[Devel] [PATCH -mm 1/4] sl[au]b: do not charge large allocations to memcg

[Greg Thelen]

On Wed, Mar 26 2014, Vladimir Davydov <vdavydov at parallels.com> wrote:

> We don't track any random page allocation, so we shouldn't track kmalloc
> that falls back to the page allocator.

This seems like a change which will leads to confusing (and arguably
improper) kernel behavior.  I prefer the behavior prior to this patch.

Before this change both of the following allocations are charged to
memcg (assuming kmem accounting is enabled):
 a = kmalloc(KMALLOC_MAX_CACHE_SIZE, GFP_KERNEL)
 b = kmalloc(KMALLOC_MAX_CACHE_SIZE + 1, GFP_KERNEL)

After this change only 'a' is charged; 'b' goes directly to page
allocator which no longer does accounting.

> Signed-off-by: Vladimir Davydov <vdavydov at parallels.com>
> Cc: Johannes Weiner <hannes at cmpxchg.org>
> Cc: Michal Hocko <mhocko at suse.cz>
> Cc: Glauber Costa <glommer at gmail.com>
> Cc: Christoph Lameter <cl at linux-foundation.org>
> Cc: Pekka Enberg <penberg at kernel.org>
> ---
>  include/linux/slab.h |    2 +-
>  mm/memcontrol.c      |   27 +--------------------------
>  mm/slub.c            |    4 ++--
>  3 files changed, 4 insertions(+), 29 deletions(-)
>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 3dd389aa91c7..8a928ff71d93 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -363,7 +363,7 @@ kmalloc_order(size_t size, gfp_t flags, unsigned int order)
>  {
>  	void *ret;
>  
> -	flags |= (__GFP_COMP | __GFP_KMEMCG);
> +	flags |= __GFP_COMP;
>  	ret = (void *) __get_free_pages(flags, order);
>  	kmemleak_alloc(ret, size, 1, flags);
>  	return ret;
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b4b6aef562fa..81a162d01d4d 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3528,35 +3528,10 @@ __memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **_memcg, int order)
>  
>  	*_memcg = NULL;
>  
> -	/*
> -	 * Disabling accounting is only relevant for some specific memcg
> -	 * internal allocations. Therefore we would initially not have such
> -	 * check here, since direct calls to the page allocator that are marked
> -	 * with GFP_KMEMCG only happen outside memcg core. We are mostly
> -	 * concerned with cache allocations, and by having this test at
> -	 * memcg_kmem_get_cache, we are already able to relay the allocation to
> -	 * the root cache and bypass the memcg cache altogether.
> -	 *
> -	 * There is one exception, though: the SLUB allocator does not create
> -	 * large order caches, but rather service large kmallocs directly from
> -	 * the page allocator. Therefore, the following sequence when backed by
> -	 * the SLUB allocator:
> -	 *
> -	 *	memcg_stop_kmem_account();
> -	 *	kmalloc(<large_number>)
> -	 *	memcg_resume_kmem_account();
> -	 *
> -	 * would effectively ignore the fact that we should skip accounting,
> -	 * since it will drive us directly to this function without passing
> -	 * through the cache selector memcg_kmem_get_cache. Such large
> -	 * allocations are extremely rare but can happen, for instance, for the
> -	 * cache arrays. We bring this test here.
> -	 */
> -	if (!current->mm || current->memcg_kmem_skip_account)
> +	if (!current->mm)
>  		return true;
>  
>  	memcg = get_mem_cgroup_from_mm(current->mm);
> -
>  	if (!memcg_can_account_kmem(memcg)) {
>  		css_put(&memcg->css);
>  		return true;
> diff --git a/mm/slub.c b/mm/slub.c
> index 5e234f1f8853..c2e58a787443 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3325,7 +3325,7 @@ static void *kmalloc_large_node(size_t size, gfp_t flags, int node)
>  	struct page *page;
>  	void *ptr = NULL;
>  
> -	flags |= __GFP_COMP | __GFP_NOTRACK | __GFP_KMEMCG;
> +	flags |= __GFP_COMP | __GFP_NOTRACK;
>  	page = alloc_pages_node(node, flags, get_order(size));
>  	if (page)
>  		ptr = page_address(page);
> @@ -3395,7 +3395,7 @@ void kfree(const void *x)
>  	if (unlikely(!PageSlab(page))) {
>  		BUG_ON(!PageCompound(page));
>  		kfree_hook(x);
> -		__free_memcg_kmem_pages(page, compound_order(page));
> +		__free_pages(page, compound_order(page));
>  		return;
>  	}
>  	slab_free(page->slab_cache, page, object, _RET_IP_);
> -- 
> 1.7.10.4