Re: [PATCH bpf-next v2 00/12] bpf: Introduce selectable memcg for bpf map

[Yafang Shao <laoar.shao@gmail.com>]

On Fri, Aug 19, 2022 at 6:33 AM Tejun Heo <tj@kernel.org> wrote:
>
> On Thu, Aug 18, 2022 at 12:20:33PM -1000, Tejun Heo wrote:
> > We have the exact same problem for any resources which span multiple
> > instances of a service including page cache, tmpfs instances and any other
> > thing which can persist longer than procss life time. My current opinion is
>
> To expand a bit more on this point, once we start including page cache and
> tmpfs, we now get entangled with memory reclaim which then brings in IO and
> not-yet-but-eventually CPU usage.

Introduce-a-new-layer vs introduce-a-new-cgroup, which one is more overhead?

> Once you start splitting the tree like
> you're suggesting here, all those will break down and now we have to worry
> about how to split resource accounting and control for the same entities
> across two split branches of the tree, which doesn't really make any sense.
>

The k8s has already been broken thanks to the memcg accounting on  bpf memory.
If you ignored it, I paste it below.
[0]"1. The memory usage is not consistent between the first generation and
new generations."

This issue will persist even if you introduce a new layer.

> So, we *really* don't wanna paint ourselves into that kind of a corner. This
> is a dead-end. Please ditch it.
>

It makes non-sensen to ditch it.
Because, the hierarchy I described in the commit log is *one* use case
of the selectable memcg, but not *the only one* use case of it. If you
dislike that hierarchy, I will remove it to avoid misleading you.

Even if you introduce a new layer, you still need the selectable memcg.
For example, to avoid the issue I described in [0],  you still need to
charge to the parent cgroup instead of the current cgroup.

That's why I described in the commit log that the selectable memcg is flexible.

-- 
Regards
Yafang