scx_bpf_online_cmask(9)

📦 version 7.3.1, in nixpkgs unstable

💅 scx_bpf_online_cmask 💅

okay so bestie, welcome to the Kernel API Manual — yes that's the manual name, we're fancy here — and today's main character is scx_bpf_online_cmask, a little kfunc with big "here's your mask" energy. let's get into it 🎀

✨ NAME ✨

scx_bpf_online_cmask — returns the online cid mask from the scheduler arena. that's it, that's the whole vibe 🌸

🦋 SYNOPSIS 🦋

__bpf_kfunc const void * scx_bpf_online_cmask (const struct bpf_prog_aux
*aux );

literally just hand it the aux and it hands you back a const void *. no cap. ✨

🍓 ARGUMENTS 🍓

  • aux — the implicit BPF argument that gets you into bpf_prog_aux. it's hidden from BPF progs, so like… it's there, you just don't get to see it.🙈

👑 DESCRIPTION 👑

so what does it actually do. it returns a kernel-maintained cmask that covers [0, scx_bpf_nr_cids). that's the range, periodt. 🔥

but — and this is the important part — it returns NULL in two situations:

  • if the calling program is not associated with a live cid-form scheduler, or
  • if the mask is not allocated yet, as in ops.init_cids.

so like, vibe check your return value before you go dereferencing anything, bestie 😭

treat the mask as read-only — even though arena memory stays writable by the BPF scheduler. the memory being writable does not mean you should write to it. hands off. 💅

the mask follows the SCX hotplug notifications: a cid's bit gets updated before ops.cid_online/ops.offline runs for it. so the ordering is already handled for you, slay. 🫶

the pointer is valid from ops.init through ops.exit. that's the window. don't go holding onto it past that, that's not the vibe.

also: root ops.init runs with hotplug excluded. but other contexts can observe concurrent updates — so yeah, concurrency is real, stay aware. 👀

🌈 SEE ALSO 🌈

this is the part where the manpage goes absolutely feral with cross-references. buckle up babes, we're listing all of them fr fr 💀

kernel file ./kernel/sched/ext/ext.c, plus:

and that's the page, bestie 🫶 coming at you straight from the kernel source at ./kernel/sched/ext, born October 2026, manual name Kernel API Manual. go forth and read your mask (read-only!!) 💖✨🎀

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