Re: [PATCH] sched, fair: Allow a small degree of load imbalance between SD_NUMA domains v2

[Vincent Guittot]

On Fri, 3 Jan 2020 at 15:31, Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Dec 23, 2019 at 02:31:40PM +0100, Vincent Guittot wrote:
> > > +               /* Consider allowing a small imbalance between NUMA groups */
> > > +               if (env->sd->flags & SD_NUMA) {
> > > +                       unsigned int imbalance_adj, imbalance_max;
> > > +
> > > +                       /*
> > > +                        * imbalance_adj is the allowable degree of imbalance
> > > +                        * to exist between two NUMA domains. It's calculated
> > > +                        * relative to imbalance_pct with a minimum of two
> > > +                        * tasks or idle CPUs. The choice of two is due to
> > > +                        * the most basic case of two communicating tasks
> > > +                        * that should remain on the same NUMA node after
> > > +                        * wakeup.
> > > +                        */
> > > +                       imbalance_adj = max(2U, (busiest->group_weight *
> > > +                               (env->sd->imbalance_pct - 100) / 100) >> 1);
> > > +
> > > +                       /*
> > > +                        * Ignore small imbalances unless the busiest sd has
> > > +                        * almost half as many busy CPUs as there are
> > > +                        * available CPUs in the busiest group. Note that
> > > +                        * it is not exactly half as imbalance_adj must be
> > > +                        * accounted for or the two domains do not converge
> > > +                        * as equally balanced if the number of busy tasks is
> > > +                        * roughly the size of one NUMA domain.
> > > +                        */
> > > +                       imbalance_max = (busiest->group_weight >> 1) + imbalance_adj;
> > > +                       if (env->imbalance <= imbalance_adj &&
> >
> > AFAICT, env->imbalance is undefined there. I have tried your patch
> > with the below instead
> >
>
> Even when fixed, other corner cases were hit for parallelised loads that
> benefit from spreading early. At the moment I'm testing a variant of the
> first approach except it adjust small balances at low utilisation and
> otherwise balances at normal. It appears to work for basic communicating
> tasks at relatively low utitisation that fits within a node without
> obviously impacting higher utilisation non-communicating workloads but
> more testing time will be needed to be sure.
>
> It's still based on sum_nr_running as a cut-off instead of idle_cpus as
> at low utilisation, there is not much of a material difference in the
> cut-offs given that either approach can be misleading depending on the
> load of the individual tasks, any cpu binding configurations and the
> degree the tasks are memory-bound.
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ba749f579714..58ba2f0c6363 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8648,10 +8648,6 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>         /*
>          * Try to use spare capacity of local group without overloading it or
>          * emptying busiest.
> -        * XXX Spreading tasks across NUMA nodes is not always the best policy
> -        * and special care should be taken for SD_NUMA domain level before
> -        * spreading the tasks. For now, load_balance() fully relies on
> -        * NUMA_BALANCING and fbq_classify_group/rq to override the decision.
>          */
>         if (local->group_type == group_has_spare) {
>                 if (busiest->group_type > group_fully_busy) {
> @@ -8691,16 +8686,39 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>                         env->migration_type = migrate_task;
>                         lsub_positive(&nr_diff, local->sum_nr_running);
>                         env->imbalance = nr_diff >> 1;
> -                       return;
> -               }
> +               } else {
>
> -               /*
> -                * If there is no overload, we just want to even the number of
> -                * idle cpus.
> -                */
> -               env->migration_type = migrate_task;
> -               env->imbalance = max_t(long, 0, (local->idle_cpus -
> +                       /*
> +                        * If there is no overload, we just want to even the number of
> +                        * idle cpus.
> +                        */
> +                       env->migration_type = migrate_task;
> +                       env->imbalance = max_t(long, 0, (local->idle_cpus -
>                                                  busiest->idle_cpus) >> 1);
> +               }
> +
> +               /* Consider allowing a small imbalance between NUMA groups */
> +               if (env->sd->flags & SD_NUMA) {
> +                       long imbalance_adj, imbalance_max;
> +
> +                       /*
> +                        * imbalance_adj is the allowable degree of imbalance
> +                        * to exist between two NUMA domains. imbalance_pct
> +                        * is used to estimate the number of active tasks
> +                        * needed before memory bandwidth may be as important
> +                        * as memory locality.
> +                        */
> +                       imbalance_adj = (100 / (env->sd->imbalance_pct - 100)) - 1;

This looks weird to me because you use imbalance_pct, which is
meaningful only compare a ratio, to define a number that will be then
compared to a number of tasks without taking into account the weight
of the node. So whatever the node size, 32 or 128 CPUs, the
imbalance_adj will be the same: 3 with the default imbalance_pct of
NUMA level which is 125 AFAICT

> +
> +                       /*
> +                        * Allow small imbalances when the busiest group has
> +                        * low utilisation.
> +                        */
> +                       imbalance_max = imbalance_adj << 1;

Why do you add this shift ?

So according to the above, imbalance_max = 6 whatever the size of the node


Regards,
Vincent

> +                       if (busiest->sum_nr_running < imbalance_max)
> +                               env->imbalance -= min(env->imbalance, imbalance_adj);
> +               }
> +
>                 return;
>         }
>
>
> --
> Mel Gorman
> SUSE Labs