linux-kernel - Re: [PATCH v3 13/28] slub: create duplicate cache

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Date:	Tue, 29 May 2012 19:56:23 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Christoph Lameter <cl@...ux.com>
CC:	<linux-kernel@...r.kernel.org>, <cgroups@...r.kernel.org>,
	<linux-mm@...ck.org>, <kamezawa.hiroyu@...fujitsu.com>,
	Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
	Greg Thelen <gthelen@...gle.com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>, <devel@...nvz.org>,
	David Rientjes <rientjes@...gle.com>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH v3 13/28] slub: create duplicate cache

On 05/29/2012 06:36 PM, Christoph Lameter wrote:
> On Fri, 25 May 2012, Glauber Costa wrote:
>
>> index dacd1fb..4689034 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -467,6 +467,23 @@ struct cg_proto *tcp_proto_cgroup(struct mem_cgroup *memcg)
>>   EXPORT_SYMBOL(tcp_proto_cgroup);
>>   #endif /* CONFIG_INET */
>>
>> +char *mem_cgroup_cache_name(struct mem_cgroup *memcg, struct kmem_cache *cachep)
>> +{
>> +	char *name;
>> +	struct dentry *dentry;
>> +
>> +	rcu_read_lock();
>> +	dentry = rcu_dereference(memcg->css.cgroup->dentry);
>> +	rcu_read_unlock();
>> +
>> +	BUG_ON(dentry == NULL);
>> +
>> +	name = kasprintf(GFP_KERNEL, "%s(%d:%s)",
>> +	    cachep->name, css_id(&memcg->css), dentry->d_name.name);
>> +
>> +	return name;
>> +}
>
> Function allocates a string that is supposed to be disposed of by the
> caller. That needs to be documented and maybe even the name needs to
> reflect that.

Okay, I can change it.

>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -4002,6 +4002,38 @@ struct kmem_cache *kmem_cache_create(const char *name, size_t size,
>>   }
>>   EXPORT_SYMBOL(kmem_cache_create);
>>
>> +#ifdef CONFIG_CGROUP_MEM_RES_CTLR_KMEM
>> +struct kmem_cache *kmem_cache_dup(struct mem_cgroup *memcg,
>> +				  struct kmem_cache *s)
>> +{
>> +	char *name;
>> +	struct kmem_cache *new;
>> +
>> +	name = mem_cgroup_cache_name(memcg, s);
>> +	if (!name)
>> +		return NULL;
>> +
>> +	new = kmem_cache_create_memcg(memcg, name, s->objsize, s->align,
>> +				      (s->allocflags&  ~SLAB_PANIC), s->ctor);
>
> Hmmm... A full duplicate of the slab cache? We may have many sparsely
> used portions of the per node and per cpu structure as a result.

I've already commented on patch 0, but I will repeat it here. This 
approach leads to more fragmentation, yes, but this is exactly to be 
less intrusive.

With a full copy, all I need to do is:

1) relay the allocation to the right cache.
2) account for a new page when it is needed.

How does the cache work from inside? I don't care.

Accounting pages seems just crazy to me. If new allocators come in the 
future, organizing the pages in a different way, instead of patching it 
here and there, we need to totally rewrite this.

If those allocators happen to depend on a specific placement for 
performance, then we're destroying this as well too.

>
>> +	 * prevent it from being deleted. If kmem_cache_destroy() is
>> +	 * called for the root cache before we call it for a child cache,
>> +	 * it will be queued for destruction when we finally drop the
>> +	 * reference on the child cache.
>> +	 */
>> +	if (new) {
>> +		down_write(&slub_lock);
>> +		s->refcount++;
>> +		up_write(&slub_lock);
>> +	}
>
> Why do you need to increase the refcount? You made a full copy right?

Yes, but I don't want this copy to go away while we have other caches 
around.

So, in the memcg internals, I used a different reference counter, to 
avoid messing with this one. I could use that, and leave the original 
refcnt alone. Would you prefer this?
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