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: <b132dd1f-984b-452e-b19b-18cdecb2842a@linux.dev>
Date: Wed, 11 Jun 2025 13:26:52 +0800
From: Hao Ge <hao.ge@...ux.dev>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Kent Overstreet <kent.overstreet@...ux.dev>, 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 2025/6/10 00:39, Suren Baghdasaryan wrote:
> On Sun, Jun 8, 2025 at 11:08 PM Hao Ge <hao.ge@...ux.dev> wrote:
>>
>> 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?
> Hi Hao,
> The problem here is that ARCH_NEEDS_WEAK_PER_CPU is not a Kconfig
> option, it gets defined only on 2 architectures and only when building
> modules here https://elixir.bootlin.com/linux/v6.15.1/source/arch/alpha/include/asm/percpu.h#L14
> and here https://elixir.bootlin.com/linux/v6.15.1/source/arch/s390/include/asm/percpu.h#L21.
> A nicer way to deal with that is to make if a Kconfig option which is
> enabled only for alpha and s390 and then do something like this:
>
>   #if defined(MODULE) && defined(ARCH_NEEDS_WEAK_PER_CPU)
> #define MODULE_NEEDS_WEAK_PER_CPU
> #endif
>
> and change all the usages of ARCH_NEEDS_WEAK_PER_CPU with
> MODULE_NEEDS_WEAK_PER_CPU.
> Did I explain the idea clearly?
> Thanks,
> Suren.
>
Hi Suren

Thanks for your guidance.
I understand this train of thought.

I've been thinking about a problem: I only added the 
ARCH_NEEDS_WEAK_PER_CPU

macro to isolate the definition of _shared_alloc_tag.

Since s390 defines this macro, why did this build error occur?

Could you please help explain it again?

Thanks
Best Regards
Hao

>>
>> Thanks
>>
>> Best Regards
>>
>> Hao
>>
>>
>>
>>