Re: [RFC][PATCH 4/14] delay page_cgroup freeing

[Balbir Singh <balbir@linux.vnet.ibm.com>]

KAMEZAWA Hiroyuki wrote:
> On Wed, 27 Aug 2008 06:47:59 +0530
> Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> 
>> KAMEZAWA Hiroyuki wrote:
>>> On Tue, 26 Aug 2008 17:16:20 +0530
>>> Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
>>>
>>>>> +/*
>>>>> + * per-cpu slot for freeing page_cgroup in lazy manner.
>>>>> + * All page_cgroup linked to this list is OBSOLETE.
>>>>> + */
>>>>> +struct mem_cgroup_sink_list {
>>>>> +	int count;
>>>>> +	struct page_cgroup *next;
>>>>> +};
>>>> Can't we reuse the lru field in page_cgroup to build a list? Do we need them on
>>>> the memory controller LRU if they are obsolete? I want to do something similar
>>>> for both additions and deletions - reuse pagevec style, basically. I am OK,
>>>> having a list as well, in that case we can just reuse the LRU pointer.
>>>>
>>> reusing page_cgroup->lru is not a choice because this patch is for avoid
>>> locking on mz->lru_lock (and kfree).
>>> But using vector can be a choice. I'll try in the next version.
>> Kame,
>>
>> Do we need to use the lru_lock? If we do an atomic check on PcgObsolete(), can't
>> we use another lock for obsolete pages and still use the lru list head?
> 
> To reuse that, we'll have to modify lru.prev or lru.next pointer. 
> 
> And there will be race with 
>  - move_list,
>  - isolate_pages,
>  - (new) force_empty
> 

I was suggesting that we could mark the page as obsolete and then move it on to
another queue, if the page_cgroup was marked as obsolete.

> move_list and (new)force_empty modifies lru.prev/lru.next.
> So, I think it's dangerous at this stage. (We can revist this when it's
> necessary (if vector seems bad.)

OK, lets try with the vector and see how that turns out.

> Anyway, I think I'll be able to remove page_cgroup->next pointer I added.
> 

Yes, with the vector you should not need it.

> Thanks,
> -Kame
> 


-- 
	Balbir

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