Re: [PATCH] mm/alloc_tag: add the ARCH_NEEDS_WEAK_PER_CPU macro when statically defining the percpu variable alloc_tag_counters.

[Hao Ge <hao.ge@linux.dev>]

On 2025/6/11 23:24, Suren Baghdasaryan wrote:
> On Tue, Jun 10, 2025 at 10:27 PM Hao Ge <hao.ge@linux.dev> wrote:
>>
>> On 2025/6/10 00:39, Suren Baghdasaryan wrote:
>>> On Sun, Jun 8, 2025 at 11:08 PM Hao Ge <hao.ge@linux.dev> wrote:
>>>> On 2025/5/29 15:35, Hao Ge wrote:
>>>>> From: Hao Ge <gehao@kylinos.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@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..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
> Hi Hao,
>
>> 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?
Hi Suren
> The problem is that ARCH_NEEDS_WEAK_PER_CPU is not a Kconfig option,
> it's just a definition, for s390 it's here:
> https://elixir.bootlin.com/linux/v6.15.1/source/arch/s390/include/asm/percpu.h#L21
> So, even for s390 if you are building core kernel code (not a module),
> ARCH_NEEDS_WEAK_PER_CPU will be undefined, however if you are building
> a module on s390 then it is defined. So, your change effectively
> results in _shared_alloc_tag being compiled out in the core kernel
> while it's used when you build a module. Therefore during linking
> modules can't link to that symbol in the core kernel. Hope this
> explains the issue.


Thank you so, so, so much! I understand now, and thank you for such a 
detailed explanation.


> The way I would fix this is by making ARCH_NEEDS_WEAK_PER_CPU a
> Kconfig option and enable it for s390 and alpha, would replace old
> definitions from
> https://elixir.bootlin.com/linux/v6.15.1/source/arch/s390/include/asm/percpu.h#L21
> and https://elixir.bootlin.com/linux/v6.15.1/source/arch/alpha/include/asm/percpu.h#L14
> with:
>
> #if defined(MODULE) && defined(ARCH_NEEDS_WEAK_PER_CPU)
> #define MODULE_NEEDS_WEAK_PER_CPU
> #endif
>
> Then use MODULE_NEEDS_WEAK_PER_CPU instead of ARCH_NEEDS_WEAK_PER_CPU
> in all the current places in the kernel code. Lastly, to compile out
> _shared_alloc_tag your current patch should work fine because on s390
> and alpha ARCH_NEEDS_WEAK_PER_CPU will be defined after all these
> changes.
> Does that make sense?


I quite agree with this approach.

Thanks
Best Regards
Hao

>> Could you please help explain it again?
>>
>> Thanks
>> Best Regards
>> Hao
>>
>>>> Thanks
>>>>
>>>> Best Regards
>>>>
>>>> Hao
>>>>
>>>>
>>>>
>>>>