Re: [PATCH v2 6/7] memcg: replace cgroup_lock with memcg specific memcg_lock

[Michal Hocko <mhocko@suse.cz>]

On Fri 11-01-13 13:45:26, Glauber Costa wrote:
> After the preparation work done in earlier patches, the
> cgroup_lock can be trivially replaced with a memcg-specific lock. This
> is an automatic translation in every site the values involved were
> queried.
> 
> The sites were values are written, however, used to be naturally called
> under cgroup_lock. This is the case for instance of the css_online
> callback. For those, we now need to explicitly add the memcg_lock.
> 
> With this, all the calls to cgroup_lock outside cgroup core are gone.
> 
> Signed-off-by: Glauber Costa <glommer@parallels.com>

Please add a doc for memcg_mutex

Acked-by: Michal Hocko <mhocko@suse.cz>

> ---
>  mm/memcontrol.c | 43 ++++++++++++++++++++++---------------------
>  1 file changed, 22 insertions(+), 21 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c024614..5f3adbc 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -472,6 +472,8 @@ enum res_type {
>  #define MEM_CGROUP_RECLAIM_SHRINK_BIT	0x1
>  #define MEM_CGROUP_RECLAIM_SHRINK	(1 << MEM_CGROUP_RECLAIM_SHRINK_BIT)
>  
  /*
   * This lock protects TODO
   */
> +static DEFINE_MUTEX(memcg_mutex);
> +
>  static void mem_cgroup_get(struct mem_cgroup *memcg);
>  static void mem_cgroup_put(struct mem_cgroup *memcg);
>  
> @@ -2909,7 +2911,7 @@ int memcg_cache_id(struct mem_cgroup *memcg)
>   * operation, because that is its main call site.
>   *
>   * But when we create a new cache, we can call this as well if its parent
> - * is kmem-limited. That will have to hold cgroup_lock as well.
> + * is kmem-limited. That will have to hold memcg_mutex as well.
>   */
>  int memcg_update_cache_sizes(struct mem_cgroup *memcg)
>  {
> @@ -2924,7 +2926,7 @@ int memcg_update_cache_sizes(struct mem_cgroup *memcg)
>  	 * the beginning of this conditional), is no longer 0. This
>  	 * guarantees only one process will set the following boolean
>  	 * to true. We don't need test_and_set because we're protected
> -	 * by the cgroup_lock anyway.
> +	 * by the memcg_mutex anyway.
>  	 */
>  	memcg_kmem_set_activated(memcg);
>  
> @@ -3265,9 +3267,9 @@ void kmem_cache_destroy_memcg_children(struct kmem_cache *s)
>  	 *
>  	 * Still, we don't want anyone else freeing memcg_caches under our
>  	 * noses, which can happen if a new memcg comes to life. As usual,
> -	 * we'll take the cgroup_lock to protect ourselves against this.
> +	 * we'll take the memcg_mutex to protect ourselves against this.
>  	 */
> -	cgroup_lock();
> +	mutex_lock(&memcg_mutex);
>  	for (i = 0; i < memcg_limited_groups_array_size; i++) {
>  		c = s->memcg_params->memcg_caches[i];
>  		if (!c)
> @@ -3290,7 +3292,7 @@ void kmem_cache_destroy_memcg_children(struct kmem_cache *s)
>  		cancel_work_sync(&c->memcg_params->destroy);
>  		kmem_cache_destroy(c);
>  	}
> -	cgroup_unlock();
> +	mutex_unlock(&memcg_mutex);
>  }
>  
>  struct create_work {
> @@ -4816,7 +4818,7 @@ static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft,
>  	if (parent)
>  		parent_memcg = mem_cgroup_from_cont(parent);
>  
> -	cgroup_lock();
> +	mutex_lock(&memcg_mutex);
>  
>  	if (memcg->use_hierarchy == val)
>  		goto out;
> @@ -4839,7 +4841,7 @@ static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft,
>  		retval = -EINVAL;
>  
>  out:
> -	cgroup_unlock();
> +	mutex_unlock(&memcg_mutex);
>  
>  	return retval;
>  }
> @@ -4939,13 +4941,10 @@ static int memcg_update_kmem_limit(struct cgroup *cont, u64 val)
>  	 * After it first became limited, changes in the value of the limit are
>  	 * of course permitted.
>  	 *
> -	 * Taking the cgroup_lock is really offensive, but it is so far the only
> -	 * way to guarantee that no children will appear. There are plenty of
> -	 * other offenders, and they should all go away. Fine grained locking
> -	 * is probably the way to go here. When we are fully hierarchical, we
> -	 * can also get rid of the use_hierarchy check.
> +	 * We are protected by the memcg_mutex, so no other cgroups can appear
> +	 * in the mean time.
>  	 */
> -	cgroup_lock();
> +	mutex_lock(&memcg_mutex);
>  	if (!memcg->kmem_account_flags && val != RESOURCE_MAX) {
>  		if (cgroup_task_count(cont) || memcg_has_children(memcg)) {
>  			ret = -EBUSY;
> @@ -4970,7 +4969,7 @@ static int memcg_update_kmem_limit(struct cgroup *cont, u64 val)
>  	} else
>  		ret = res_counter_set_limit(&memcg->kmem, val);
>  out:
> -	cgroup_unlock();
> +	mutex_unlock(&memcg_mutex);
>  
>  	/*
>  	 * We are by now familiar with the fact that we can't inc the static
> @@ -5027,9 +5026,9 @@ static int memcg_propagate_kmem(struct mem_cgroup *memcg)
>  	mem_cgroup_get(memcg);
>  	static_key_slow_inc(&memcg_kmem_enabled_key);
>  
> -	cgroup_lock();
> +	mutex_lock(&memcg_mutex);
>  	ret = memcg_update_cache_sizes(memcg);
> -	cgroup_unlock();
> +	mutex_unlock(&memcg_mutex);
>  #endif
>  out:
>  	return ret;
> @@ -5359,17 +5358,17 @@ static int mem_cgroup_swappiness_write(struct cgroup *cgrp, struct cftype *cft,
>  
>  	parent = mem_cgroup_from_cont(cgrp->parent);
>  
> -	cgroup_lock();
> +	mutex_lock(&memcg_mutex);
>  
>  	/* If under hierarchy, only empty-root can set this value */
>  	if ((parent->use_hierarchy) || memcg_has_children(memcg)) {
> -		cgroup_unlock();
> +		mutex_unlock(&memcg_mutex);
>  		return -EINVAL;
>  	}
>  
>  	memcg->swappiness = val;
>  
> -	cgroup_unlock();
> +	mutex_unlock(&memcg_mutex);
>  
>  	return 0;
>  }
> @@ -5695,7 +5694,7 @@ static int mem_cgroup_oom_control_write(struct cgroup *cgrp,
>  
>  	parent = mem_cgroup_from_cont(cgrp->parent);
>  
> -	cgroup_lock();
> +	mutex_lock(&memcg_mutex);
>  	/* oom-kill-disable is a flag for subhierarchy. */
>  	if ((parent->use_hierarchy) ||
>  	    (memcg->use_hierarchy && !list_empty(&cgrp->children))) {
> @@ -5705,7 +5704,7 @@ static int mem_cgroup_oom_control_write(struct cgroup *cgrp,
>  	memcg->oom_kill_disable = val;
>  	if (!val)
>  		memcg_oom_recover(memcg);
> -	cgroup_unlock();
> +	mutex_unlock(&memcg_mutex);
>  	return 0;
>  }
>  
> @@ -6148,6 +6147,7 @@ mem_cgroup_css_online(struct cgroup *cont)
>  		return 0;
>  	}
>  
> +	mutex_lock(&memcg_mutex);
>  	parent = mem_cgroup_from_cont(cont->parent);
>  
>  	memcg->use_hierarchy = parent->use_hierarchy;
> @@ -6182,6 +6182,7 @@ mem_cgroup_css_online(struct cgroup *cont)
>  	atomic_set(&memcg->refcnt, 1);
>  
>  	error = memcg_init_kmem(memcg, &mem_cgroup_subsys);
> +	mutex_unlock(&memcg_mutex);
>  	if (error) {
>  		/*
>  		 * We call put now because our (and parent's) refcnts
> -- 
> 1.7.11.7
> 

-- 
Michal Hocko
SUSE Labs

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