Re: [PATCH v2 3/6] memcg: introduce memsw_accounting_users

[Jeff Liu <jeff.liu@oracle.com>]

On 01/29/2013 05:46 PM, Lord Glauber Costa of Sealand wrote:
> Hi,
> 
> On 01/28/2013 02:54 PM, Jeff Liu wrote:
>> As we don't account the swap stat number for the root_mem_cgroup anymore,
>> here we can just return an invalid CSS ID if there is no non-root memcg
>> is alive.  Also, introduce memsw_accounting_users to track the number of
>> active non-root memcgs.
>>
>> Signed-off-by: Jie Liu <jeff.liu@oracle.com>
>> CC: Glauber Costa <glommer@parallels.com>
>> CC: Michal Hocko <mhocko@suse.cz>
>> CC: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>> CC: Johannes Weiner <hannes@cmpxchg.org>
>> CC: Mel Gorman <mgorman@suse.de>
>> CC: Andrew Morton <akpm@linux-foundation.org>
>> CC: Sha Zhengju <handai.szj@taobao.com>
>>
>> ---
>>  mm/page_cgroup.c |   16 +++++++++++++++-
>>  1 file changed, 15 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
>> index c945254..189fbf5 100644
>> --- a/mm/page_cgroup.c
>> +++ b/mm/page_cgroup.c
>> @@ -336,6 +336,8 @@ struct swap_cgroup {
>>  };
>>  #define SC_PER_PAGE	(PAGE_SIZE/sizeof(struct swap_cgroup))
>>  
>> +static atomic_t memsw_accounting_users = ATOMIC_INIT(0);
>> +
> 
> I am not seeing this being incremented or decremented. I can only guess
> that it comes in later patches. However, they are clearly used as a
> global reference counter.
> 
> This is precisely one of the use cases static branches solve very
> neatly. Did you consider using them?
Yep, I'll try to replace it with the static branches if there are no
objections from the others. :)

Thanks,
-Jeff
> 
> True, they will help a lot more when we are touching hot paths, and swap
> is hardly a hot path.
> 
> However, since one of the main complaints about memcg has been that we
> inflict "death by a thousand cuts", maybe it wouldn't hurt everything
> else being the same.
> 
> Michal and others, do you have any feelings here?
> 

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