Re: [PATCH v3 09/19] mm: memcg/slab: charge individual slab objects instead of pages

[Vlastimil Babka]

On 4/22/20 10:46 PM, Roman Gushchin wrote:
> Switch to per-object accounting of non-root slab objects.
> 
> Charging is performed using obj_cgroup API in the pre_alloc hook.
> Obj_cgroup is charged with the size of the object and the size
> of metadata: as now it's the size of an obj_cgroup pointer.
> If the amount of memory has been charged successfully, the actual
> allocation code is executed. Otherwise, -ENOMEM is returned.
> 
> In the post_alloc hook if the actual allocation succeeded,
> corresponding vmstats are bumped and the obj_cgroup pointer is saved.
> Otherwise, the charge is canceled.
> 
> On the free path obj_cgroup pointer is obtained and used to uncharge
> the size of the releasing object.
> 
> Memcg and lruvec counters are now representing only memory used
> by active slab objects and do not include the free space. The free
> space is shared and doesn't belong to any specific cgroup.
> 
> Global per-node slab vmstats are still modified from (un)charge_slab_page()
> functions. The idea is to keep all slab pages accounted as slab pages
> on system level.
> 
> Signed-off-by: Roman Gushchin <guro@xxxxxx>

Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>

Suggestion below:

> @@ -568,32 +548,33 @@ static __always_inline int charge_slab_page(struct page *page,
>  					    gfp_t gfp, int order,
>  					    struct kmem_cache *s)
>  {
> -	int ret;
> -
> -	if (is_root_cache(s)) {
> -		mod_node_page_state(page_pgdat(page), cache_vmstat_idx(s),
> -				    PAGE_SIZE << order);
> -		return 0;
> -	}
> +#ifdef CONFIG_MEMCG_KMEM
> +	if (!is_root_cache(s)) {

This could also benefit from memcg_kmem_enabled() static key test AFAICS. Maybe
even have a wrapper for both tests together?

> +		int ret;
>  
> -	ret = memcg_alloc_page_obj_cgroups(page, gfp, objs_per_slab(s));
> -	if (ret)
> -		return ret;
> +		ret = memcg_alloc_page_obj_cgroups(page, gfp, objs_per_slab(s));

You created memcg_alloc_page_obj_cgroups() empty variant for !CONFIG_MEMCG_KMEM
but now the only caller is under CONFIG_MEMCG_KMEM.

> +		if (ret)
> +			return ret;
>  
> -	return memcg_charge_slab(page, gfp, order, s);
> +		percpu_ref_get_many(&s->memcg_params.refcnt, 1 << order);

Perhaps moving this refcount into memcg_alloc_page_obj_cgroups() (maybe the name
should be different then) will allow you to not add #ifdef CONFIG_MEMCG_KMEM in
this function.

Maybe this is all moot after patch 12/19, will find out :)

> +	}
> +#endif
> +	mod_node_page_state(page_pgdat(page), cache_vmstat_idx(s),
> +			    PAGE_SIZE << order);
> +	return 0;
>  }
>  
>  static __always_inline void uncharge_slab_page(struct page *page, int order,
>  					       struct kmem_cache *s)
>  {
> -	if (is_root_cache(s)) {
> -		mod_node_page_state(page_pgdat(page), cache_vmstat_idx(s),
> -				    -(PAGE_SIZE << order));
> -		return;
> +#ifdef CONFIG_MEMCG_KMEM
> +	if (!is_root_cache(s)) {

Everything from above also applies here.

> +		memcg_free_page_obj_cgroups(page);
> +		percpu_ref_put_many(&s->memcg_params.refcnt, 1 << order);
>  	}
> -
> -	memcg_free_page_obj_cgroups(page);
> -	memcg_uncharge_slab(page, order, s);
> +#endif
> +	mod_node_page_state(page_pgdat(page), cache_vmstat_idx(s),
> +			    -(PAGE_SIZE << order));
>  }
>  
>  static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)