Re: [PATCH v3 1/5] memcg: disable kmem charging in nmi for unsupported arch

[Shakeel Butt]

On Sat, May 17, 2025 at 7:06 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> On Fri, May 16, 2025 at 11:32:27AM -0700, Shakeel Butt wrote:
> > The memcg accounting and stats uses this_cpu* and atomic* ops. There are
> > archs which define CONFIG_HAVE_NMI but does not define
> > CONFIG_ARCH_HAS_NMI_SAFE_THIS_CPU_OPS and ARCH_HAVE_NMI_SAFE_CMPXCHG, so
> > memcg accounting for such archs in nmi context is not possible to
> > support. Let's just disable memcg accounting in nmi context for such
> > archs.
> >
> > Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> > ---
> > Changes since v2:
> > - reorder the in_nmi() check as suggested by Vlastimil
> >
> >  include/linux/memcontrol.h |  5 +++++
> >  mm/memcontrol.c            | 15 +++++++++++++++
> >  2 files changed, 20 insertions(+)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index f7848f73f41c..53920528821f 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -62,6 +62,11 @@ struct mem_cgroup_reclaim_cookie {
> >
> >  #ifdef CONFIG_MEMCG
> >
> > +#if defined(CONFIG_ARCH_HAS_NMI_SAFE_THIS_CPU_OPS) || \
> > +     !defined(CONFIG_HAVE_NMI) || defined(ARCH_HAVE_NMI_SAFE_CMPXCHG)
>
>                                              CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG?
>
> > +#define MEMCG_SUPPORTS_NMI_CHARGING
> > +#endif
>
> Since it's derived from config symbols, it's better to make this an
> internal symbol as well. Something like:
>
>         config MEMCG_NMI_UNSAFE
>                 bool
>                 depends on HAVE_NMI
>                 depends on !ARCH_HAS_NMI_SAFE_THIS_CPU_OPS && !ARCH_HAVE_NMI_SAFE_CMPXCHG
>
> >  #define MEM_CGROUP_ID_SHIFT  16
> >
> >  struct mem_cgroup_id {
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index e17b698f6243..0f182e4a9da0 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2647,11 +2647,26 @@ static struct obj_cgroup *current_objcg_update(void)
> >       return objcg;
> >  }
> >
> > +#ifdef MEMCG_SUPPORTS_NMI_CHARGING
> > +static inline bool nmi_charging_allowed(void)
> > +{
> > +     return true;
> > +}
> > +#else
> > +static inline bool nmi_charging_allowed(void)
> > +{
> > +     return false;
> > +}
> > +#endif
>
> ...drop these...
>
> > +
> >  __always_inline struct obj_cgroup *current_obj_cgroup(void)
> >  {
> >       struct mem_cgroup *memcg;
> >       struct obj_cgroup *objcg;
> >
> > +     if (!nmi_charging_allowed() && in_nmi())
> > +             return NULL;
>
> ..and finally do
>
>         if (IS_ENABLED(CONFIG_MEMCG_NMI_UNSAFE && in_nmi())
>                 return NULL;
>
> here.

Thanks Johannes, will do in the next version.