                 if (dst_cpu == cpu)
>                         util_est += _task_util_est(p);
> -               else if (unlikely(task_on_rq_queued(p) || current == p))
> +               else if (p && unlikely(task_on_rq_queued(p) || current == p))
>                         lsub_positive(&util_est, _task_util_est(p));
>
>                 util = max(util, util_est);
> @@ -7207,6 +7237,11 @@ static unsigned long cpu_util_next(int cpu, struct task_struct *p, int dst_cpu)
>         return min(util, capacity_orig_of(cpu));
>  }
>
> +unsigned long cpu_util_cfs(int cpu)
> +{
> +       return cpu_util(cpu, NULL, -1);
> +}
> +
>  /*
>   * cpu_util_without: compute cpu utilization without any contributions from *p
>   * @cpu: the CPU which utilization is requested
> @@ -7224,9 +7259,9 @@ static unsigned long cpu_util_without(int cpu, struct task_struct *p)
>  {
>         /* Task has no contribution or is new */
>         if (cpu != task_cpu(p) || !READ_ONCE(p->se.avg.last_update_time))
> -               return cpu_util_cfs(cpu);
> +               p = NULL;
>
> -       return cpu_util_next(cpu, p, -1);
> +       return cpu_util(cpu, p, -1);
>  }
>
>  /*
> @@ -7273,7 +7308,7 @@ static inline void eenv_task_busy_time(struct energy_env *eenv,
>   * 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
> + * energy cost is removed (by cpu_util()) 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
> @@ -7294,7 +7329,7 @@ static inline void eenv_pd_busy_time(struct energy_env *eenv,
>         int cpu;
>
>         for_each_cpu(cpu, pd_cpus) {
> -               unsigned long util = cpu_util_next(cpu, p, -1);
> +               unsigned long util = cpu_util(cpu, p, -1);
>
>                 busy_time += effective_cpu_util(cpu, util, ENERGY_UTIL, NULL);
>         }
> @@ -7318,7 +7353,7 @@ eenv_pd_max_util(struct energy_env *eenv, struct cpumask *pd_cpus,
>
>         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 util = cpu_util(cpu, p, dst_cpu);
>                 unsigned long cpu_util;
>
>                 /*
> @@ -7464,7 +7499,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>                         if (!cpumask_test_cpu(cpu, p->cpus_ptr))
>                                 continue;
>
> -                       util = cpu_util_next(cpu, p, cpu);
> +                       util = cpu_util(cpu, p, cpu);
>                         cpu_cap = capacity_of(cpu);
>
>                         /*
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index ec7b3e0a2b20..f78c0f85cc76 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2946,53 +2946,8 @@ static inline unsigned long cpu_util_dl(struct rq *rq)
>         return READ_ONCE(rq->avg_dl.util_avg);
>  }
>
> -/**
> - * cpu_util_cfs() - Estimates the amount of CPU capacity used by CFS tasks.
> - * @cpu: the CPU to get the utilization for.
> - *
> - * The unit of the return value must be the same as the one of CPU capacity
> - * so that CPU utilization can be compared with CPU capacity.
> - *
> - * CPU utilization is the sum of running time of runnable tasks plus the
> - * recent utilization of currently non-runnable tasks on that CPU.
> - * It represents the amount of CPU capacity currently used by CFS tasks in
> - * the range [0..max CPU capacity] with max CPU capacity being the CPU
> - * capacity at f_max.
> - *
> - * The estimated CPU utilization is defined as the maximum between CPU
> - * utilization and sum of the estimated utilization of the currently
> - * runnable tasks on that CPU. It preserves a utilization "snapshot" of
> - * previously-executed tasks, which helps better deduce how busy a CPU will
> - * be when a long-sleeping task wakes up. The contribution to CPU utilization
> - * of such a task would be significantly decayed at this point of time.
> - *
> - * CPU utilization can be higher than the current CPU capacity
> - * (f_curr/f_max * max CPU capacity) or even the max CPU capacity because
> - * of rounding errors as well as task migrations or wakeups of new tasks.
> - * CPU utilization has to be capped to fit into the [0..max CPU capacity]
> - * range. Otherwise a group of CPUs (CPU0 util = 121% + CPU1 util = 80%)
> - * could be seen as over-utilized even though CPU1 has 20% of spare CPU
> - * capacity. CPU utilization is allowed to overshoot current CPU capacity
> - * though since this is useful for predicting the CPU capacity required
> - * after task migrations (scheduler-driven DVFS).
> - *
> - * Return: (Estimated) utilization for the specified CPU.
> - */
> -static inline unsigned long cpu_util_cfs(int cpu)
> -{
> -       struct cfs_rq *cfs_rq;
> -       unsigned long util;
> -
> -       cfs_rq = &cpu_rq(cpu)->cfs;
> -       util = READ_ONCE(cfs_rq->avg.util_avg);
>
> -       if (sched_feat(UTIL_EST)) {
> -               util = max_t(unsigned long, util,
> -                            READ_ONCE(cfs_rq->avg.util_est.enqueued));
> -       }
> -
> -       return min(util, capacity_orig_of(cpu));
> -}
> +extern unsigned long cpu_util_cfs(int cpu);
>
>  static inline unsigned long cpu_util_rt(struct rq *rq)
>  {
> --
> 2.25.1
>