[PATCH v2] sched_ext: Add cmask, a base-windowed bitmap over cid space

[Tejun Heo <tj@kernel.org>]

Sub-scheduler code built on cids needs bitmaps scoped to a slice of cid
space (e.g. the idle cids of a shard). A cpumask sized for NR_CPUS wastes
most of its bits for a small window and is awkward in BPF.

scx_cmask covers [base, base + nr_bits). bits[] is aligned to the global
64-cid grid: bits[0] spans [base & ~63, (base & ~63) + 64). Any two
cmasks therefore address bits[] against the same global windows, so
cross-cmask word ops reduce to

	dest->bits[i] OP= operand->bits[i - delta]

with no bit-shifting, at the cost of up to one extra storage word for
head misalignment. This alignment guarantee is the reason binary ops
can stay word-level; every mutating helper preserves it.

Kernel side in ext_cid.[hc]; BPF side in tools/sched_ext/include/scx/
cid.bpf.h. BPF side drops the scx_ prefix (redundant in BPF code) and
adds the extra helpers that basic idle-cpu selection needs.

No callers yet.

v2: Narrow to helpers that will be used in the planned changes;
    set/bit/find/zero ops will be added as usage develops.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext_cid.h                   |   63 +++
 tools/sched_ext/include/scx/cid.bpf.h    |  595 +++++++++++++++++++++++++++++++
 tools/sched_ext/include/scx/common.bpf.h |    1 
 3 files changed, 659 insertions(+)

--- a/kernel/sched/ext_cid.h
+++ b/kernel/sched/ext_cid.h
@@ -145,4 +145,67 @@ static inline s32 scx_cpu_to_cid(struct
 	return __scx_cpu_to_cid(cpu);
 }
 
+/*
+ * cmask: variable-length, base-windowed bitmap over cid space
+ * -----------------------------------------------------------
+ *
+ * A cmask covers the cid range [base, base + nr_bits). bits[] is aligned to the
+ * global 64-cid grid: bits[0] spans [base & ~63, (base & ~63) + 64), so the
+ * first (base & 63) bits of bits[0] are head padding and any tail past base +
+ * nr_bits is tail padding. Both must stay zero for the lifetime of the mask;
+ * all mutating helpers preserve that invariant.
+ *
+ * Grid alignment means two cmasks always address bits[] against the same global
+ * 64-cid windows, so cross-cmask word ops (AND, OR, ...) reduce to
+ *
+ *	dest->bits[i] OP= operand->bits[i - delta]
+ *
+ * with no bit-shifting, regardless of how the two bases relate mod 64.
+ */
+struct scx_cmask {
+	u32 base;
+	u32 nr_bits;
+	DECLARE_FLEX_ARRAY(u64, bits);
+};
+
+/*
+ * Number of u64 words of bits[] storage that covers @nr_bits regardless of base
+ * alignment. The +1 absorbs up to 63 bits of head padding when base is not
+ * 64-aligned - always allocating one extra word beats branching on base or
+ * splitting the compute.
+ */
+#define SCX_CMASK_NR_WORDS(nr_bits)	(((nr_bits) + 63) / 64 + 1)
+
+/*
+ * Define an on-stack cmask for up to @cap_bits. @name is a struct scx_cmask *
+ * aliasing zero-initialized storage; call scx_cmask_init() to set base/nr_bits.
+ */
+#define SCX_CMASK_DEFINE(name, cap_bits)	\
+	DEFINE_RAW_FLEX(struct scx_cmask, name, bits, SCX_CMASK_NR_WORDS(cap_bits))
+
+static inline bool __scx_cmask_contains(const struct scx_cmask *m, u32 cid)
+{
+	return likely(cid >= m->base && cid < m->base + m->nr_bits);
+}
+
+/* Word in bits[] covering @cid. @cid must satisfy __scx_cmask_contains(). */
+static inline u64 *__scx_cmask_word(const struct scx_cmask *m, u32 cid)
+{
+	return (u64 *)&m->bits[cid / 64 - m->base / 64];
+}
+
+static inline void scx_cmask_init(struct scx_cmask *m, u32 base, u32 nr_bits)
+{
+	m->base = base;
+	m->nr_bits = nr_bits;
+	memset(m->bits, 0, SCX_CMASK_NR_WORDS(nr_bits) * sizeof(u64));
+}
+
+static inline void __scx_cmask_set(struct scx_cmask *m, u32 cid)
+{
+	if (!__scx_cmask_contains(m, cid))
+		return;
+	*__scx_cmask_word(m, cid) |= BIT_U64(cid & 63);
+}
+
 #endif /* _KERNEL_SCHED_EXT_CID_H */
--- /dev/null
+++ b/tools/sched_ext/include/scx/cid.bpf.h
@@ -0,0 +1,595 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * BPF-side helpers for cids and cmasks. See kernel/sched/ext_cid.h for the
+ * authoritative layout and semantics. The BPF-side helpers use the cmask_*
+ * naming (no scx_ prefix); cmask is the SCX bitmap type so the prefix is
+ * redundant in BPF code. Atomics use __sync_val_compare_and_swap and every
+ * helper is inline (no .c counterpart).
+ *
+ * Included by scx/common.bpf.h; don't include directly.
+ *
+ * Copyright (c) 2026 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2026 Tejun Heo <tj@kernel.org>
+ */
+#ifndef __SCX_CID_BPF_H
+#define __SCX_CID_BPF_H
+
+#include "bpf_arena_common.bpf.h"
+
+#ifndef BIT_U64
+#define BIT_U64(nr)		(1ULL << (nr))
+#endif
+#ifndef GENMASK_U64
+#define GENMASK_U64(h, l)	((~0ULL << (l)) & (~0ULL >> (63 - (h))))
+#endif
+
+/*
+ * Storage cap for bounded loops over bits[]. Sized to cover NR_CPUS=8192 with
+ * one extra word for head-misalignment. Increase if deployment targets larger
+ * NR_CPUS.
+ */
+#ifndef CMASK_MAX_WORDS
+#define CMASK_MAX_WORDS 129
+#endif
+
+#define CMASK_NR_WORDS(nr_bits)		(((nr_bits) + 63) / 64 + 1)
+
+static __always_inline bool __cmask_contains(const struct scx_cmask __arena *m, u32 cid)
+{
+	return cid >= m->base && cid < m->base + m->nr_bits;
+}
+
+static __always_inline u64 __arena *__cmask_word(const struct scx_cmask __arena *m, u32 cid)
+{
+	return (u64 __arena *)&m->bits[cid / 64 - m->base / 64];
+}
+
+static __always_inline void cmask_init(struct scx_cmask __arena *m, u32 base, u32 nr_bits)
+{
+	u32 nr_words = CMASK_NR_WORDS(nr_bits), i;
+
+	m->base = base;
+	m->nr_bits = nr_bits;
+
+	bpf_for(i, 0, CMASK_MAX_WORDS) {
+		if (i >= nr_words)
+			break;
+		m->bits[i] = 0;
+	}
+}
+
+static __always_inline bool cmask_test(const struct scx_cmask __arena *m, u32 cid)
+{
+	if (!__cmask_contains(m, cid))
+		return false;
+	return *__cmask_word(m, cid) & BIT_U64(cid & 63);
+}
+
+/*
+ * x86 BPF JIT rejects BPF_OR | BPF_FETCH and BPF_AND | BPF_FETCH on arena
+ * pointers (see bpf_jit_supports_insn() in arch/x86/net/bpf_jit_comp.c). Only
+ * BPF_CMPXCHG / BPF_XCHG / BPF_ADD with FETCH are allowed. Implement
+ * test_and_{set,clear} and the atomic set/clear via a cmpxchg loop.
+ *
+ * CMASK_CAS_TRIES is far above what any non-pathological contention needs.
+ * Exhausting it means the bit update was lost, which corrupts the caller's view
+ * of the bitmap, so raise scx_bpf_error() to abort the scheduler.
+ */
+#define CMASK_CAS_TRIES		1024
+
+static __always_inline void cmask_set(struct scx_cmask __arena *m, u32 cid)
+{
+	u64 __arena *w;
+	u64 bit, old, new;
+	u32 i;
+
+	if (!__cmask_contains(m, cid))
+		return;
+	w = __cmask_word(m, cid);
+	bit = BIT_U64(cid & 63);
+	bpf_for(i, 0, CMASK_CAS_TRIES) {
+		old = *w;
+		if (old & bit)
+			return;
+		new = old | bit;
+		if (__sync_val_compare_and_swap(w, old, new) == old)
+			return;
+	}
+	scx_bpf_error("cmask_set CAS exhausted at cid %u", cid);
+}
+
+static __always_inline void cmask_clear(struct scx_cmask __arena *m, u32 cid)
+{
+	u64 __arena *w;
+	u64 bit, old, new;
+	u32 i;
+
+	if (!__cmask_contains(m, cid))
+		return;
+	w = __cmask_word(m, cid);
+	bit = BIT_U64(cid & 63);
+	bpf_for(i, 0, CMASK_CAS_TRIES) {
+		old = *w;
+		if (!(old & bit))
+			return;
+		new = old & ~bit;
+		if (__sync_val_compare_and_swap(w, old, new) == old)
+			return;
+	}
+	scx_bpf_error("cmask_clear CAS exhausted at cid %u", cid);
+}
+
+static __always_inline bool cmask_test_and_set(struct scx_cmask __arena *m, u32 cid)
+{
+	u64 __arena *w;
+	u64 bit, old, new;
+	u32 i;
+
+	if (!__cmask_contains(m, cid))
+		return false;
+	w = __cmask_word(m, cid);
+	bit = BIT_U64(cid & 63);
+	bpf_for(i, 0, CMASK_CAS_TRIES) {
+		old = *w;
+		if (old & bit)
+			return true;
+		new = old | bit;
+		if (__sync_val_compare_and_swap(w, old, new) == old)
+			return false;
+	}
+	scx_bpf_error("cmask_test_and_set CAS exhausted at cid %u", cid);
+	return false;
+}
+
+static __always_inline bool cmask_test_and_clear(struct scx_cmask __arena *m, u32 cid)
+{
+	u64 __arena *w;
+	u64 bit, old, new;
+	u32 i;
+
+	if (!__cmask_contains(m, cid))
+		return false;
+	w = __cmask_word(m, cid);
+	bit = BIT_U64(cid & 63);
+	bpf_for(i, 0, CMASK_CAS_TRIES) {
+		old = *w;
+		if (!(old & bit))
+			return false;
+		new = old & ~bit;
+		if (__sync_val_compare_and_swap(w, old, new) == old)
+			return true;
+	}
+	scx_bpf_error("cmask_test_and_clear CAS exhausted at cid %u", cid);
+	return false;
+}
+
+static __always_inline void __cmask_set(struct scx_cmask __arena *m, u32 cid)
+{
+	if (!__cmask_contains(m, cid))
+		return;
+	*__cmask_word(m, cid) |= BIT_U64(cid & 63);
+}
+
+static __always_inline void __cmask_clear(struct scx_cmask __arena *m, u32 cid)
+{
+	if (!__cmask_contains(m, cid))
+		return;
+	*__cmask_word(m, cid) &= ~BIT_U64(cid & 63);
+}
+
+static __always_inline bool __cmask_test_and_set(struct scx_cmask __arena *m, u32 cid)
+{
+	u64 bit = BIT_U64(cid & 63);
+	u64 __arena *w;
+	u64 prev;
+
+	if (!__cmask_contains(m, cid))
+		return false;
+	w = __cmask_word(m, cid);
+	prev = *w & bit;
+	*w |= bit;
+	return prev;
+}
+
+static __always_inline bool __cmask_test_and_clear(struct scx_cmask __arena *m, u32 cid)
+{
+	u64 bit = BIT_U64(cid & 63);
+	u64 __arena *w;
+	u64 prev;
+
+	if (!__cmask_contains(m, cid))
+		return false;
+	w = __cmask_word(m, cid);
+	prev = *w & bit;
+	*w &= ~bit;
+	return prev;
+}
+
+static __always_inline void cmask_zero(struct scx_cmask __arena *m)
+{
+	u32 nr_words = CMASK_NR_WORDS(m->nr_bits), i;
+
+	bpf_for(i, 0, CMASK_MAX_WORDS) {
+		if (i >= nr_words)
+			break;
+		m->bits[i] = 0;
+	}
+}
+
+/*
+ * BPF_-prefixed to avoid colliding with the kernel's anonymous CMASK_OP_*
+ * enum in ext_cid.c, which is exported via BTF and reachable through
+ * vmlinux.h.
+ */
+enum {
+	BPF_CMASK_OP_AND,
+	BPF_CMASK_OP_OR,
+	BPF_CMASK_OP_COPY,
+};
+
+static __always_inline void cmask_op_word(struct scx_cmask __arena *dest,
+					  const struct scx_cmask __arena *operand,
+					  u32 di, u32 oi, u64 mask, int op)
+{
+	u64 dv = dest->bits[di];
+	u64 ov = operand->bits[oi];
+	u64 rv;
+
+	if (op == BPF_CMASK_OP_AND)
+		rv = dv & ov;
+	else if (op == BPF_CMASK_OP_OR)
+		rv = dv | ov;
+	else
+		rv = ov;
+
+	dest->bits[di] = (dv & ~mask) | (rv & mask);
+}
+
+static __always_inline void cmask_op(struct scx_cmask __arena *dest,
+				     const struct scx_cmask __arena *operand, int op)
+{
+	u32 d_end = dest->base + dest->nr_bits;
+	u32 o_end = operand->base + operand->nr_bits;
+	u32 lo = dest->base > operand->base ? dest->base : operand->base;
+	u32 hi = d_end < o_end ? d_end : o_end;
+	u32 d_base = dest->base / 64;
+	u32 o_base = operand->base / 64;
+	u32 lo_word, hi_word, i;
+	u64 head_mask, tail_mask;
+
+	if (lo >= hi)
+		return;
+
+	lo_word = lo / 64;
+	hi_word = (hi - 1) / 64;
+	head_mask = GENMASK_U64(63, lo & 63);
+	tail_mask = GENMASK_U64((hi - 1) & 63, 0);
+
+	bpf_for(i, 0, CMASK_MAX_WORDS) {
+		u32 w = lo_word + i;
+		u64 m;
+
+		if (w > hi_word)
+			break;
+
+		m = GENMASK_U64(63, 0);
+		if (w == lo_word)
+			m &= head_mask;
+		if (w == hi_word)
+			m &= tail_mask;
+
+		cmask_op_word(dest, operand, w - d_base, w - o_base, m, op);
+	}
+}
+
+/*
+ * cmask_and/or/copy only modify @dest bits that lie in the intersection of
+ * [@dest->base, @dest->base + @dest->nr_bits) and [@operand->base,
+ * @operand->base + @operand->nr_bits). Bits in @dest outside that window
+ * keep their prior values - in particular, cmask_copy() does NOT zero @dest
+ * bits that lie outside @operand's range.
+ */
+static __always_inline void cmask_and(struct scx_cmask __arena *dest,
+				      const struct scx_cmask __arena *operand)
+{
+	cmask_op(dest, operand, BPF_CMASK_OP_AND);
+}
+
+static __always_inline void cmask_or(struct scx_cmask __arena *dest,
+				     const struct scx_cmask __arena *operand)
+{
+	cmask_op(dest, operand, BPF_CMASK_OP_OR);
+}
+
+static __always_inline void cmask_copy(struct scx_cmask __arena *dest,
+				       const struct scx_cmask __arena *operand)
+{
+	cmask_op(dest, operand, BPF_CMASK_OP_COPY);
+}
+
+/**
+ * cmask_next_set - find the first set bit at or after @cid
+ * @m: cmask to search
+ * @cid: starting cid (clamped to @m->base if below)
+ *
+ * Returns the smallest set cid in [@cid, @m->base + @m->nr_bits), or
+ * @m->base + @m->nr_bits if none (the out-of-range sentinel matches the
+ * termination condition used by cmask_for_each()).
+ */
+static __always_inline u32 cmask_next_set(const struct scx_cmask __arena *m, u32 cid)
+{
+	u32 end = m->base + m->nr_bits;
+	u32 base = m->base / 64;
+	u32 last_wi = (end - 1) / 64 - base;
+	u32 start_wi, start_bit, i;
+
+	if (cid < m->base)
+		cid = m->base;
+	if (cid >= end)
+		return end;
+
+	start_wi = cid / 64 - base;
+	start_bit = cid & 63;
+
+	bpf_for(i, 0, CMASK_MAX_WORDS) {
+		u32 wi = start_wi + i;
+		u64 word;
+		u32 found;
+
+		if (wi > last_wi)
+			break;
+
+		word = m->bits[wi];
+		if (i == 0)
+			word &= GENMASK_U64(63, start_bit);
+		if (!word)
+			continue;
+
+		found = (base + wi) * 64 + __builtin_ctzll(word);
+		if (found >= end)
+			return end;
+		return found;
+	}
+	return end;
+}
+
+static __always_inline u32 cmask_first_set(const struct scx_cmask __arena *m)
+{
+	return cmask_next_set(m, m->base);
+}
+
+#define cmask_for_each(cid, m)							\
+	for ((cid) = cmask_first_set(m);					\
+	     (cid) < (m)->base + (m)->nr_bits;					\
+	     (cid) = cmask_next_set((m), (cid) + 1))
+
+/*
+ * Population count over [base, base + nr_bits). Padding bits in the head/tail
+ * words are guaranteed zero by the mutating helpers, so a flat popcount over
+ * all words is correct.
+ */
+static __always_inline u32 cmask_weight(const struct scx_cmask __arena *m)
+{
+	u32 nr_words = CMASK_NR_WORDS(m->nr_bits), i;
+	u32 count = 0;
+
+	bpf_for(i, 0, CMASK_MAX_WORDS) {
+		if (i >= nr_words)
+			break;
+		count += __builtin_popcountll(m->bits[i]);
+	}
+	return count;
+}
+
+/*
+ * True if @a and @b share any set bit. Walk only the intersection of their
+ * ranges, matching the semantics of cmask_and().
+ */
+static __always_inline bool cmask_intersects(const struct scx_cmask __arena *a,
+					     const struct scx_cmask __arena *b)
+{
+	u32 a_end = a->base + a->nr_bits;
+	u32 b_end = b->base + b->nr_bits;
+	u32 lo = a->base > b->base ? a->base : b->base;
+	u32 hi = a_end < b_end ? a_end : b_end;
+	u32 a_base = a->base / 64;
+	u32 b_base = b->base / 64;
+	u32 lo_word, hi_word, i;
+	u64 head_mask, tail_mask;
+
+	if (lo >= hi)
+		return false;
+
+	lo_word = lo / 64;
+	hi_word = (hi - 1) / 64;
+	head_mask = GENMASK_U64(63, lo & 63);
+	tail_mask = GENMASK_U64((hi - 1) & 63, 0);
+
+	bpf_for(i, 0, CMASK_MAX_WORDS) {
+		u32 w = lo_word + i;
+		u64 mask, av, bv;
+
+		if (w > hi_word)
+			break;
+
+		mask = GENMASK_U64(63, 0);
+		if (w == lo_word)
+			mask &= head_mask;
+		if (w == hi_word)
+			mask &= tail_mask;
+
+		av = a->bits[w - a_base] & mask;
+		bv = b->bits[w - b_base] & mask;
+		if (av & bv)
+			return true;
+	}
+	return false;
+}
+
+/*
+ * Find the next cid set in both @a and @b at or after @start, bounded by the
+ * intersection of the two ranges. Return a->base + a->nr_bits if none found.
+ *
+ * Building block for cmask_next_and_set_wrap(). Callers that want a bounded
+ * scan without wrap call this directly.
+ */
+static __always_inline u32 cmask_next_and_set(const struct scx_cmask __arena *a,
+					      const struct scx_cmask __arena *b,
+					      u32 start)
+{
+	u32 a_end = a->base + a->nr_bits;
+	u32 b_end = b->base + b->nr_bits;
+	u32 a_wbase = a->base / 64;
+	u32 b_wbase = b->base / 64;
+	u32 lo = a->base > b->base ? a->base : b->base;
+	u32 hi = a_end < b_end ? a_end : b_end;
+	u32 last_wi, start_wi, start_bit, i;
+
+	if (lo >= hi)
+		return a_end;
+	if (start < lo)
+		start = lo;
+	if (start >= hi)
+		return a_end;
+
+	last_wi = (hi - 1) / 64;
+	start_wi = start / 64;
+	start_bit = start & 63;
+
+	bpf_for(i, 0, CMASK_MAX_WORDS) {
+		u32 abs_wi = start_wi + i;
+		u64 word;
+		u32 found;
+
+		if (abs_wi > last_wi)
+			break;
+
+		word = a->bits[abs_wi - a_wbase] & b->bits[abs_wi - b_wbase];
+		if (i == 0)
+			word &= GENMASK_U64(63, start_bit);
+		if (!word)
+			continue;
+
+		found = abs_wi * 64 + __builtin_ctzll(word);
+		if (found >= hi)
+			return a_end;
+		return found;
+	}
+	return a_end;
+}
+
+/*
+ * Find the next set cid in @m at or after @start, wrapping to @m->base if no
+ * set bit is found in [start, m->base + m->nr_bits). Return m->base +
+ * m->nr_bits if @m is empty.
+ *
+ * Callers do round-robin distribution by passing (last_cid + 1) as @start.
+ */
+static __always_inline u32 cmask_next_set_wrap(const struct scx_cmask __arena *m,
+					       u32 start)
+{
+	u32 end = m->base + m->nr_bits;
+	u32 found;
+
+	found = cmask_next_set(m, start);
+	if (found < end || start <= m->base)
+		return found;
+
+	found = cmask_next_set(m, m->base);
+	return found < start ? found : end;
+}
+
+/*
+ * Find the next cid set in both @a and @b at or after @start, wrapping to
+ * @a->base if none found in the forward half. Return a->base + a->nr_bits
+ * if the intersection is empty.
+ *
+ * Callers do round-robin distribution by passing (last_cid + 1) as @start.
+ */
+static __always_inline u32 cmask_next_and_set_wrap(const struct scx_cmask __arena *a,
+						   const struct scx_cmask __arena *b,
+						   u32 start)
+{
+	u32 a_end = a->base + a->nr_bits;
+	u32 found;
+
+	found = cmask_next_and_set(a, b, start);
+	if (found < a_end || start <= a->base)
+		return found;
+
+	found = cmask_next_and_set(a, b, a->base);
+	return found < start ? found : a_end;
+}
+
+/**
+ * cmask_from_cpumask - translate a kernel cpumask to a cid-space cmask
+ * @m: cmask to fill. Zeroed first; only bits within [@m->base, @m->base +
+ *     @m->nr_bits) are updated - cpus mapping to cids outside that range
+ *     are ignored.
+ * @cpumask: kernel cpumask to translate
+ *
+ * For each cpu in @cpumask, set the cpu's cid in @m. Caller must ensure
+ * @cpumask stays stable across the call (e.g. RCU read lock for
+ * task->cpus_ptr).
+ */
+static __always_inline void cmask_from_cpumask(struct scx_cmask __arena *m,
+					       const struct cpumask *cpumask)
+{
+	u32 nr_cpu_ids = scx_bpf_nr_cpu_ids();
+	s32 cpu;
+
+	cmask_zero(m);
+	bpf_for(cpu, 0, nr_cpu_ids) {
+		s32 cid;
+
+		if (!bpf_cpumask_test_cpu(cpu, cpumask))
+			continue;
+		cid = scx_bpf_cpu_to_cid(cpu);
+		if (cid >= 0)
+			__cmask_set(m, cid);
+	}
+}
+
+/**
+ * cmask_copy_from_kernel - copy a kernel-memory scx_cmask into an arena cmask
+ * @dst: arena cmask to fill. Must be sized for at least @src's bit count.
+ * @src: kernel-memory cmask (e.g. the @cmask arg delivered to ops.set_cmask()).
+ *       Kernel guarantees @src->base == 0.
+ *
+ * Probe the kernel header for nr_bits, zero @dst, then copy @src->bits[]
+ * word by word via bpf_probe_read_kernel. Call scx_bpf_error() on any probe
+ * failure. Intended for set_cmask callbacks where @src is kernel memory that
+ * BPF cmask helpers (which expect __arena pointers) can't touch directly.
+ */
+static __always_inline void cmask_copy_from_kernel(struct scx_cmask __arena *dst,
+						   const struct scx_cmask *src)
+{
+	u32 nr_bits = 0, nr_words, dst_nr_words, wi;
+
+	if (bpf_probe_read_kernel(&nr_bits, sizeof(nr_bits), &src->nr_bits)) {
+		scx_bpf_error("probe-read cmask->nr_bits failed");
+		return;
+	}
+
+	nr_words = CMASK_NR_WORDS(nr_bits);
+	dst_nr_words = CMASK_NR_WORDS(dst->nr_bits);
+	if (nr_words > dst_nr_words) {
+		scx_bpf_error("src cmask nr_bits=%u exceeds dst capacity",
+			      nr_bits);
+		return;
+	}
+
+	cmask_zero(dst);
+	bpf_for(wi, 0, CMASK_MAX_WORDS) {
+		u64 word = 0;
+		if (wi >= nr_words)
+			break;
+		if (bpf_probe_read_kernel(&word, sizeof(u64), &src->bits[wi])) {
+			scx_bpf_error("probe-read cmask->bits[%u] failed", wi);
+			return;
+		}
+		dst->bits[wi] = word;
+	}
+}
+
+#endif /* __SCX_CID_BPF_H */
--- a/tools/sched_ext/include/scx/common.bpf.h
+++ b/tools/sched_ext/include/scx/common.bpf.h
@@ -1055,5 +1055,6 @@ static inline u64 scx_clock_irq(u32 cpu)
 
 #include "compat.bpf.h"
 #include "enums.bpf.h"
+#include "cid.bpf.h"
 
 #endif	/* __SCX_COMMON_BPF_H */