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

[Suren Baghdasaryan <surenb@google.com>]

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.


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