[Devel] Re: [PATCH v4 10/14] memcg: use static branches when code not in use

[Kamezawa Hiroyuki]

(2012/10/12 16:47), Glauber Costa wrote:
> On 10/11/2012 05:40 PM, Michal Hocko wrote:
>> On Mon 08-10-12 14:06:16, Glauber Costa wrote:
>>> We can use static branches to patch the code in or out when not used.
>>>
>>> Because the _ACTIVE bit on kmem_accounted is only set after the
>>> increment is done, we guarantee that the root memcg will always be
>>> selected for kmem charges until all call sites are patched (see
>>> memcg_kmem_enabled).  This guarantees that no mischarges are applied.
>>>
>>> static branch decrement happens when the last reference count from the
>>> kmem accounting in memcg dies. This will only happen when the charges
>>> drop down to 0.
>>>
>>> When that happen, we need to disable the static branch only on those
>>> memcgs that enabled it. To achieve this, we would be forced to
>>> complicate the code by keeping track of which memcgs were the ones
>>> that actually enabled limits, and which ones got it from its parents.
>>>
>>> It is a lot simpler just to do static_key_slow_inc() on every child
>>> that is accounted.
>>>
>>> [ v4: adapted this patch to the changes in kmem_accounted ]
>>>
>>> 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>
>>
>> Looks reasonable to me
>> Acked-by: Michal Hocko <mhocko at suse.cz>
>>
>> Just a little nit.
>>
>> [...]
>>
>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>> index 634c7b5..724a08b 100644
>>> --- a/mm/memcontrol.c
>>> +++ b/mm/memcontrol.c
>>> @@ -344,11 +344,15 @@ struct mem_cgroup {
>>>   /* internal only representation about the status of kmem accounting. */
>>>   enum {
>>>   	KMEM_ACCOUNTED_ACTIVE = 0, /* accounted by this cgroup itself */
>>> +	KMEM_ACCOUNTED_ACTIVATED, /* static key enabled. */
>>>   	KMEM_ACCOUNTED_DEAD, /* dead memcg, pending kmem charges */
>>>   };
>>>
>>> -/* first bit */
>>> -#define KMEM_ACCOUNTED_MASK 0x1
>>> +/*
>>> + * first two bits. We account when limit is on, but only after
>>> + * call sites are patched
>>> + */
>>> +#define KMEM_ACCOUNTED_MASK 0x3
>>
>> The names are long but why not use KMEM_ACCOUNTED_ACTIVE*
>> #define KMEM_ACCOUNTED_MASK 1<<KMEM_ACCOUNTED_ACTIVE | 1<<KMEM_ACCOUNTED_ACTIVATED
>>
> Because the names are long! =)
>

please use "long" macros ;) it's not bad.

Anyway,

Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>