linux-kernel - 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

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Message-ID: <CAJuCfpGkUk5RXm-HDLB2mr_mRbSxPV+kx116FFBhyYsf6pXNCg@mail.gmail.com>
Date: Fri, 13 Jun 2025 12:21:32 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Hao Ge <hao.ge@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...hat.com>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>, 
	Richard Henderson <richard.henderson@...aro.org>, Matt Turner <mattst88@...il.com>, 
	Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>, 
	Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>, 
	Alexander Gordeev <agordeev@...ux.ibm.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>, 
	Sven Schnelle <svens@...ux.ibm.com>, Kent Overstreet <kent.overstreet@...ux.dev>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org, 
	linux-s390@...r.kernel.org, Hao Ge <gehao@...inos.cn>
Subject: 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

On Thu, Jun 12, 2025 at 8:06 PM Hao Ge <hao.ge@...ux.dev> wrote:
>
> From: Hao Ge <gehao@...inos.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@...gle.com>
> Signed-off-by: Hao Ge <gehao@...inos.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
>