linux-kernel - Re: [PATCH v4 09/14] memcg: kmem accounting lifecycle management

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Message-ID: <5077CB05.907@parallels.com>
Date:	Fri, 12 Oct 2012 11:47:17 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Tejun Heo <tj@...nel.org>, <cgroups@...r.kernel.org>,
	<kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Greg Thelen <gthelen@...gle.com>, <devel@...nvz.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH v4 09/14] memcg: kmem accounting lifecycle management

On 10/11/2012 05:11 PM, Michal Hocko wrote:
> On Mon 08-10-12 14:06:15, Glauber Costa wrote:
>> Because kmem charges can outlive the cgroup, we need to make sure that
>> we won't free the memcg structure while charges are still in flight.
>> For reviewing simplicity, the charge functions will issue
>> mem_cgroup_get() at every charge, and mem_cgroup_put() at every
>> uncharge.
>>
>> This can get expensive, however, and we can do better. mem_cgroup_get()
>> only really needs to be issued once: when the first limit is set. In the
>> same spirit, we only need to issue mem_cgroup_put() when the last charge
>> is gone.
>>
>> We'll need an extra bit in kmem_accounted for that: KMEM_ACCOUNTED_DEAD.
>> it will be set when the cgroup dies, if there are charges in the group.
>> If there aren't, we can proceed right away.
>>
>> Our uncharge function will have to test that bit every time the charges
>> drop to 0. Because that is not the likely output of
>> res_counter_uncharge, this should not impose a big hit on us: it is
>> certainly much better than a reference count decrease at every
>> operation.
>>
>> [ v3: merged all lifecycle related patches in one ]
>>
>> Signed-off-by: Glauber Costa <glommer@...allels.com>
>> CC: Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
>> CC: Christoph Lameter <cl@...ux.com>
>> CC: Pekka Enberg <penberg@...helsinki.fi>
>> CC: Michal Hocko <mhocko@...e.cz>
>> CC: Johannes Weiner <hannes@...xchg.org>
>> CC: Suleiman Souhlal <suleiman@...gle.com>
> 
> OK, I like the optimization. I have just one comment to the
> memcg_kmem_dead naming but other than that
> 
> Acked-by: Michal Hocko <mhocko@...e.cz>
> 
> [...]
>> +static bool memcg_kmem_dead(struct mem_cgroup *memcg)
> 
> The name is tricky because it doesn't tell you that it clears the flag
> which made me scratch my head when reading comment in kmem_cgroup_destroy
> 
memcg_kmem_finally_kill_that_bastard() ?

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