[Devel] Re: [PATCH 17/23] kmem controller charge/uncharge infrastructure

[Frederic Weisbecker]

On Mon, Apr 23, 2012 at 03:25:59PM -0700, David Rientjes wrote:
> On Sun, 22 Apr 2012, Glauber Costa wrote:
> 
> > +/*
> > + * Return the kmem_cache we're supposed to use for a slab allocation.
> > + * If we are in interrupt context or otherwise have an allocation that
> > + * can't fail, we return the original cache.
> > + * Otherwise, we will try to use the current memcg's version of the cache.
> > + *
> > + * If the cache does not exist yet, if we are the first user of it,
> > + * we either create it immediately, if possible, or create it asynchronously
> > + * in a workqueue.
> > + * In the latter case, we will let the current allocation go through with
> > + * the original cache.
> > + *
> > + * This function returns with rcu_read_lock() held.
> > + */
> > +struct kmem_cache *__mem_cgroup_get_kmem_cache(struct kmem_cache *cachep,
> > +					     gfp_t gfp)
> > +{
> > +	struct mem_cgroup *memcg;
> > +	int idx;
> > +
> > +	gfp |=  cachep->allocflags;
> > +
> > +	if ((current->mm == NULL))
> > +		return cachep;
> > +
> > +	if (cachep->memcg_params.memcg)
> > +		return cachep;
> > +
> > +	idx = cachep->memcg_params.id;
> > +	VM_BUG_ON(idx == -1);
> > +
> > +	memcg = mem_cgroup_from_task(current);
> > +	if (!mem_cgroup_kmem_enabled(memcg))
> > +		return cachep;
> > +
> > +	if (rcu_access_pointer(memcg->slabs[idx]) == NULL) {
> > +		memcg_create_cache_enqueue(memcg, cachep);
> > +		return cachep;
> > +	}
> > +
> > +	return rcu_dereference(memcg->slabs[idx]);
> > +}
> > +EXPORT_SYMBOL(__mem_cgroup_get_kmem_cache);
> > +
> > +void mem_cgroup_remove_child_kmem_cache(struct kmem_cache *cachep, int id)
> > +{
> > +	rcu_assign_pointer(cachep->memcg_params.memcg->slabs[id], NULL);
> > +}
> > +
> > +bool __mem_cgroup_charge_kmem(gfp_t gfp, size_t size)
> > +{
> > +	struct mem_cgroup *memcg;
> > +	bool ret = true;
> > +
> > +	rcu_read_lock();
> > +	memcg = mem_cgroup_from_task(current);
> 
> This seems horribly inconsistent with memcg charging of user memory since 
> it charges to p->mm->owner and you're charging to p.  So a thread attached 
> to a memcg can charge user memory to one memcg while charging slab to 
> another memcg?

Charging to the thread rather than the process seem to me the right behaviour:
you can have two threads of a same process attached to different cgroups.

Perhaps it is the user memory memcg that needs to be fixed?

> 
> > +
> > +	if (!mem_cgroup_kmem_enabled(memcg))
> > +		goto out;
> > +
> > +	mem_cgroup_get(memcg);
> > +	ret = memcg_charge_kmem(memcg, gfp, size) == 0;
> > +	if (ret)
> > +		mem_cgroup_put(memcg);
> > +out:
> > +	rcu_read_unlock();
> > +	return ret;
> > +}
> > +EXPORT_SYMBOL(__mem_cgroup_charge_kmem);
> > +
> > +void __mem_cgroup_uncharge_kmem(size_t size)
> > +{
> > +	struct mem_cgroup *memcg;
> > +
> > +	rcu_read_lock();
> > +	memcg = mem_cgroup_from_task(current);
> > +
> > +	if (!mem_cgroup_kmem_enabled(memcg))
> > +		goto out;
> > +
> > +	mem_cgroup_put(memcg);
> > +	memcg_uncharge_kmem(memcg, size);
> > +out:
> > +	rcu_read_unlock();
> > +}
> > +EXPORT_SYMBOL(__mem_cgroup_uncharge_kmem);