Re: [PATCH 3/3] sched/numa: Limit the amount of imbalance that can exist at fork time

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

On Tue, 17 Nov 2020 at 16:17, Mel Gorman <mgorman@techsingularity.net> wrote:
>
> On Tue, Nov 17, 2020 at 03:31:19PM +0100, Vincent Guittot wrote:
> > On Tue, 17 Nov 2020 at 15:18, Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Tue, Nov 17, 2020 at 01:42:22PM +0000, Mel Gorman wrote:
> > > > -                     if (local_sgs.idle_cpus)
> > > > +                     if (local_sgs.idle_cpus >= (sd->span_weight >> 2))
> > > >                               return NULL;
> > >
> > > Is that the same 25% ?
> >
> > same question for me
>
> It's the same 25%. It's in the comment as -- utilisation is not too high

utilization is misleading, it usually refers to pelt utilization
whereas whet you check is the number of busy cpus

> where "high" is related to adjust_numa_imbalance.
>
> > could we encapsulate this 25% allowed imbalance like for adjust_numa_imbalance
>
> Using adjust_numa_imbalance() directly in this context would be awkward

Would be good to use the same function to say if we allow or not the imbalance

something like numa_allow_imbalance(sg_lb_stats * group_stats)

which would return how much margin remains available before not
allowing the imbalance

and use the same metrics in all cases


> but the threshold could be shared with something like the additional
> diff below. Is that what you had in mind or something different?
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index adfab218a498..49ef3484c56c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5878,6 +5878,8 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p,
>  static struct sched_group *
>  find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu);
>
> +static inline int numa_imbalance_threshold(int weight);
> +
>  /*
>   * find_idlest_group_cpu - find the idlest CPU among the CPUs in the group.
>   */
> @@ -8894,7 +8896,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
>                          * If there is a real need of migration, periodic load
>                          * balance will take care of it.
>                          */
> -                       if (local_sgs.idle_cpus >= (sd->span_weight >> 2))

also here you use idle_cpus and on the other part you use nr_running.
Can't we use the same metrics on both sides?

> +                       if (local_sgs.idle_cpus >= numa_imbalance_threshold(sd->span_weight))
>                                 return NULL;
>                 }
>
> @@ -8998,6 +9000,14 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>
>  #define NUMA_IMBALANCE_MIN 2
>
> +/* Allows imbalances until active CPUs hits 25% of a domain */
> +#define IMBALANCE_THRESHOLD_SHIFT 2
> +
> +static inline int numa_imbalance_threshold(int weight)
> +{
> +       return weight >> IMBALANCE_THRESHOLD_SHIFT;
> +}
> +
>  static inline long adjust_numa_imbalance(int imbalance,
>                                 int dst_running, int dst_weight)
>  {
> @@ -9006,8 +9016,10 @@ static inline long adjust_numa_imbalance(int imbalance,
>          * when the destination is lightly loaded so that pairs of
>          * communicating tasks may remain local.
>          */
> -       if (dst_running < (dst_weight >> 2) && imbalance <= NUMA_IMBALANCE_MIN)
> +       if (dst_running < numa_imbalance_threshold(dst_weight) &&
> +           imbalance <= NUMA_IMBALANCE_MIN) {
>                 return 0;
> +       }
>
>         return imbalance;
>  }
>
> --
> Mel Gorman
> SUSE Labs