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

[Joel Fernandes]

On Fri, Apr 20, 2018 at 1:13 AM, Joel Fernandes <joelaf@xxxxxxxxxx> wrote:
> On Wed, Apr 18, 2018 at 4:17 AM, Quentin Perret <quentin.perret@xxxxxxx> wrote:
>> On Friday 13 Apr 2018 at 16:56:39 (-0700), Joel Fernandes wrote:
>>> 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.
>>>
>>
>> So at the moment in cpu_overutilized() we comapre cpu_util() to
>> capacity_of() which should include RT and IRQ pressure IIRC. But
>> you're right, we might be able to do more here... Perhaps we
>> could also use cpu_util_dl() which is available in sched.h now ?
>
> Yes, should be Ok, and then when RT utilization stuff is available,
> then that can be included in the equation as well (probably for now
> you could use rt_avg).
>
> Another crazy idea is to check the contribution of higher classes in
> one-shot with (capacity_orig_of - capacity_of) although I think that
> method would be less instantaneous/accurate.

Just to add to the last point, the capacity_of also factors in the IRQ
contribution if I remember correctly, which is probably a good thing?

- Joel