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References: <20211118164240.623551-1-dietmar.eggemann@arm.com>
In-Reply-To: <20211118164240.623551-1-dietmar.eggemann@arm.com>
From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Fri, 19 Nov 2021 09:26:42 +0100
Message-ID: <CAKfTPtC4URVKZvJfgGs9ZaxBr+x1H7239aTYAUi=2gX5c15aqw@mail.gmail.com>
Subject: Re: [PATCH v2] sched/fair: Replace CFS internal cpu_util() with cpu_util_cfs()
To:     Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <Valentin.Schneider@arm.com>,
        linux-kernel@vger.kernel.org
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Precedence: bulk

On Thu, 18 Nov 2021 at 17:42, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
>
> cpu_util_cfs() was created by commit d4edd662ac16 ("sched/cpufreq: Use
> the DEADLINE utilization signal") to enable the access to CPU
> utilization from the Schedutil CPUfreq governor.
>
> Commit a07630b8b2c1 ("sched/cpufreq/schedutil: Use util_est for OPP
> selection") added util_est support later.
>
> The only thing cpu_util() is doing on top of what cpu_util_cfs() already
> does is to clamp the return value to the [0..capacity_orig] capacity
> range of the CPU. Integrating this into cpu_util_cfs() is not harming
> the existing users (Schedutil and CPUfreq cooling (latter via
> sched_cpu_util() wrapper)).
>
> For straightforwardness, prefer to keep using `int cpu` as the function
> parameter over using `struct rq *rq` which might avoid some calls to
> cpu_rq(cpu) -> per_cpu(runqueues, cpu) -> RELOC_HIDE().
> Update cfs_util()'s documentation and reuse it for cpu_util_cfs().
> Remove cpu_util().
>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

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

Thanks

> ---
>  kernel/sched/core.c              |  2 +-
>  kernel/sched/cpufreq_schedutil.c |  2 +-
>  kernel/sched/fair.c              | 71 ++++----------------------------
>  kernel/sched/sched.h             | 44 ++++++++++++++++++--
>  4 files changed, 50 insertions(+), 69 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f2611b9cf503..a86865ebbe2f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7123,7 +7123,7 @@ unsigned long effective_cpu_util(int cpu, unsigned long util_cfs,
>
>  unsigned long sched_cpu_util(int cpu, unsigned long max)
>  {
> -       return effective_cpu_util(cpu, cpu_util_cfs(cpu_rq(cpu)), max,
> +       return effective_cpu_util(cpu, cpu_util_cfs(cpu), max,
>                                   ENERGY_UTIL, NULL);
>  }
>  #endif /* CONFIG_SMP */
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index e7af18857371..26778884d9ab 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -168,7 +168,7 @@ static void sugov_get_util(struct sugov_cpu *sg_cpu)
>
>         sg_cpu->max = max;
>         sg_cpu->bw_dl = cpu_bw_dl(rq);
> -       sg_cpu->util = effective_cpu_util(sg_cpu->cpu, cpu_util_cfs(rq), max,
> +       sg_cpu->util = effective_cpu_util(sg_cpu->cpu, cpu_util_cfs(sg_cpu->cpu), max,
>                                           FREQUENCY_UTIL, NULL);
>  }
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 13950beb01a2..6ddc2013e033 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1502,7 +1502,6 @@ struct task_numa_env {
>
>  static unsigned long cpu_load(struct rq *rq);
>  static unsigned long cpu_runnable(struct rq *rq);
> -static unsigned long cpu_util(int cpu);
>  static inline long adjust_numa_imbalance(int imbalance,
>                                         int dst_running, int dst_weight);
>
> @@ -1569,7 +1568,7 @@ static void update_numa_stats(struct task_numa_env *env,
>
>                 ns->load += cpu_load(rq);
>                 ns->runnable += cpu_runnable(rq);
> -               ns->util += cpu_util(cpu);
> +               ns->util += cpu_util_cfs(cpu);
>                 ns->nr_running += rq->cfs.h_nr_running;
>                 ns->compute_capacity += capacity_of(cpu);
>
> @@ -3240,7 +3239,7 @@ static inline void cfs_rq_util_change(struct cfs_rq *cfs_rq, int flags)
>                  * As is, the util number is not freq-invariant (we'd have to
>                  * implement arch_scale_freq_capacity() for that).
>                  *
> -                * See cpu_util().
> +                * See cpu_util_cfs().
>                  */
>                 cpufreq_update_util(rq, flags);
>         }
> @@ -5509,11 +5508,9 @@ static inline void hrtick_update(struct rq *rq)
>  #endif
>
>  #ifdef CONFIG_SMP
> -static inline unsigned long cpu_util(int cpu);
> -
>  static inline bool cpu_overutilized(int cpu)
>  {
> -       return !fits_capacity(cpu_util(cpu), capacity_of(cpu));
> +       return !fits_capacity(cpu_util_cfs(cpu), capacity_of(cpu));
>  }
>
>  static inline void update_overutilized_status(struct rq *rq)
> @@ -6456,58 +6453,6 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>         return target;
>  }
>
> -/**
> - * cpu_util - Estimates the amount of capacity of a CPU used by CFS tasks.
> - * @cpu: the CPU to get the utilization of
> - *
> - * The unit of the return value must be the one of capacity so we can compare
> - * the utilization with the capacity of the CPU that is available for CFS task
> - * (ie cpu_capacity).
> - *
> - * cfs_rq.avg.util_avg is the sum of running time of runnable tasks plus the
> - * recent utilization of currently non-runnable tasks on a CPU. It represents
> - * the amount of utilization of a CPU in the range [0..capacity_orig] where
> - * capacity_orig is the cpu_capacity available at the highest frequency
> - * (arch_scale_freq_capacity()).
> - * The utilization of a CPU converges towards a sum equal to or less than the
> - * current capacity (capacity_curr <= capacity_orig) of the CPU because it is
> - * the running time on this CPU scaled by capacity_curr.
> - *
> - * The estimated utilization of a CPU is defined to be the maximum between its
> - * cfs_rq.avg.util_avg and the sum of the estimated utilization of the tasks
> - * currently RUNNABLE on that CPU.
> - * This allows to properly represent the expected utilization of a CPU which
> - * has just got a big task running since a long sleep period. At the same time
> - * however it preserves the benefits of the "blocked utilization" in
> - * describing the potential for other tasks waking up on the same CPU.
> - *
> - * Nevertheless, cfs_rq.avg.util_avg can be higher than capacity_curr or even
> - * higher than capacity_orig because of unfortunate rounding in
> - * cfs.avg.util_avg or just after migrating tasks and new task wakeups until
> - * the average stabilizes with the new running time. We need to check that the
> - * utilization stays within the range of [0..capacity_orig] and cap it if
> - * necessary. Without utilization capping, a group could be seen as overloaded
> - * (CPU0 utilization at 121% + CPU1 utilization at 80%) whereas CPU1 has 20% of
> - * available capacity. We allow utilization to overshoot capacity_curr (but not
> - * capacity_orig) as it useful for predicting the capacity required after task
> - * migrations (scheduler-driven DVFS).
> - *
> - * Return: the (estimated) utilization for the specified CPU
> - */
> -static inline unsigned long cpu_util(int cpu)
> -{
> -       struct cfs_rq *cfs_rq;
> -       unsigned int util;
> -
> -       cfs_rq = &cpu_rq(cpu)->cfs;
> -       util = READ_ONCE(cfs_rq->avg.util_avg);
> -
> -       if (sched_feat(UTIL_EST))
> -               util = max(util, READ_ONCE(cfs_rq->avg.util_est.enqueued));
> -
> -       return min_t(unsigned long, util, capacity_orig_of(cpu));
> -}
> -
>  /*
>   * cpu_util_without: compute cpu utilization without any contributions from *p
>   * @cpu: the CPU which utilization is requested
> @@ -6528,7 +6473,7 @@ 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(cpu);
> +               return cpu_util_cfs(cpu);
>
>         cfs_rq = &cpu_rq(cpu)->cfs;
>         util = READ_ONCE(cfs_rq->avg.util_avg);
> @@ -6592,7 +6537,7 @@ static unsigned long cpu_util_without(int cpu, struct task_struct *p)
>         /*
>          * Utilization (estimated) can exceed the CPU capacity, thus let's
>          * clamp to the maximum CPU capacity to ensure consistency with
> -        * the cpu_util call.
> +        * cpu_util.
>          */
>         return min_t(unsigned long, util, capacity_orig_of(cpu));
>  }
> @@ -6624,7 +6569,7 @@ static unsigned long cpu_util_next(int cpu, struct task_struct *p, int dst_cpu)
>                  * During wake-up, the task isn't enqueued yet and doesn't
>                  * appear in the cfs_rq->avg.util_est.enqueued of any rq,
>                  * so just add it (if needed) to "simulate" what will be
> -                * cpu_util() after the task has been enqueued.
> +                * cpu_util after the task has been enqueued.
>                  */
>                 if (dst_cpu == cpu)
>                         util_est += _task_util_est(p);
> @@ -8681,7 +8626,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>                 struct rq *rq = cpu_rq(i);
>
>                 sgs->group_load += cpu_load(rq);
> -               sgs->group_util += cpu_util(i);
> +               sgs->group_util += cpu_util_cfs(i);
>                 sgs->group_runnable += cpu_runnable(rq);
>                 sgs->sum_h_nr_running += rq->cfs.h_nr_running;
>
> @@ -9699,7 +9644,7 @@ static struct rq *find_busiest_queue(struct lb_env *env,
>                         break;
>
>                 case migrate_util:
> -                       util = cpu_util(cpu_of(rq));
> +                       util = cpu_util_cfs(i);
>
>                         /*
>                          * Don't try to pull utilization from a CPU with one
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index f0b249ec581d..2733f15c5859 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2942,16 +2942,52 @@ static inline unsigned long cpu_util_dl(struct rq *rq)
>         return READ_ONCE(rq->avg_dl.util_avg);
>  }
>
> -static inline unsigned long cpu_util_cfs(struct rq *rq)
> +/**
> + * 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)
>  {
> -       unsigned long util = READ_ONCE(rq->cfs.avg.util_avg);
> +       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(rq->cfs.avg.util_est.enqueued));
> +                            READ_ONCE(cfs_rq->avg.util_est.enqueued));
>         }
>
> -       return util;
> +       return min(util, capacity_orig_of(cpu));
>  }
>
>  static inline unsigned long cpu_util_rt(struct rq *rq)
> --
> 2.25.1
>