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

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