[PATCH v2 5/8] sched/topology: Avoid redundant O(N^3) group capacity walks and unused allocations in build_sched_domains()

[sashiko-bot at kernel.org]

> During sequential CPU hotplug on large systems, build_sched_domains() is
> invoked for every CPU offline and online step. Two bottlenecks in the
> domain construction path scale poorly with CPU count:
> 
> 1. On non-overlapping topology levels, all CPUs sharing a domain span
>    share the same circular ring of sched_group instances, and on
>    overlapping NUMA levels, groups with identical spans share
>    sched_group_capacity (sgc). However, init_sched_groups_capacity()
>    walks the entire group ring for every CPU in the domain and repeatedly
>    recomputes group_weight, cores (walking the group cpumask and clearing
>    cpu_smt_mask), and asym_prefer_cpu (O(N^3) bitmap operations per
>    hotplug step). Similarly, init_overlap_sched_group() recomputes
>    build_balance_mask() and cpumask_weight(sg_span) even when the shared
>    sgc was already initialized.
> 
> [ ... ]
>   Total:     7.66 s ->  5.29 s (-30.9%)
>   build_sched_domains() (cpu511 offline / online):
>     11.69 ms -> 5.61 ms (-52.0%) / 12.24 ms -> 5.84 ms (-52.3%)
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin at soleen.com>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261019011446.834487-1-pasha.tatashin@soleen.com?part=5