Re: [PATCH v2 2/3] mm: Force update of mem cgroup soft limit tree on usage excess

[Tim Chen]

On 2/22/21 9:41 AM, Tim Chen wrote:
> 
> 
> On 2/22/21 12:40 AM, Michal Hocko wrote:
>> On Fri 19-02-21 10:59:05, Tim Chen wrote:
>  occurrence.
>>>>
>>>> Soft limit is evaluated every THRESHOLDS_EVENTS_TARGET * SOFTLIMIT_EVENTS_TARGET.
>>>> If all events correspond with a newly charged memory and the last event
>>>> was just about the soft limit boundary then we should be bound by 128k
>>>> pages (512M and much more if this were huge pages) which is a lot!
>>>> I haven't realized this was that much. Now I see the problem. This would
>>>> be a useful information for the changelog.
>>>>
>>>> Your fix is focusing on the over-the-limit boundary which will solve the
>>>> problem but wouldn't that lead to to updates happening too often in
>>>> pathological situation when a memcg would get reclaimed immediatelly?
>>>
>>> Not really immediately.  The memcg that has the most soft limit excess will
>>> be chosen for page reclaim, which is the way it should be.  
>>> It is less likely that a memcg that just exceeded
>>> the soft limit becomes the worst offender immediately. 
>>
>> Well this all depends on when the the soft limit reclaim triggeres. In
>> other words how often you see the global memory reclaim. If we have a
>> memcg with a sufficient excess then this will work mostly fine. I was more
>> worried about a case when you have memcgs just slightly over the limit
>> and the global memory pressure is a regular event. You can easily end up
>> bouncing memcgs off and on the tree in a rapid fashion. 
>>
> 
> If you are concerned about such a case, we can add an excess threshold,
> say 4 MB (or 1024 4K pages), before we trigger a forced update. You think
> that will cover this concern?
> 

Michal,

How about modifiying this patch with a threshold? Like the following?

Tim

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