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

[Kamezawa Hiroyuki]

(2012/10/12 17:41), Michal Hocko wrote:
> On Fri 12-10-12 11:47:17, Glauber Costa wrote:
>> 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 at parallels.com>
>>>> CC: Kamezawa Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
>>>> CC: Christoph Lameter <cl at linux.com>
>>>> CC: Pekka Enberg <penberg at cs.helsinki.fi>
>>>> CC: Michal Hocko <mhocko at suse.cz>
>>>> CC: Johannes Weiner <hannes at cmpxchg.org>
>>>> CC: Suleiman Souhlal <suleiman at 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 at 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() ?
>
> memcg_kmem_test_and_clear_dead? I know long but at least clear that the
> flag is cleared. Or just open code it.
>

I agree. Ack by me with that naming.

Thanks,
-Kame