Re: [PATCH 5/5] mm/alloc_tag: add the CONFIG_ARCH_NEEDS_WEAK_PER_CPU macro when statically defining the percpu variable _shared_alloc_tag

[Heiko Carstens <hca@linux.ibm.com>]

On Thu, Jun 12, 2025 at 11:57:49AM +0300, Mike Rapoport wrote:
> On Thu, Jun 12, 2025 at 04:27:30PM +0800, Hao Ge wrote:
> > From: Hao Ge <gehao@kylinos.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.
> > 
> > Enclose the definition of _shared_alloc_tag within the
> > CONFIG_ARCH_NEEDS_WEAK_PER_CPU condition.
> > 
> > Suggested-by: Suren Baghdasaryan <surenb@google.com>
> > 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..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
> 
> It should be enough to add #ifdef ARCH_NEEDS_WEAK_PER_CPU here instead of
> all the churn.

That won't work since ARCH_NEEDS_WEAK_PER_CPU is only defined if MODULE is
also defined, which is not the case for core kernel code like lib/alloc_tag.c.

As a side note: I'm wondering if s390 still needs ARCH_NEEDS_WEAK_PER_CPU due
to all the compiler option changes we had recently. But that's a different
story and independent of this series.