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SubjectRe: kernfs memcg accounting
Hello.

On Mon, May 02, 2022 at 10:37:49PM +0300, Vasily Averin <vvs@openvz.org> wrote:
> I did not understand your statement. Could you please explain it in more details?

Sure, let me expand my perhaps ambiguous and indefinite sentence.

> I see that cgroup_mkdir()->cgroup_create() creates new kernfs node for new
> sub-directory, and with my patch account memory of kernfs node to memcg
> of current process.

Indeed. The variants I'm comparing here are: a) charge to the creator's
memcg, b) charge to the parent (memcg ancestor) of created cgroup.

When struct mem_cgroup charging was introduced, there was a similar
discussion [1].

I can see following aspects here:
1) absolute size of kernfs_objects,
2) practical difference between a) and b),
3) consistency with memcg,
4) v1 vs v2 behavior.

Ad 1) -- normally, I'd treat this as negligible (~120B struct
kernfs_node * there are ~10 of them per subsys * ~10 subsystems ~ 12
KB/cgroup). But I guess the point of this change are exploitative users
where this doesn't hold [2], so absolute size is not so important.

Ad 2) -- in the typical workloads, only top-level cgroup are created by
some management entity and lower level are managed from within, i.e.
there is little difference whom to charge the created objects.

Ad 3) -- struct mem_cgroup objects are charged to their hierarchical
parent, so that dying memcgs can be associated to a subtree which is
where the reclaim can deal with it (in contrast with creator's cgroup).

Now, if I'm looking correctly, the kernfs_node objects are not pinned by
any residual state (subsystems kill_css()->css_clear_dir() synchronously
from rmdir, cgroup itself may be RCU delayed). So the memcg argument
remains purely for consistency (but no practical reason).

Ad 4) -- the variant b) becomes slightly awkward when mkdir'ing a cgroup
in a non-memcg hierarchy (bubbles up to root, despite creator in a
non-root memcg).

How do these reasonings align with your original intention of net
devices accounting? (Are the creators of net devices inside the
container?)


> Do you think it is incorrect and new kernfs node should be accounted
> to memcg of parent cgroup, as mem_cgroup_css_alloc()-> mem_cgroup_alloc() does?

I don't think either variant is incorrect. I'd very much prefer the
consistency with memcg behavior (variant a)) but as I've listed the
arguments above, it seems such a consistency can't be easily justified.


> Perhaps you mean that in this case kernfs should not be counted at all,
> as almost all neighboring allocations do?

No, I think it wouldn't help here [2]. (Or which neighboring allocations
do you mean? There must be at least nr_cgroups of them.)

(Of course, then there's the traditional performance argument, cgroup's
kernfs_node object shouldn't be problematic but I can't judge others
(sysfs) but that's nothing to prevent any form of kernfs_node accounting
going forward in my eyes.)

HTH,
Michal

[1] https://lore.kernel.org/all/20200729171039.GA22229@blackbody.suse.cz/
[2] Unless this could be constraint by something even bigger and
accounted. But only struct mem_cgroup (recursively its percpu stats)
comes to my mind.

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