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

[KAMEZAWA Hiroyuki]

(2012/04/24 23:22), Frederic Weisbecker wrote:

> 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?
> 

There is a problem of OOM-Kill.
To free memory by killing process, 'mm' should be released by kill.
So, oom-killer just finds a leader of process.

Assume A process X consists of thread A, B and A is thread-group-leader.

Put thread A into cgroup/Gold
    thread B into cgroup/Silver.

If we do accounting based on threads, we can't do anything at OOM in cgroup/Silver.
An idea 'Killing thread-A to kill thread-B'..... breaks isolation.
 
As far as resources used by process, I think accounting should be done per process.
It's not tied to thread.

About kmem, if we count task_struct, page tables, etc...which can be freed by
OOM-Killer i.e. it's allocated for 'process', should be aware of OOM problem.
Using mm->owner makes sense to me until someone finds a great idea to handle
OOM situation rather than task killing.

Thanks,
-Kame