On Thu 27-06-19 17:16:04, Waiman Long wrote:

On 6/27/19 11:15 AM, Michal Hocko wrote:

quoted

On Mon 24-06-19 13:42:19, Waiman Long wrote:

quoted

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)?
-- 
Michal Hocko
SUSE Labs