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

Peter Zijlstra
2019-12-19T10:02:58Z

On Wed, Dec 18, 2019 at 06:50:52PM +0000, Valentin Schneider wrote:
> I'm quite sure you have reasons to have written it that way, but I was
> hoping we could squash it down to something like:
> ---
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 08a233e97a01..f05d09a8452e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8680,16 +8680,27 @@ 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 -
> + busiest->idle_cpus) >> 1);
> }
>
> /*
> - * If there is no overload, we just want to even the number of
> - * idle cpus.
> + * Allow for a small imbalance between NUMA groups; don't do any
> + * of it if there is at least half as many tasks / busy CPUs as
> + * there are available CPUs in the busiest group
> */
> - env->migration_type = migrate_task;
> - env->imbalance = max_t(long, 0, (local->idle_cpus -
> - busiest->idle_cpus) >> 1);
> + if (env->sd->flags & SD_NUMA &&
> + (busiest->sum_nr_running < busiest->group_weight >> 1) &&
> + (env->imbalance < busiest->group_weight * (env->sd->imbalance_pct - 100) / 100))

Note that this form allows avoiding the division. Every time I see that
/100 I'm thinking we should rename and make imbalance_pct a base-2
thing.

> + env->imbalance = 0;
> +
> return;
> }
>