cpuset_cpus_...wed_fallback(9)

💅 cpuset_cpus_allowed_fallback 💅

okay bestie, welcome to the Kernel API Manual ✨ — this one's from the kernel cpuset corner of the world and it is lowkey the most dramatic function in the whole scheduler's contact list. sit down, hydrate, let's get into it 🫶

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🌸 NAME 🌸

cpuset_cpus_allowed_fallback — the final fallback before complete catastrophe. fr fr, "catastrophe" is the actual word in the docs, this is not me being extra 😭

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✨ SYNOPSIS ✨

bool cpuset_cpus_allowed_fallback (struct task_struct *tsk );

so it takes one thing and gives you back a bool. simple. iconic. no cap. 👑

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🎀 ARGUMENTS 🎀

• tsk — pointer to a task_struct, and specifically the one the scheduler is struggling with. like, actively fighting for its life over. that poor little task. 🙈

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🦋 DESCRIPTION 🦋

okay so picture this: the scheduler is out here trying to find an allowed cpu in tsk->cpus_allowed, and it just… can't. nothing. zero. the vibe check failed 💀. and in that moment of crisis, we fall back to task_cs(tsk)->cpus_allowed.

but wait — legacy mode said "not so fast" 😭. in legacy mode, that value is the same as task_cs(tsk)->effective_cpus, and that one will not contain a sane cpumask during cases such as cpu hotplugging. so like, it's giving "we have a backup plan" but the backup plan is also going through it.

this is the absolute last resort for the scheduler, bestie. the bottom of the barrel. it's only used if _every_ other avenue has been traveled — and yes, the original really emphasised every like that, because it means it 🌈

it returns true if the affinity of tsk was changed, and false otherwise. groundbreaking, I know, but we stan a clear return value. 💖

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🔥 SEE ALSO 🔥

the whole cpuset squad pulled up, and they're all in ./kernel/cgroup/cpuset.c — go say hi besties ✨

• 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_mems_allowed(9)
• cpuset_nodemask_valid_mems_allowed(9)
• cpuset_nodes_allowed(9)
• cpuset_spread_node(9)
• cpuset_mem_spread_node(9)
• cpuset_mems_allowed_intersects(9)
• cpuset_print_current_mems_allowed(9)

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and that's the whole page, bestie 💜 this came out of the ./kernel/cgroup directory, dated October 2026, and it's filed under cpuset_cpus_...wed_fallback(9) — yes the title is literally truncated mid-word in the header, we respect the source around here 💅 mwah, go touch grass, then come back and read some more kernel source 🌸