cpuset_nodes_allowed(9)

💅 cpuset_nodes_allowed — the kernel API manual, bestie edition

okay so you're looking at cpuset_nodes_allowed(9), and yes, it's exactly as niche as it sounds. It lives in the Kernel API Manual. Sit down, hydrate, and let's get into it 🫶

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✨ NAME ✨

cpuset_nodes_allowed — grab the effective_mems mask out of a cgroup cpuset. that's the whole gig 💖

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🌸 SYNOPSIS 🌸

void cpuset_nodes_allowed (struct cgroup *cgroup , nodemask_t *mask );

Like, look at her. Void return, two args, zero drama.

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🎀 ARGUMENTS 🎀

• cgroup — a pointer to struct cgroup. your cgroup of interest, your main character ✨
• mask — a pointer to struct nodemask_t that gets returned to you. this is where the answer lands 💌

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🦋 DESCRIPTION 🦋

So here's what actually happens 👑: this returns the effective_mems mask from a cgroup cpuset if it's cgroup v2 and it has the cpuset subsys. Otherwise? You get node_states[N_MEMORY]. That's the fallback, no cap 💀

Now the part you need to know, and I'm gonna say it with my whole chest:

This function intentionally avoids taking the cpuset_mutex or callback_lock when it accesses effective_mems. Not a bug. Not an oopsie. On purpose.

Why tho?? Because the effective_mems you get is stale immediately after the query anyway. Like, genuinely instantly. The example given is exactly that: effective_mems gets updated right after the lock is released but before the function returns. So locking would've been doing all that work for literally nothing 😭

Consequences, and please read these fr fr:

• the returned mask may be empty, because cs->effective_mems can be rebound during this very call 🌪️
• also, nodes in mask are not guaranteed to be online due to hot plugins 🔥

So like… callers beware. You should check the mask for validity on return, based on whatever you're planning to do with it next. Your subsequent use, your responsibility, iykyk 💅

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🌈 SEE ALSO 🌈

Kernel file ./kernel/cgroup/cpuset.c, plus the whole friend group:

• cpuset_lock(9)
• cpuset_full_lock(9)
• partition_is_populated(9)
• alloc_cpumasks(9)
• alloc_tmpmasks(9)
• free_tmpmasks(9)
• dup_or_alloc_cpuset(9)
• free_cpuset(9)
• cpus_excl_conflict(9)
• cpuset_update_tasks_cpumask(9)
• compute_effective_cpumask(9)
• compute_effective_nodemask(9)
• rm_siblings_excl_cpus(9)
• update_parent_effective_cpumask(9)
• compute_partition_effective_cpumask(9)
• update_sibling_cpumasks(9)
• validate_partition(9)
• partition_cpus_change(9)
• update_cpumask(9)
• update_exclusive_cpumask(9)
• cpuset_update_tasks_nodemask(9)
• update_prstate(9)
• cpuset_css_alloc(9)
• cpuset_init(9)
• cpuset_hotplug_update_tasks(9)
• cpuset_handle_hotplug(9)
• cpuset_init_smp(9)
• cpuset_cpus_allowed_locked(9)
• cpuset_cpus_allowed(9)
• cpuset_cpus_allowed_fallback(9)
• cpuset_mems_allowed(9)
• cpuset_nodemask_valid_mems_allowed(9)
• cpuset_spread_node(9)
• cpuset_mem_spread_node(9)
• cpuset_mems_allowed_intersects(9)
• cpuset_print_current_mems_allowed(9)

like, the entire cpuset extended universe, slay 🧚

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and that's the page!! mwah 💋 this little icon came out of ./kernel/cgroup, dated October 2026, and it answers to the name cpuset_nodes_allowed(9). go forth and check your masks for validity bestie 🍓💜