Re: [PATCH 1/3] mm: memcg: remote memcg charging for kmem allocations

[Shakeel Butt <shakeelb@google.com>]

On Tue, Jun 19, 2018 at 9:22 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Mon, Jun 18, 2018 at 10:13:25PM -0700, Shakeel Butt wrote:
> > @@ -248,6 +248,30 @@ static inline void memalloc_noreclaim_restore(unsigned int flags)
> >       current->flags = (current->flags & ~PF_MEMALLOC) | flags;
> >  }
> >
> > +#ifdef CONFIG_MEMCG
> > +static inline struct mem_cgroup *memalloc_memcg_save(struct mem_cgroup *memcg)
> > +{
> > +     struct mem_cgroup *old_memcg = current->target_memcg;
> > +
> > +     current->target_memcg = memcg;
> > +     return old_memcg;
> > +}
> > +
> > +static inline void memalloc_memcg_restore(struct mem_cgroup *memcg)
> > +{
> > +     current->target_memcg = memcg;
> > +}
>
> The use_mm() and friends naming scheme would be better here:
> memalloc_use_memcg(), memalloc_unuse_memcg(), current->active_memcg
>

Ack. Though do you still think <linux/sched/mm.h> is the right place
for these functions?

> > @@ -375,6 +376,27 @@ static __always_inline void kfree_bulk(size_t size, void **p)
> >       kmem_cache_free_bulk(NULL, size, p);
> >  }
> >
> > +/*
> > + * Calling kmem_cache_alloc_memcg implicitly assumes that the caller wants
> > + * a __GFP_ACCOUNT allocation. However if memcg is NULL then
> > + * kmem_cache_alloc_memcg is same as kmem_cache_alloc.
> > + */
> > +static __always_inline void *kmem_cache_alloc_memcg(struct kmem_cache *cachep,
> > +                                                 gfp_t flags,
> > +                                                 struct mem_cgroup *memcg)
> > +{
> > +     struct mem_cgroup *old_memcg;
> > +     void *ptr;
> > +
> > +     if (!memcg)
> > +             return kmem_cache_alloc(cachep, flags);
> > +
> > +     old_memcg = memalloc_memcg_save(memcg);
> > +     ptr = kmem_cache_alloc(cachep, flags | __GFP_ACCOUNT);
> > +     memalloc_memcg_restore(old_memcg);
> > +     return ptr;
>
> I'm not a big fan of these functions as an interface because it
> implies that kmem_cache_alloc() et al wouldn't charge a memcg - but
> they do, just using current's memcg.
>
> It's also a lot of churn to duplicate all the various slab functions.
>
> Can you please inline the save/restore (or use/unuse) functions into
> the callsites? If you make them handle NULL as parameters, it merely
> adds two bracketing lines around the allocation call in the callsites,
> which I think would be better to understand - in particular with a
> comment on why we are charging *that* group instead of current's.
>

Ack.

> > +static __always_inline struct mem_cgroup *get_mem_cgroup(
> > +                             struct mem_cgroup *memcg, struct mm_struct *mm)
> > +{
> > +     if (unlikely(memcg)) {
> > +             rcu_read_lock();
> > +             if (css_tryget_online(&memcg->css)) {
> > +                     rcu_read_unlock();
> > +                     return memcg;
> > +             }
> > +             rcu_read_unlock();
> > +     }
> > +     return get_mem_cgroup_from_mm(mm);
> > +}
> > +
> >  /**
> >   * mem_cgroup_iter - iterate over memory cgroup hierarchy
> >   * @root: hierarchy root
> > @@ -2260,7 +2274,7 @@ struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep)
> >       if (current->memcg_kmem_skip_account)
> >               return cachep;
> >
> > -     memcg = get_mem_cgroup_from_mm(current->mm);
> > +     memcg = get_mem_cgroup(current->target_memcg, current->mm);
>
> get_mem_cgroup_from_current(), which uses current->active_memcg if set
> and current->mm->memcg otherwise, would be a nicer abstraction IMO.

Ack.

thanks,
Shakeel