Re: [PATCH 2/2] mm: memcg/slab: pre-allocate obj_cgroups for slab caches with SLAB_ACCOUNT

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Nov 10, 2020 at 11:57:53AM -0800, Roman Gushchin wrote:
> In general it's unknown in advance if a slab page will contain
> accounted objects or not. In order to avoid memory waste, an
> obj_cgroup vector is allocated dynamically when a need to account
> of a new object arises. Such approach is memory efficient, but
> requires an expensive cmpxchg() to set up the memcg/objcgs pointer,
> because an allocation can race with a different allocation on another
> cpu.
> 
> But in some common cases it's known for sure that a slab page will
> contain accounted objects: if the page belongs to a slab cache with a
> SLAB_ACCOUNT flag set. It includes such popular objects like
> vm_area_struct, anon_vma, task_struct, etc.
> 
> In such cases we can pre-allocate the objcgs vector and simple assign
> it to the page without any atomic operations, because at this early
> stage the page is not visible to anyone else.
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>

That's a nice optimization!

Some comments inline:

> @@ -485,14 +485,20 @@ static inline struct obj_cgroup **page_objcgs_check(struct page *page)
>   * set_page_objcgs - associate a page with a object cgroups vector
>   * @page: a pointer to the page struct
>   * @objcgs: a pointer to the object cgroups vector
> + * @atomic: save the value atomically
>   *
>   * Atomically associates a page with a vector of object cgroups.
>   */
>  static inline bool set_page_objcgs(struct page *page,
> -					struct obj_cgroup **objcgs)
> +				   struct obj_cgroup **objcgs, bool atomic)

bool parameters make callsites pretty hard to understand - unless the
function interface obviously has two modes (read vs write etc.), which
isn't the case here.

> -	return !cmpxchg(&page->memcg_data, 0, (unsigned long)objcgs |
> -			MEMCG_DATA_OBJCGS);
> +	unsigned long memcg_data = (unsigned long) objcgs | MEMCG_DATA_OBJCGS;
> +
> +	if (atomic)
> +		return !cmpxchg(&page->memcg_data, 0, memcg_data);
> +
> +	page->memcg_data = memcg_data;
> +	return true;
>  }
>  #else
>  static inline struct obj_cgroup **page_objcgs(struct page *page)
> @@ -506,7 +512,7 @@ static inline struct obj_cgroup **page_objcgs_check(struct page *page)
>  }
>  
>  static inline bool set_page_objcgs(struct page *page,
> -					struct obj_cgroup **objcgs)
> +				   struct obj_cgroup **objcgs, bool atomic)
>  {
>  	return true;
>  }
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 69a2893a6455..37bffd336235 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2874,7 +2874,7 @@ static void commit_charge(struct page *page, struct mem_cgroup *memcg)
>  
>  #ifdef CONFIG_MEMCG_KMEM
>  int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
> -				 gfp_t gfp)
> +				 gfp_t gfp, bool atomic)
>  {
>  	unsigned int objects = objs_per_slab_page(s, page);
>  	void *vec;
> @@ -2884,7 +2884,7 @@ int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
>  	if (!vec)
>  		return -ENOMEM;
>  
> -	if (!set_page_objcgs(page, vec))
> +	if (!set_page_objcgs(page, vec, atomic))
>  		kfree(vec);
>  	else
>  		kmemleak_not_leak(vec);

The life of page->memcg_data and this optimization could use a central
comment somewhere, because it's hard to understand what's going on
from the code alone. This function here seems like a good place?

I don't see a way to eliminate the bool on the allocation function,
but maybe it could be more descriptive. Maybe bool slab_account?

set_page_objcgs() can be inlined at this point. It made some sense to
abstract away the atomics with setter and matching getter, but with a
non-atomic mode, inlining makes things clearer and allows for better
in-place documentation in the sole callsite.

How about something like this?

	vec = kcalloc(...);

	memcg_data = (unsigned long)vec | MEMCG_DATA_OBJCGS;
	/*
	 * Set up the objcg vector for the page.
	 *
	 * When only some objects in a slab are tracked (think GFP_ACCOUNT
	 * kmalloc allocations), the objcg vector is set up when the first
	 * tracked object in the slab page is allocated. Multiple concurrent
	 * slab allocations can race to this, so synchronization is required.
	 *
	 * When SLAB_ACCOUNT is set on the cache, however, all objects in the
	 * slab page will be tracked, and the vector is allocated along with
	 * the page itself, while it's still exclusive; no atomics necessary.
	 */
	if (slab_account) {
		page->memcg_data = memcg_data;
	} else {
		if (cmpxchg(&page->memcg_data, 0, memcg_data)) {
			/* Somebody else beat us, use their vec */
			kfree(vec);
			return 0;
		}
	}
	kmemleak_not_leak(vec);
	return 0;