Re: [PATCH bpf-next v1 5/5] bpf: add a selftest for cgroup hierarchical stats collection

[Michal Koutný]

On Fri, Jun 03, 2022 at 12:52:27PM -0700, Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> Good catch. I get confused between cgrp->subsys and
> task->cgroups->subsys sometimes because of different fallback
> behavior. IIUC cgrp->subsys should have NULL if the memory controller
> is not enabled (no nearest ancestor fallback), and hence I can use
> memory_subsys_enabled() that I defined just above task_memcg() to test
> for this (I have no idea why I am not already using it here). Is my
> understanding correct?

You're correct, css_set (task->cgroups) has a css (memcg) always defined
(be it root only (or even a css from v1 hierarchy but that should not
relevant here)). A particular cgroup can have the css set to NULL.

When I think about your stats collecting example now, task_memcg() looks
more suitable to achieve proper hierarchical counting in the end (IOW
you'd lose info from tasks who don't reside in memcg-enabled leaf).

(It's just that task_memcg won't return NULL. Unless the kernel is
compiled without memcg support completely, which makes me think how do
the config-dependent values propagate to BPF programs?)

Thanks,
Michal