linux-kernel - Re: [PATCH v2] tools/sched_ext: Compatible testing of SCX_ENQ_CPU

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Message-ID: <Z6hO0toLJPN9PL-w@slm.duckdns.org>
Date: Sat, 8 Feb 2025 20:44:34 -1000
From: Tejun Heo <tj@...nel.org>
To: Changwoo Min <changwoo@...lia.com>
Cc: void@...ifault.com, arighi@...dia.com, kernel-dev@...lia.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] tools/sched_ext: Compatible testing of
 SCX_ENQ_CPU_SELECTED

On Sun, Feb 09, 2025 at 10:53:53AM +0900, Changwoo Min wrote:
> This provides compatible testing of SCX_ENQ_CPU_SELECTED.
> More specifically, it handles two cases:
> 
>   1. a BPF scheduler is compiled against vmlinux.h where
>   SCX_ENQ_CPU_SELECTED is defined, but it runs on a kernel that does not
>   have SCX_ENQ_CPU_SELECTED. In this case, the test result of
>   'enq_flags & SCX_ENQ_CPU_SELECTED' will always be false. That test result
>   is semantically incorrect because the kernel before SCX_ENQ_CPU_SELECTED
>   has never skipped select_task_rq_scx(), so the result should be true.
> 
>   2. a BPF scheduler is compiling against vmlinux.h where
>   SCX_ENQ_CPU_SELECTED is not defined. In this case, directly using
>   SCX_ENQ_CPU_SELECTED causes compilation errors.
> 
> To hide such complexity, introduce __COMPAT_is_enq_cpu_selected(),
> which checks if SCX_ENQ_CPU_SELECTED exists in runtime using BPF CO-RE.
> This consists of three parts:
> 
>   1. Add enum_defs.autogen.h, which has macros (HAVE_{enum name}) denoting
>   whether SCX enums are defined in the vmlinux.h or not.
> 
>   2. Implement __COMPAT_is_enq_cpu_selected(), which provide the test of
>   SCX_ENQ_CPU_SELECTED in a compatible way.
> 
>   3. Use  __COMPAT_is_enq_cpu_selected() in scx_qmap.
> 
> Note that this is a sync of the relevant PR [1] in the scx repo.
> 
>   [1] https://github.com/sched-ext/scx/pull/1314
> 
> Signed-off-by: Changwoo Min <changwoo@...lia.com>

Applied to sched_ext/for-6.15 with a very minor edit:

> diff --git a/tools/sched_ext/include/scx/enum_defs.autogen.h b/tools/sched_ext/include/scx/enum_defs.autogen.h
> new file mode 100644
> index 000000000000..a32e534a4012
> --- /dev/null
> +++ b/tools/sched_ext/include/scx/enum_defs.autogen.h
...
> +#endif /* __ENUM_DEFS_AUTOGEN_H__ */
> +

The extra newline triggers git warning on commit. I dropped the extra
newline. Maybe adjust the script too? Also, if we aren't including the
script in the kernel tree, maybe a good idea for the generated file to point
to where the script is?

Thanks.

-- 
tejun