Re: [PATCH v2 3/3] mm/alloc_tag: add the CONFIG_ARCH_NEEDS_WEAK_PER_CPU macro when statically defining the percpu variable _shared_alloc_tag

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Jun 12, 2025 at 8:06 PM Hao Ge <hao.ge@linux.dev> wrote:
>
> From: Hao Ge <gehao@kylinos.cn>
>
> Recently discovered this entry while checking kallsyms on ARM64:
> ffff800083e509c0 D _shared_alloc_tag
>
> If CONFIG_ARCH_NEEDS_WEAK_PER_CPU is not defined(it is only defined for
> s390 and alpha architectures),there's no need to statically define
> the percpu variable _shared_alloc_tag. As the number of CPUs
> increases,the wasted memory will grow correspondingly.

I don't think this patch needs all this background. Just say that
_shared_alloc_tag is not used when CONFIG_ARCH_NEEDS_WEAK_PER_CPU=n,
so you want to compile it out.

>
> Enclose the definition of _shared_alloc_tag within the
> CONFIG_ARCH_NEEDS_WEAK_PER_CPU condition.
>
> Suggested-by: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Hao Ge <gehao@kylinos.cn>
> ---
>  lib/alloc_tag.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
> index c7f602fa7b23..14fd66f26e42 100644
> --- a/lib/alloc_tag.c
> +++ b/lib/alloc_tag.c
> @@ -24,8 +24,10 @@ static bool mem_profiling_support;
>
>  static struct codetag_type *alloc_tag_cttype;
>
> +#ifdef CONFIG_ARCH_NEEDS_WEAK_PER_CPU
>  DEFINE_PER_CPU(struct alloc_tag_counters, _shared_alloc_tag);
>  EXPORT_SYMBOL(_shared_alloc_tag);
> +#endif
>
>  DEFINE_STATIC_KEY_MAYBE(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT,
>                         mem_alloc_profiling_key);
> --
> 2.25.1
>