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

[Mel Gorman <mgorman@techsingularity.net>]

On Thu, Dec 09, 2021 at 02:23:40PM +0000, Valentin Schneider wrote:
> 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 guess we could but I don't see any specific advantage to doing that.

> > I'll test this and should have results tomorrow.
> >

The test results indicated that there was still a problem with
communicating tasks being pulled apart so am testing a new version.

-- 
Mel Gorman
SUSE Labs