Re: [PATCH] sched_ext:SCX is preemptible by the Fair scheduler

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Inserting SCX_CPU_PREEMPT_FAIR shifts the value of SCX_CPU_PREEMPT_UNKNOWN without updating tools/sched_ext autogenerated headers, breaking feature detection for BPF schedulers.
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commit 19f50c7bbf7f31acfd835724319513f85b45249d
Author: Cui Jian <cjian720@163.com>

sched_ext:SCX is preemptible by the Fair scheduler

This commit adds the SCX_CPU_PREEMPT_FAIR enum value to distinguish
preemptions caused by the Fair scheduler. It updates the core sched_ext
code to report this reason when the next scheduled task belongs to the
fair_sched_class.

> diff --git a/kernel/sched/ext_internal.h b/kernel/sched/ext_internal.h
> index a075732d4430d..3ce07bec58a55 100644
> --- a/kernel/sched/ext_internal.h
> +++ b/kernel/sched/ext_internal.h
> @@ -241,6 +241,8 @@ enum scx_cpu_preempt_reason {
>  	SCX_CPU_PREEMPT_DL,
>  	/* next task is being scheduled by &sched_class_stop */
>  	SCX_CPU_PREEMPT_STOP,
> +	/* next task is being scheduled by &sched_class_fair*/
> +	SCX_CPU_PREEMPT_FAIR,

[Severity: Medium]
Does inserting SCX_CPU_PREEMPT_FAIR here shift the value of
SCX_CPU_PREEMPT_UNKNOWN and potentially break existing BPF programs that rely
on its previous value or use it as a maximum for array sizing?

Also, should tools/sched_ext/include/scx/enum_defs.autogen.h be updated to
include #define HAVE_SCX_CPU_PREEMPT_FAIR? Without it, BPF schedulers might
fail to detect this new feature via #ifdef, potentially mishandling the new
enum value.

>  	/* unknown reason for SCX being preempted */
>  	SCX_CPU_PREEMPT_UNKNOWN,
>  };

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Sashiko AI review · https://sashiko.dev/#/patchset/20260612175113.110447-1-cjian720@163.com?part=1