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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, May 16, 2025 at 11:30:17AM +0200, Vlastimil Babka wrote:
> On 5/16/25 08:49, 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@linux.dev>
> > ---
> >  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)
> > +#define MEMCG_SUPPORTS_NMI_CHARGING
> > +#endif
> > +
> >  #define MEM_CGROUP_ID_SHIFT	16
> >  
> >  struct mem_cgroup_id {
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index e17b698f6243..dface07f69bb 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
> > +
> >  __always_inline struct obj_cgroup *current_obj_cgroup(void)
> >  {
> >  	struct mem_cgroup *memcg;
> >  	struct obj_cgroup *objcg;
> >  
> > +	if (in_nmi() && !nmi_charging_allowed())
> 
> Exchange the two as the latter is compile-time constant, so it can shortcut
> the in_nmi() check away in all the good cases?
> 

Oh I thought compiler would figure that out but now that I think about
it, it can only do so if the first condition does not have any
side-effects and though in_nmi() does not, I am not sure if compiler can
extract that information.

I will fix this and make sure that compiler is doing the right thing.