Re: [RFC PATCH v2 3/6] sched: Add over-utilization/tipping point indicator

[Joel Fernandes]

Hi,

On Fri, Apr 6, 2018 at 8:36 AM, Dietmar Eggemann
<dietmar.eggemann@xxxxxxx> wrote:
> From: Thara Gopinath <thara.gopinath@xxxxxxxxxx>
>
> Energy-aware scheduling should only operate when the system is not
> overutilized. There must be cpu time available to place tasks based on
> utilization in an energy-aware fashion, i.e. to pack tasks on
> energy-efficient cpus without harming the overall throughput.
>
> In case the system operates above this tipping point the tasks have to
> be placed based on task and cpu load in the classical way of spreading
> tasks across as many cpus as possible.
>
> The point in which a system switches from being not overutilized to
> being overutilized is called the tipping point.
>
> Such a tipping point indicator on a sched domain as the system
> boundary is introduced here. As soon as one cpu of a sched domain is
> overutilized the whole sched domain is declared overutilized as well.
> A cpu becomes overutilized when its utilization is higher that 80%
> (capacity_margin) of its capacity.
>
> The implementation takes advantage of the shared sched domain which is
> shared across all per-cpu views of a sched domain level. The new
> overutilized flag is placed in this shared sched domain.
>
> Load balancing is skipped in case the energy model is present and the
> sched domain is not overutilized because under this condition the
> predominantly load-per-capacity driven load-balancer should not
> interfere with the energy-aware wakeup placement based on utilization.
>
> In case the total utilization of a sched domain is greater than the
> total sched domain capacity the overutilized flag is set at the parent
> sched domain level to let other sched groups help getting rid of the
> overutilization of cpus.
>
> Signed-off-by: Thara Gopinath <thara.gopinath@xxxxxxxxxx>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> ---
>  include/linux/sched/topology.h |  1 +
>  kernel/sched/fair.c            | 62 ++++++++++++++++++++++++++++++++++++++++--
>  kernel/sched/sched.h           |  1 +
>  kernel/sched/topology.c        | 12 +++-----
>  4 files changed, 65 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index 26347741ba50..dd001c232646 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -72,6 +72,7 @@ struct sched_domain_shared {
>         atomic_t        ref;
>         atomic_t        nr_busy_cpus;
>         int             has_idle_cores;
> +       int             overutilized;
>  };
>
>  struct sched_domain {
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0a76ad2ef022..6960e5ef3c14 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5345,6 +5345,28 @@ static inline void hrtick_update(struct rq *rq)
>  }
>  #endif
>
> +#ifdef CONFIG_SMP
> +static inline int cpu_overutilized(int cpu);
> +
> +static inline int sd_overutilized(struct sched_domain *sd)
> +{
> +       return READ_ONCE(sd->shared->overutilized);
> +}
> +
> +static inline void update_overutilized_status(struct rq *rq)
> +{
> +       struct sched_domain *sd;
> +
> +       rcu_read_lock();
> +       sd = rcu_dereference(rq->sd);
> +       if (sd && !sd_overutilized(sd) && cpu_overutilized(rq->cpu))
> +               WRITE_ONCE(sd->shared->overutilized, 1);
> +       rcu_read_unlock();
> +}
> +#else
> +static inline void update_overutilized_status(struct rq *rq) {}
> +#endif /* CONFIG_SMP */
> +
>  /*
>   * The enqueue_task method is called before nr_running is
>   * increased. Here we update the fair scheduling stats and
> @@ -5394,8 +5416,10 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>                 update_cfs_group(se);
>         }
>
> -       if (!se)
> +       if (!se) {
>                 add_nr_running(rq, 1);
> +               update_overutilized_status(rq);
> +       }

I'm wondering if it makes sense for considering scenarios whether
other classes cause CPUs in the domain to go above the tipping point.
Then in that case also, it makes sense to not to do EAS in that domain
because of the overutilization.

I guess task_fits using cpu_util which is PELT only at the moment...
so may require some other method like aggregation of CFS PELT, with
RT-PELT and DL running bw or something.

thanks,

- Joel