linux-kernel - Re: [PATCH v2 08/11] memcg: disable kmem code when not in use.

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Message-ID: <502DEC32.6070807@parallels.com>
Date:	Fri, 17 Aug 2012 11:01:06 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	<cgroups@...r.kernel.org>, <devel@...nvz.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	<kamezawa.hiroyu@...fujitsu.com>, Christoph Lameter <cl@...ux.com>,
	David Rientjes <rientjes@...gle.com>,
	Pekka Enberg <penberg@...nel.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Suleiman Souhlal <suleiman@...gle.com>
Subject: Re: [PATCH v2 08/11] memcg: disable kmem code when not in use.

On 08/17/2012 11:02 AM, Michal Hocko wrote:
> On Thu 09-08-12 17:01:16, Glauber Costa wrote:
>> We can use jump labels to patch the code in or out when not used.
>>
>> Because the assignment: memcg->kmem_accounted = true is done after the
>> jump labels increment, we guarantee that the root memcg will always be
>> selected until all call sites are patched (see memcg_kmem_enabled).
> 
> Not that it would be really important because kmem_accounted goes away
> in a subsequent patch but I think the wording is a bit misleading here.
> First of all there is no guanratee that kmem_accounted=true is seen
> before atomic_inc(&key->enabled) because there is no memory barrier and
> the lock serves just a leave barrier. But I do not think this is
> important at all because key->enabled is what matters here. Even if
> memcg_kmem_enabled is true we do not consider it if the key is disabled,
> right?
> 

Right.

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