Re: [PATCH 2/2] mm, slab: Extend vm/drop_caches to shrink kmem slabs

[Waiman Long]

On 6/28/19 3:31 AM, Michal Hocko wrote:
> On Thu 27-06-19 17:16:04, Waiman Long wrote:
>> On 6/27/19 11:15 AM, Michal Hocko wrote:
>>> On Mon 24-06-19 13:42:19, Waiman Long wrote:
>>>> With the slub memory allocator, the numbers of active slab objects
>>>> reported in /proc/slabinfo are not real because they include objects
>>>> that are held by the per-cpu slab structures whether they are actually
>>>> used or not.  The problem gets worse the more CPUs a system have. For
>>>> instance, looking at the reported number of active task_struct objects,
>>>> one will wonder where all the missing tasks gone.
>>>>
>>>> I know it is hard and costly to get a real count of active objects.
>>> What exactly is expensive? Why cannot slabinfo reduce the number of
>>> active objects by per-cpu cached objects?
>>>
>> The number of cachelines that needs to be accessed in order to get an
>> accurate count will be much higher if we need to iterate through all the
>> per-cpu structures. In addition, accessing the per-cpu partial list will
>> be racy.
> Why is all that a problem for a root only interface that should be used
> quite rarely (it is not something that you should be reading hundreds
> time per second, right)?

That can be true. Anyway, I have posted a new patch to use the existing
<slab>/shrink sysfs file to perform memcg cache shrinking as well. So I
am not going to pursue this patch.

Thanks,
Longman