[PATCH v1 7/8] sched/topology: Avoid redundant percpu allocations and bitmap walks in build_sched_domains()

[sashiko-bot at kernel.org]

> During CPU hotplug, build_sched_domains() is invoked on every CPU
> activation and deactivation. Profiling build_sched_domains() on a
> 480-CPU Intel Xeon 6985P-C and a 512-CPU AMD EPYC 9B45 shows several
> sources of avoidable O(N^2) and O(N) work per step:
> 
> 1. build_sched_groups() and build_overlap_sched_groups() iterate over
>    every bit of sched_domain_span(sd) via for_each_cpu_wrap(i, span, cpu)
>    and check cpumask_test_cpu(i, covered) one CPU at a time, even after
>    entire child groups (e.g., 120-CPU NUMA nodes) have been added to
>    covered. Instead, track the remaining uncovered CPUs
>    (cpumask_copy(uncovered, span)), clear each built group's span with
>    cpumask_andnot(), and advance directly to the next uncovered CPU.
> 
> 2. __visit_domain_allocation_hell() and __free_domain_allocs() allocate
>    and free 3 * nr_levels + 2 temporary per-CPU pointer arrays via
> [ ... ]
>   Total:     4.30 s ->  3.46 s (-19.5%)
>   build_sched_domains() (cpu511 offline / online):
>     6.55 ms -> 3.04 ms (-53.7%) / 5.28 ms -> 3.20 ms (-39.4%)
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin at soleen.com>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20261015153331.1872641-1-pasha.tatashin@soleen.com?part=7