Re: [PATCH 1/3] sched_ext: Order single-cid cmask helpers as (cid, mask)

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Incomplete refactoring of cmask helpers argument order, causing an API divergence between kernel and BPF implementations.
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commit cae9caa6c3655ef0cb4c126820c635529b2d5e19
Author: Tejun Heo <tj@kernel.org>

sched_ext: Order single-cid cmask helpers as (cid, mask)

This commit reorders the parameters of single-cid cmask helpers to take the
cid before the mask. This change is intended to make the parameters consistent
with standard kernel predicates like cpumask_test_cpu(cpu, mask).

> diff --git a/kernel/sched/ext_cid.h b/kernel/sched/ext_cid.h
> index abea22ba2cc25..46fd8eda04435 100644
> --- a/kernel/sched/ext_cid.h
> +++ b/kernel/sched/ext_cid.h

[ ... ]

> @@ -218,11 +218,11 @@ static inline void scx_cmask_reframe(struct scx_cmask *m, u32 base, u32 nr_cids)
>  	m->nr_cids = nr_cids;
>  }
>  
> -static inline void __scx_cmask_set(struct scx_cmask *m, u32 cid)
> +static inline void __scx_cmask_set(u32 cid, struct scx_cmask *m)
>  {
> -	if (!__scx_cmask_contains(m, cid))
> +	if (!__scx_cmask_contains(cid, m))
>  		return;
> -	*__scx_cmask_word(m, cid) |= BIT_U64(cid & 63);
> +	*__scx_cmask_word(cid, m) |= BIT_U64(cid & 63);
>  }
>  

[Severity: Medium]
The commit message states the intent to reorder the cmask helpers for
consistency, but does this refactoring inadvertently omit the BPF-facing
helpers?

The kernel-side helpers in ext_cid.h are updated here to use the new
(cid, mask) order, but the equivalent functions in
tools/sched_ext/include/scx/cid.bpf.h appear to still use the old
(mask, cid) ordering:

tools/sched_ext/include/scx/cid.bpf.h:
static __always_inline void cmask_set(struct scx_cmask __arena *m, u32 cid)
static __always_inline bool cmask_test(const struct scx_cmask __arena *m, u32 cid)
static __always_inline bool __cmask_contains(const struct scx_cmask __arena *m, u32 cid)

Could this introduce an API divergence between the kernel and BPF
implementations for developers writing BPF schedulers?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260603172153.3081861-1-tj@kernel.org?part=1