Re: [PATCH v2 2/5] memcg: provide root figures from system totals

[Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

(2013/03/13 15:58), Sha Zhengju wrote:
> On Wed, Mar 6, 2013 at 6:59 PM, Kamezawa Hiroyuki
> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>> (2013/03/06 19:52), Glauber Costa wrote:
>>> On 03/06/2013 02:45 PM, Kamezawa Hiroyuki wrote:
>>>> (2013/03/06 17:30), Glauber Costa wrote:
>>>>> On 03/06/2013 04:27 AM, Kamezawa Hiroyuki wrote:
>>>>>> (2013/03/05 22:10), Glauber Costa wrote:
>>>>>>> + case _MEMSWAP: {
>>>>>>> +         struct sysinfo i;
>>>>>>> +         si_swapinfo(&i);
>>>>>>> +
>>>>>>> +         return ((memcg_read_root_rss() +
>>>>>>> +         atomic_long_read(&vm_stat[NR_FILE_PAGES])) << PAGE_SHIFT) +
>>>>>>> +         i.totalswap - i.freeswap;
>>>>>>
>>>>>> How swapcache is handled ? ...and How kmem works with this calc ?
>>>>>>
>>>>> I am ignoring kmem, because we don't account kmem for the root cgroup
>>>>> anyway.
>>>>>
>>>>> Setting the limit is invalid, and we don't account until the limit is
>>>>> set. Then it will be 0, always.
>>>>>
>>>>> For swapcache, I am hoping that totalswap - freeswap will cover
>>>>> everything swap related. If you think I am wrong, please enlighten me.
>>>>>
>>>>
>>>> i.totalswap - i.freeswap = # of used swap entries.
>>>>
>>>> SwapCache can be rss and used swap entry at the same time.
>>>>
>>>
>>> Well, yes, but the rss entries would be accounted for in get_mm_rss(),
>>> won't they ?
>>>
>>> What am I missing ?
>>
>>
>> I think the correct caluculation is
>>
>>    Sum of all RSS + All file caches + (i.total_swap - i.freeswap - # of mapped SwapCache)
>>
>>
>> In the patch, mapped SwapCache is counted as both of rss and swap.
>>
>
> After a quick look, swapcache is counted as file pages and meanwhile
> use a swap entry at the same time(__add_to{delete_from}_swap_cache()).
> Even though, I think we still do not need to exclude swapcache out,
> because it indeed uses two copy of resource: one is swap entry, one is
> cache, so the usage should count both of them in.
>
> What I think it matters is that swapcache may be counted as both file
> pages and rss(if it's a process's anonymous page), which we need to
> subtract # of swapcache to avoid double-counting. But it isn't always
> so: a shmem/tmpfs page may use swapcache and be counted as file pages
> but not a rss, then we can not subtract swapcache... Is there anything
> I lost?
>


Please don't think difficult. All pages for user/caches are counted in
LRU. All swap-entry usage can be cauht by total_swap_pages - nr_swap_pages.
We just need to subtract number of swap-cache which is double counted
as swap-entry and a page in LRU.

NR_ACTIVE_ANON + NR_INACTIVE_ANON + NR_ACTIVE_FILE + NR_INACTIVE_FILE
+ NR_UNEVICTABLE + total_swap_pages - nr_swap_pages - NR_SWAP_CACHE

is the number we whant for memsw.usage_in_bytes.

Thanks,
-Kame




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