Re: [PATCH] memcg: make threshold index in the right position

[Sha Zhengju <handai.szj@gmail.com>]

On 02/02/2012 06:54 PM, Sha Zhengju wrote:
> On 02/02/2012 06:14 PM, Kirill A. Shutemov wrote:
>> On Thu, Feb 02, 2012 at 05:45:19PM +0800, Sha Zhengju wrote:
>>> From: Sha Zhengju<handai.szj@taobao.com>
>>>
>>> Index current_threshold may point to threshold that just equal to
>>> usage after __mem_cgroup_threshold is triggerd.
>> I don't see it. Could you describe conditions?
>>
> It is because of the following code path in __mem_cgroup_threshold:
> {
>     ...
>         i = t->current_threshold;
>
>         for (; i >= 0 && unlikely(t->entries[i].threshold > usage); i--)
>                 eventfd_signal(t->entries[i].eventfd, 1);
>         i++;
>
>         for (; i < t->size && unlikely(t->entries[i].threshold <= 
> usage); i++)
>                 eventfd_signal(t->entries[i].eventfd, 1);
>
>         t->current_threshold = i - 1;
>     ...
> }
>
> For example:
> now:
>     threshold array:  3  5  7  9   (usage = 6)
>                                    ^
>                                 index
>
> next turn:
>     threshold array:  3  5  7  9   (usage = 7)
>                                        ^
>                                     index
>
> after registering a new event(threshold = 10):
>     threshold array:  3  5  7  9  10 (usage = 7)
>                                    ^
>                                 index
Err.. Sorry for showing inaccurate index position... (may because of the 
mail format)

now:
     threshold array:  3  [5]  7  9   (usage = 6, index = 5)

next turn:
     threshold array:  3  5  [7]  9   (usage = 7, index = 7)

after registering a new event(threshold = 10):
     threshold array:  3  [5]  7  9  10 (usage = 7, index = 5)

>>> But after registering
>>> a new event, it will change (pointing to threshold just below usage).
>>> So make it consistent here.
>>>
>>> Cc: KAMEZAWA Hiroyuki<kamezawa.hiroyu@jp.fujitsu.com>
>>> Cc: Kirill A. Shutemov<kirill@shutemov.name>
>>> Signed-off-by: Sha Zhengju<handai.szj@taobao.com>
>>> ---
>>>   mm/memcontrol.c |    7 ++++---
>>>   1 files changed, 4 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>> index 22d94f5..79f4a58 100644
>>> --- a/mm/memcontrol.c
>>> +++ b/mm/memcontrol.c
>>> @@ -183,7 +183,7 @@ struct mem_cgroup_threshold {
>>>
>>>   /* For threshold */
>>>   struct mem_cgroup_threshold_ary {
>>> -    /* An array index points to threshold just below usage. */
>>> +    /* An array index points to threshold just below or equal to 
>>> usage. */
>>>       int current_threshold;
>>>       /* Size of entries[] */
>>>       unsigned int size;
>>> @@ -4319,14 +4319,15 @@ static int 
>>> mem_cgroup_usage_register_event(struct cgroup *cgrp,
>>>       /* Find current threshold */
>>>       new->current_threshold = -1;
>>>       for (i = 0; i<  size; i++) {
>>> -        if (new->entries[i].threshold<  usage) {
>>> +        if (new->entries[i].threshold<= usage) {
>>>               /*
>>>                * new->current_threshold will not be used until
>>>                * rcu_assign_pointer(), so it's safe to increment
>>>                * it here.
>>>                */
>>>               ++new->current_threshold;
>>> -        }
>>> +        } else
>>> +            break;
>>>       }
>>>
>>>       /* Free old spare buffer and save old primary buffer as spare */
>>> -- 
>>> 1.7.4.1
>>>
>

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