cpumask_local_spread(9)

💅 cpumask_local_spread(9) — a Kernel API Manual moment

okay so bestie, welcome to the Kernel API Manual 💖 this one's called cpumask_local_spread and it's genuinely iconic once u get it, i promise.

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✨ NAME ✨

cpumask_local_spread — pick the i'th cpu based on NUMA distances 🗺️

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🌸 SYNOPSIS 🌸

unsigned int cpumask_local_spread (unsigned int i , int node );

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🎀 ARGUMENTS 🎀

• i — the index number, like which one u want in the lineup 💅
• node — the local numa_node, aka ur home base 🏠

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🦋 RETURN 🦋

returns an online CPU according to a numa aware policy — and the order is the whole vibe, bestie:

1. local cpus come back first 🥇
2. then the non-local ones 🚶
3. then it wraps around 🔁

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🔥 DESCRIPTION 🔥

so if ur the kind of person who wants to enumerate all the CPUs based on their NUMA distances — i.e. u wanna call this function in a loop — it looks like this:

for (i = 0; i < num_online_cpus; i++) {
    cpu = cpumask_local_spread(i, node);
    do_something(cpu);
}

but bestie, there's a better alternative based on for_each-like iterators:

for_each_numa_hop_mask(mask, node) {
    for_each_cpu_andnot(cpu, mask, prev)
        do_something(cpu);
    prev = mask;
}

the docs say it's simpler and more verbose than the above (yes, that's literally what it says 💀 just relaying the message fr fr).

as for complexity, ngl this is the part u screenshot:

• iterator-based enumeration is O(sched_domains_numa_levels * nr_cpu_ids) 👑
• cpumask_local_spread when called for each cpu is O(sched_domains_numa_levels * nr_cpu_ids * log(nr_cpu_ids)) 😭

so like... the iterator version is the one ur actually gonna want. take the hint, periodt.

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💜 SEE ALSO 💜

• Kernel file ./lib/cpumask.c
• alloc_cpumask_var_node(9)
• alloc_bootmem_cpumask_var(9)
• free_cpumask_var(9)
• free_bootmem_cpumask_var(9)
• cpumask_any_and_distribute(9)
• cpumask_any_distribute(9)

go read those next, iykwim 🫶

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and that's the page!! 🧚 this little masterpiece lives in ./lib, dated October 2026, and it's filed under cpumask_local_spread(9). mwah 💋 slay, bye! 💖✨