Re: [PATCH 10/17] sched_ext: Add cmask, a base-windowed bitmap over cid space

[Changwoo Min <changwoo@igalia.com>]

On 4/29/26 5:35 AM, Tejun Heo wrote:

> +/*
> + * 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;
> +}

Exiting a BPF scheduler when CAS retries fail seems too brutal, while it
is extremely rare to happen. What about adding a kfunc for the slow path
that runs only when the CAS retry fails? For example,
scx_bpf_cmask_test_and_clear( ) does the same thing as
cmask_test_and_clear(), but it never fails. cmask_test_and_clear() calls
scx_bpf_cmask_test_and_clear( ) only when the CAS retry fails. In this
way, we can use the fast path in the BPF implementation while ensuring
the operation succeeds.

> +/**
> + * 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);

Some compiler versions (e.g., clang-18 or older) don’t support
__builtin_ctzll(). To handle this gracefully, there is already a
wrapper, ctzll(), in common.bpf.h. So, I suggest using ctzll()
for compatibility.

Reviewed-by: Changwoo Min <changwoo@igalia.com>