scx_cmask_ref_shard(9)

📦 version 7.3.1, in nixpkgs unstable

💅 scx_cmask_ref_shard(9) — the one from the Kernel API Manual, bestie

Okay so bestie, listen. This is a page out of the Kernel API Manual, and it's about scx_cmask_ref_shard — literally just "read one little shard out of @ref and drop it into @out", and honestly? Iconic. Lowkey doing the most with the least. Let's get into it 🎀

✨ NAME ✨

scx_cmask_ref_shard — Read one shard from @ref into @out. That's it, that's the whole vibe. 🦋

🌸 SYNOPSIS 🌸

void scx_cmask_ref_shard (const struct scx_cmask_ref *ref , s32
shard_idx , struct scx_cmask *out );

Yeah she's a void, she takes three args, she does not return anything, she simply exists and does her job. Main character energy, no notes. 👑

🫶 ARGUMENTS 🫶

  • ref — a validated ref. Emphasis on validated, bestie, this is not the place to be handing over your little unvetted situationship of a pointer 💀
  • shard_idx — the target shard, and it has to live in [ref->shard_first, ref->shard_end). Note that half-open interval, the closing bracket is doing real work there 🙈
  • out — your output cmask, and out->alloc_words must already hold the shard. Like, actually. If you didn't allocate enough, that's a you problem and the universe (and scx_errors) will let you know 😭

💖 DESCRIPTION 💖

So what she actually does: she sets out to the intersection of ref's range with shard_idx's cid range, and the bits[] get read from ref->src via READ_ONCE. Very demure, very mindful, very concurrency-aware of her ⭐

If the intersection comes out empty? Chill. She just sets out->nr_cids to 0. No drama, no crash, just a cute little zero 🌸

BUT — and this is the part where you pay attention, bestie — if out can't hold the shard, she calls scx_errors on ref's sched. So please, for the love of everything, size your out right the first time 💅

🔥 SEE ALSO 🔥

The whole friend group, fr fr — plus the kernel file they all live in:

Kernel file ./kernel/sched/ext/cid.c:

ngl bestie, if you're reading this page you're probably already deep in the cmask ref lore. Go read the rest of them, iykyk 🧚

And that's a wrap, babe 🍓 — this lil transmission came to you from ./kernel/sched/ext, dated October 2026, and yes she is still scx_cmask_ref_shard(9). Slay, stay hydrated, allocate your out properly 💜✨

📦 where this page comes from