Re: [PATCH 2/2] sched/fair: Adjust the allowed NUMA imbalance when SD_NUMA spans multiple LLCs

[Valentin Schneider <valentin.schneider@arm.com>]

On 06/12/21 15:12, Mel Gorman wrote:
> Gautham had similar reasoning to calculate the imbalance at each
> higher-level domain instead of using a static value throughout and
> it does make sense. For each level and splitting the imbalance between
> two domains, this works out as
>
>
>       /*
>        * Calculate an allowed NUMA imbalance such that LLCs do not get
>        * imbalanced.
>        */
>       for_each_cpu(i, cpu_map) {
>               for (sd = *per_cpu_ptr(d.sd, i); sd; sd = sd->parent) {
>                       struct sched_domain *child = sd->child;
>
>                       if (!(sd->flags & SD_SHARE_PKG_RESOURCES) && child &&
>                           (child->flags & SD_SHARE_PKG_RESOURCES)) {
>                               struct sched_domain *top = sd;
>                               unsigned int llc_sq;
>
>                               /*
>                                * nr_llcs = (top->span_weight / llc_weight);
>                                * imb = (child_weight / nr_llcs) >> 1
>                                *
>                                * is equivalent to
>                                *
>                                * imb = (llc_weight^2 / top->span_weight) >> 1
>                                *
>                                */
>                               llc_sq = child->span_weight * child->span_weight;
>                               while (top) {
>                                       top->imb_numa_nr = max(1U,
>                                               (llc_sq / top->span_weight) >> 1);
>                                       top = top->parent;
>                               }
>
>                               break;
>                       }
>               }
>       }
>

IIRC Peter suggested punting that logic to before domains get degenerated,
but I don't see how that helps here. If you just want to grab the LLC
domain (aka highest_flag_domain(cpu, SD_SHARE_PKG_RESOURCES)) and compare
its span with that of its parents, that can happen after the degeneration,
no?

> I'll test this and should have results tomorrow.
>
> --
> Mel Gorman
> SUSE Labs