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

From: Michal Hocko
Date: Fri Jun 28 2019 - 03:31:34 EST

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)?
Michal Hocko