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

Glauber Costa
2012-10-12T07:47:31Z

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@parallels.com>
>> CC: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>> CC: Christoph Lameter <cl@linux.com>
>> CC: Pekka Enberg <penberg@cs.helsinki.fi>
>> CC: Michal Hocko <mhocko@suse.cz>
>> CC: Johannes Weiner <hannes@cmpxchg.org>
>> CC: Suleiman Souhlal <suleiman@google.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@suse.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() ?