[Devel] Re: [PATCH v2 09/11] memcg: propagate kmem limiting information to children

[Michal Hocko]

On Thu 09-08-12 17:01:17, Glauber Costa wrote:
> The current memcg slab cache management fails to present satisfatory
> hierarchical behavior in the following scenario:
> 
> -> /cgroups/memory/A/B/C
> 
> * kmem limit set at A,
> * A and B have no tasks,
> * span a new task in in C.
> 
> Because kmem_accounted is a boolean that was not set for C, no
> accounting would be done. This is, however, not what we expect.
> 
> The basic idea, is that when a cgroup is limited, we walk the tree
> upwards 

Isn't it rather downwards? We start at A and then mark all children so
we go down the tree. Moreover the walk is not atomic wrt. parallel
charges nor to a new child creation. First one seems to be acceptable
as the charges go to the root. The second one requires cgroup_lock.

It also seems that you are missing memcg_kmem_account_parent in
mem_cgroup_create (use_hierarchy path) if memcg_kmem_is_accounted(parent).

Some further "wording" comments below. Other than that the patch looks
correct.

> (something Kame and I already thought about doing for other
> purposes), and make sure that we store the information about the parent
> being limited in kmem_accounted (that is turned into a bitmap: two
> booleans would not be space efficient). 

Two booleans even don't serve the purpose because you want to test this
atomically, right?

> The code for that is taken from sched/core.c. My reasons for not
> putting it into a common place is to dodge the type issues that would
> arise from a common implementation between memcg and the scheduler -
> but I think that it should ultimately happen, so if you want me to do
> it now, let me know.

Is this really relevant for the patch?

> We do the reverse operation when a formerly limited cgroup becomes
> unlimited.
> 
> Signed-off-by: Glauber Costa <glommer at parallels.com>
> CC: Christoph Lameter <cl at linux.com>
> CC: Pekka Enberg <penberg at cs.helsinki.fi>
> CC: Michal Hocko <mhocko at suse.cz>
> CC: Kamezawa Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
> CC: Johannes Weiner <hannes at cmpxchg.org>
> CC: Suleiman Souhlal <suleiman at google.com>
> ---
>  mm/memcontrol.c | 88 +++++++++++++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 79 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 3216292..3d30b79 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -295,7 +295,8 @@ struct mem_cgroup {
>  	 * Should the accounting and control be hierarchical, per subtree?
>  	 */
>  	bool use_hierarchy;
> -	bool kmem_accounted;
> +
> +	unsigned long kmem_accounted; /* See KMEM_ACCOUNTED_*, below */
>  
>  	bool		oom_lock;
>  	atomic_t	under_oom;
> @@ -348,6 +349,38 @@ struct mem_cgroup {
>  #endif
>  };
>  
> +enum {
> +	KMEM_ACCOUNTED_THIS, /* accounted by this cgroup itself */
> +	KMEM_ACCOUNTED_PARENT, /* accounted by any of its parents. */

How it can be accounted by its parent, the charge doesn't go downwards.
Shouldn't it rather be /* a parent is accounted */

> +};
> +
> +#ifdef CONFIG_MEMCG_KMEM
> +static bool memcg_kmem_account(struct mem_cgroup *memcg)

memcg_kmem_set_account? It matches _clear_ counterpart and it makes
obvious that the value is changed actually.

[...]
> +static bool memcg_kmem_is_accounted(struct mem_cgroup *memcg)
> +{
> +	return test_bit(KMEM_ACCOUNTED_THIS, &memcg->kmem_accounted);
> +}
> +
> +static void memcg_kmem_account_parent(struct mem_cgroup *memcg)

same here _set_parent

[...]
> @@ -614,7 +647,7 @@ EXPORT_SYMBOL(__memcg_kmem_free_page);
>  
>  static void disarm_kmem_keys(struct mem_cgroup *memcg)
>  {
> -	if (memcg->kmem_accounted)
> +	if (test_bit(KMEM_ACCOUNTED_THIS, &memcg->kmem_accounted))

memcg_kmem_is_accounted. I do not see any reason to open code this.

>  		static_key_slow_dec(&memcg_kmem_enabled_key);
>  }
>  #else
> @@ -4171,17 +4204,54 @@ static ssize_t mem_cgroup_read(struct cgroup *cont, struct cftype *cft,
>  static void memcg_update_kmem_limit(struct mem_cgroup *memcg, u64 val)
>  {
>  #ifdef CONFIG_MEMCG_KMEM
> -	/*
> -	 * Once enabled, can't be disabled. We could in theory disable it if we
> -	 * haven't yet created any caches, or if we can shrink them all to
> -	 * death. But it is not worth the trouble.
> -	 */
> +	struct mem_cgroup *iter;
> +
>  	mutex_lock(&set_limit_mutex);
> -	if (!memcg->kmem_accounted && val != RESOURCE_MAX) {
> +	if ((val != RESOURCE_MAX) && memcg_kmem_account(memcg)) {
> +
> +		/*
> +		 * Once enabled, can't be disabled. We could in theory disable
> +		 * it if we haven't yet created any caches, or if we can shrink
> +		 * them all to death. But it is not worth the trouble
> +		 */
>  		static_key_slow_inc(&memcg_kmem_enabled_key);
> -		memcg->kmem_accounted = true;
> +
> +		if (!memcg->use_hierarchy)
> +			goto out;
> +
> +		for_each_mem_cgroup_tree(iter, memcg) {

for_each_mem_cgroup_tree does respect use_hierarchy so the above
shortcut is not necessary. Dunno but IMHO we should get rid of explicit
tests as much as possible. This doesn't look like a hot path anyway.

> +			if (iter == memcg)
> +				continue;
> +			memcg_kmem_account_parent(iter);
> +		}
> +	} else if ((val == RESOURCE_MAX) && memcg_kmem_clear_account(memcg)) {

Above you said "Once enabled, can't be disabled." and now you can
disable it? Say you are a leaf group with non accounted parents. This
will clear the flag and so no further accounting is done. Shouldn't
unlimited mean that we will never reach the limit? Or am I missing
something?

> +
> +		if (!memcg->use_hierarchy)
> +			goto out;
> +
> +		for_each_mem_cgroup_tree(iter, memcg) {
> +			struct mem_cgroup *parent;
> +
> +			if (iter == memcg)
> +				continue;
> +			/*
> +			 * We should only have our parent bit cleared if none
> +			 * of our parents are accounted. The transversal order
> +			 * of our iter function forces us to always look at the
> +			 * parents.
> +			 */
> +			parent = parent_mem_cgroup(iter);
> +			for (; parent != memcg; parent = parent_mem_cgroup(iter))
> +				if (memcg_kmem_is_accounted(parent))
> +					goto noclear;
> +			memcg_kmem_clear_account_parent(iter);

Brain hurts...
Yes we are iterating in the creation ordering so we cannot rely on the
first encountered accounted memcg
A(a) - B - D
     - C (a) - E


> +noclear:
> +			continue;
> +		}
>  	}
> +out:
>  	mutex_unlock(&set_limit_mutex);
> +
>  #endif
>  }
>  
> -- 
> 1.7.11.2
> 
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-- 
Michal Hocko
SUSE Labs