Re: [PATCH -mm 2/4] sl[au]b: charge slabs to memcg explicitly

[Vladimir Davydov]

On 03/27/2014 01:58 AM, Michal Hocko wrote:
> On Wed 26-03-14 19:28:05, Vladimir Davydov wrote:
>> We have only a few places where we actually want to charge kmem so
>> instead of intruding into the general page allocation path with
>> __GFP_KMEMCG it's better to explictly charge kmem there. All kmem
>> charges will be easier to follow that way.
>>
>> This is a step towards removing __GFP_KMEMCG. It removes __GFP_KMEMCG
>> from memcg caches' allocflags. Instead it makes slab allocation path
>> call memcg_charge_kmem directly getting memcg to charge from the cache's
>> memcg params.
> Yes, removing __GFP_KMEMCG is definitely a good step. I am currently at
> a conference and do not have much time to review this properly (even
> worse will be on vacation for the next 2 weeks) but where did all the
> static_key optimization go? What am I missing.

I expected this question, because I want somebody to confirm if we
really need such kind of optimization in the slab allocation path. From
my POV, since we thrash cpu caches there anyway by calling alloc_pages,
wrapping memcg_charge_slab in a static branch wouldn't result in any
noticeable performance boost.

I do admit we benefit from static branching in memcg_kmem_get_cache,
because this one is called on every kmem object allocation, but slab
allocations happen much rarer.

I don't insist on that though, so if you say "no", I'll just add
__memcg_charge_slab and make memcg_charge_slab call it if the static key
is on, but may be, we can avoid such code bloating?

Thanks.

>> Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
>> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
>> Cc: Michal Hocko <mhocko@xxxxxxx>
>> Cc: Glauber Costa <glommer@xxxxxxxxx>
>> Cc: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Pekka Enberg <penberg@xxxxxxxxxx>
>> ---
>>  include/linux/memcontrol.h |   24 +++++++++++++-----------
>>  mm/memcontrol.c            |   15 +++++++++++++++
>>  mm/slab.c                  |    7 ++++++-
>>  mm/slab_common.c           |    6 +-----
>>  mm/slub.c                  |   24 +++++++++++++++++-------
>>  5 files changed, 52 insertions(+), 24 deletions(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index e9dfcdad24c5..b8aaecc25cbf 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -512,6 +512,9 @@ void memcg_update_array_size(int num_groups);
>>  struct kmem_cache *
>>  __memcg_kmem_get_cache(struct kmem_cache *cachep, gfp_t gfp);
>>  
>> +int memcg_charge_slab(struct kmem_cache *s, gfp_t gfp, int order);
>> +void memcg_uncharge_slab(struct kmem_cache *s, int order);
>> +
>>  void mem_cgroup_destroy_cache(struct kmem_cache *cachep);
>>  int __kmem_cache_destroy_memcg_children(struct kmem_cache *s);
>>  
>> @@ -589,17 +592,7 @@ memcg_kmem_commit_charge(struct page *page, struct mem_cgroup *memcg, int order)
>>   * @cachep: the original global kmem cache
>>   * @gfp: allocation flags.
>>   *
>> - * This function assumes that the task allocating, which determines the memcg
>> - * in the page allocator, belongs to the same cgroup throughout the whole
>> - * process.  Misacounting can happen if the task calls memcg_kmem_get_cache()
>> - * while belonging to a cgroup, and later on changes. This is considered
>> - * acceptable, and should only happen upon task migration.
>> - *
>> - * Before the cache is created by the memcg core, there is also a possible
>> - * imbalance: the task belongs to a memcg, but the cache being allocated from
>> - * is the global cache, since the child cache is not yet guaranteed to be
>> - * ready. This case is also fine, since in this case the GFP_KMEMCG will not be
>> - * passed and the page allocator will not attempt any cgroup accounting.
>> + * All memory allocated from a per-memcg cache is charged to the owner memcg.
>>   */
>>  static __always_inline struct kmem_cache *
>>  memcg_kmem_get_cache(struct kmem_cache *cachep, gfp_t gfp)
>> @@ -667,6 +660,15 @@ memcg_kmem_get_cache(struct kmem_cache *cachep, gfp_t gfp)
>>  {
>>  	return cachep;
>>  }
>> +
>> +static inline int memcg_charge_slab(struct kmem_cache *s, gfp_t gfp, int order)
>> +{
>> +	return 0;
>> +}
>> +
>> +static inline void memcg_uncharge_slab(struct kmem_cache *s, int order)
>> +{
>> +}
>>  #endif /* CONFIG_MEMCG_KMEM */
>>  #endif /* _LINUX_MEMCONTROL_H */
>>  
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 81a162d01d4d..9bbc088e3107 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -3506,6 +3506,21 @@ out:
>>  }
>>  EXPORT_SYMBOL(__memcg_kmem_get_cache);
>>  
>> +int memcg_charge_slab(struct kmem_cache *s, gfp_t gfp, int order)
>> +{
>> +	if (is_root_cache(s))
>> +		return 0;
>> +	return memcg_charge_kmem(s->memcg_params->memcg, gfp,
>> +				 PAGE_SIZE << order);
>> +}
>> +
>> +void memcg_uncharge_slab(struct kmem_cache *s, int order)
>> +{
>> +	if (is_root_cache(s))
>> +		return;
>> +	memcg_uncharge_kmem(s->memcg_params->memcg, PAGE_SIZE << order);
>> +}
>> +
>>  /*
>>   * We need to verify if the allocation against current->mm->owner's memcg is
>>   * possible for the given order. But the page is not allocated yet, so we'll
>> diff --git a/mm/slab.c b/mm/slab.c
>> index eebc619ae33c..af126a37dafd 100644
>> --- a/mm/slab.c
>> +++ b/mm/slab.c
>> @@ -1664,8 +1664,12 @@ static struct page *kmem_getpages(struct kmem_cache *cachep, gfp_t flags,
>>  	if (cachep->flags & SLAB_RECLAIM_ACCOUNT)
>>  		flags |= __GFP_RECLAIMABLE;
>>  
>> +	if (memcg_charge_slab(cachep, flags, cachep->gfporder))
>> +		return NULL;
>> +
>>  	page = alloc_pages_exact_node(nodeid, flags | __GFP_NOTRACK, cachep->gfporder);
>>  	if (!page) {
>> +		memcg_uncharge_slab(cachep, cachep->gfporder);
>>  		if (!(flags & __GFP_NOWARN) && printk_ratelimit())
>>  			slab_out_of_memory(cachep, flags, nodeid);
>>  		return NULL;
>> @@ -1724,7 +1728,8 @@ static void kmem_freepages(struct kmem_cache *cachep, struct page *page)
>>  	memcg_release_pages(cachep, cachep->gfporder);
>>  	if (current->reclaim_state)
>>  		current->reclaim_state->reclaimed_slab += nr_freed;
>> -	__free_memcg_kmem_pages(page, cachep->gfporder);
>> +	__free_pages(page, cachep->gfporder);
>> +	memcg_uncharge_slab(cachep, cachep->gfporder);
>>  }
>>  
>>  static void kmem_rcu_free(struct rcu_head *head)
>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>> index f3cfccf76dda..6673597ac967 100644
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -290,12 +290,8 @@ void kmem_cache_create_memcg(struct mem_cgroup *memcg, struct kmem_cache *root_c
>>  				 root_cache->size, root_cache->align,
>>  				 root_cache->flags, root_cache->ctor,
>>  				 memcg, root_cache);
>> -	if (IS_ERR(s)) {
>> +	if (IS_ERR(s))
>>  		kfree(cache_name);
>> -		goto out_unlock;
>> -	}
>> -
>> -	s->allocflags |= __GFP_KMEMCG;
>>  
>>  out_unlock:
>>  	mutex_unlock(&slab_mutex);
>> diff --git a/mm/slub.c b/mm/slub.c
>> index c2e58a787443..6fefe3b33ce0 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -1317,17 +1317,26 @@ static inline void slab_free_hook(struct kmem_cache *s, void *x)
>>  /*
>>   * Slab allocation and freeing
>>   */
>> -static inline struct page *alloc_slab_page(gfp_t flags, int node,
>> -					struct kmem_cache_order_objects oo)
>> +static inline struct page *alloc_slab_page(struct kmem_cache *s,
>> +		gfp_t flags, int node, struct kmem_cache_order_objects oo)
>>  {
>> +	struct page *page;
>>  	int order = oo_order(oo);
>>  
>>  	flags |= __GFP_NOTRACK;
>>  
>> +	if (memcg_charge_slab(s, flags, order))
>> +		return NULL;
>> +
>>  	if (node == NUMA_NO_NODE)
>> -		return alloc_pages(flags, order);
>> +		page = alloc_pages(flags, order);
>>  	else
>> -		return alloc_pages_exact_node(node, flags, order);
>> +		page = alloc_pages_exact_node(node, flags, order);
>> +
>> +	if (!page)
>> +		memcg_uncharge_slab(s, order);
>> +
>> +	return page;
>>  }
>>  
>>  static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
>> @@ -1349,7 +1358,7 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
>>  	 */
>>  	alloc_gfp = (flags | __GFP_NOWARN | __GFP_NORETRY) & ~__GFP_NOFAIL;
>>  
>> -	page = alloc_slab_page(alloc_gfp, node, oo);
>> +	page = alloc_slab_page(s, alloc_gfp, node, oo);
>>  	if (unlikely(!page)) {
>>  		oo = s->min;
>>  		alloc_gfp = flags;
>> @@ -1357,7 +1366,7 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
>>  		 * Allocation may have failed due to fragmentation.
>>  		 * Try a lower order alloc if possible
>>  		 */
>> -		page = alloc_slab_page(alloc_gfp, node, oo);
>> +		page = alloc_slab_page(s, alloc_gfp, node, oo);
>>  
>>  		if (page)
>>  			stat(s, ORDER_FALLBACK);
>> @@ -1473,7 +1482,8 @@ static void __free_slab(struct kmem_cache *s, struct page *page)
>>  	page_mapcount_reset(page);
>>  	if (current->reclaim_state)
>>  		current->reclaim_state->reclaimed_slab += pages;
>> -	__free_memcg_kmem_pages(page, order);
>> +	__free_pages(page, order);
>> +	memcg_uncharge_slab(s, order);
>>  }
>>  
>>  #define need_reserve_slab_rcu						\
>> -- 
>> 1.7.10.4
>>

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