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

Vincent Guittot
2020-01-06T13:55:17Z

On Fri, 3 Jan 2020 at 15:31, Mel Gorman <mgorman@techsingularity.net> 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