linux-kernel - Re: [PATCH v6 18/29] Allocate memory for memcg caches whenever a new memcg appears

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Date:	Mon, 5 Nov 2012 16:23:30 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Glauber Costa <glommer@...allels.com>
Cc:	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	<kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Tejun Heo <tj@...nel.org>, Michal Hocko <mhocko@...e.cz>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Suleiman Souhlal <suleiman@...gle.com>
Subject: Re: [PATCH v6 18/29] Allocate memory for memcg caches whenever a
 new memcg appears

On Thu,  1 Nov 2012 16:07:34 +0400
Glauber Costa <glommer@...allels.com> wrote:

> Every cache that is considered a root cache (basically the "original" caches,
> tied to the root memcg/no-memcg) will have an array that should be large enough
> to store a cache pointer per each memcg in the system.
> 
> Theoreticaly, this is as high as 1 << sizeof(css_id), which is currently in the
> 64k pointers range. Most of the time, we won't be using that much.
> 
> What goes in this patch, is a simple scheme to dynamically allocate such an
> array, in order to minimize memory usage for memcg caches. Because we would
> also like to avoid allocations all the time, at least for now, the array will
> only grow. It will tend to be big enough to hold the maximum number of
> kmem-limited memcgs ever achieved.
> 
> We'll allocate it to be a minimum of 64 kmem-limited memcgs. When we have more
> than that, we'll start doubling the size of this array every time the limit is
> reached.
> 
> Because we are only considering kmem limited memcgs, a natural point for this
> to happen is when we write to the limit. At that point, we already have
> set_limit_mutex held, so that will become our natural synchronization
> mechanism.
> 
> ...
>
> +static struct ida kmem_limited_groups;

Could use DEFINE_IDA() here

>
> ...
>
>  static int memcg_init_kmem(struct mem_cgroup *memcg, struct cgroup_subsys *ss)
>  {
> +	int ret;
> +
>  	memcg->kmemcg_id = -1;
> -	memcg_propagate_kmem(memcg);
> +	ret = memcg_propagate_kmem(memcg);
> +	if (ret)
> +		return ret;
> +
> +	if (mem_cgroup_is_root(memcg))
> +		ida_init(&kmem_limited_groups);

and zap this?
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