linux-kernel - Re: [PATCH] mm/alloc_tag: add the ARCH_NEEDS_WEAK_PER_CPU macro when statically defining the percpu variable alloc_tag

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Message-ID: <177e1f6b-50f0-4c0a-bb0b-514283e009a2@linux.dev>
Date: Mon, 9 Jun 2025 14:08:03 +0800
From: Hao Ge <hao.ge@...ux.dev>
To: Andrew Morton <akpm@...ux-foundation.org>,
 Suren Baghdasaryan <surenb@...gle.com>,
 Kent Overstreet <kent.overstreet@...ux.dev>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Hao Ge <gehao@...inos.cn>
Subject: Re: [PATCH] mm/alloc_tag: add the ARCH_NEEDS_WEAK_PER_CPU macro when
 statically defining the percpu variable alloc_tag_counters.

On 2025/5/29 15:35, Hao Ge wrote:
> From: Hao Ge <gehao@...inos.cn>
>
> Recently discovered this entry while checking kallsyms on ARM64:
> ffff800083e509c0 D _shared_alloc_tag
>
> If ARCH_NEEDS_WEAK_PER_CPU is not defined,there's no need to statically
> define the percpu variable alloc_tag_counters.
>
> Therefore,add therelevant macro guards at the appropriate location.
>
> Fixes: 22d407b164ff ("lib: add allocation tagging support for memory allocation profiling")
> 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..d1dab80b70ad 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 ARCH_NEEDS_WEAK_PER_CPU
>   DEFINE_PER_CPU(struct alloc_tag_counters, _shared_alloc_tag);
>   EXPORT_SYMBOL(_shared_alloc_tag);
> +#endif /* ARCH_NEEDS_WEAK_PER_CPU */
>   
>   DEFINE_STATIC_KEY_MAYBE(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT,
>   			mem_alloc_profiling_key);

Hi Suren

I'm sorry to bother you. As mentioned in my commit message,

in fact, on the ARM64 architecture, the _shared_alloc_tag percpu 
variable is not needed.

In my understanding, it will create a copy for each CPU.

  The alloc_tag_counters variable will occupy 16 bytes,

and as the number of CPUs increases, more and more memory will be wasted 
in this segment.

I realized that this modification was a mistake. It resulted in a build 
error, and the link is as follows:

https://lore.kernel.org/all/202506080448.KWN8arrX-lkp@intel.com/

After I studied the comments of DECLARE_PER_CPU_SECTION, I roughly 
understood why this is the case.

But so far, I haven't come up with a good way to solve this problem. Do 
you have any suggestions?

Thanks

Best Regards

Hao