Re: [RFC PATCH] memcg: expose children memory usage for root

[Yosry Ahmed <yosryahmed@google.com>]

On Fri, Jul 26, 2024 at 10:30 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Fri, Jul 26, 2024 at 09:25:27AM GMT, Yosry Ahmed wrote:
> > On Fri, Jul 26, 2024 at 8:48 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > >
> [...]
> > >
> > > No, the workloads running in non-root memcgs will not see any
> > > difference. Only the workloads running in root will see charging
> > > overhead.
> >
> > Oh yeah we already charge the root's page counters hierarchically in
> > the upstream kernel, we just do not charge them if the origin of the
> > charge is the root itself.
> >
> > We also have workloads that iterate top-level memcgs to calculate the
> > total charged memory, so memory.children_usage for the root memcg
> > would help.
> >
> > As for memory.current, do you have any data about how much memory is
> > charged to the root itself? We think of the memory charged to the root
> > as system overhead, while the memory charged to top-level memcgs
> > isn't.
> >
> > So basically total_memory - root::memory.children_usage would be a
> > fast way to get a rough estimation of system overhead. The same would
> > not apply for total_memory - root::memory.current if I understand
> > correctly.
>
> Please note that root::memory.children_usage will have top level zombies
> included as well (at least until lru reparenting is not done). So, for
> your example it would provide good estimation of top level zombie memory
> through root::memory.children_usage - top_level(alive)::memory.current.

Good point. The fact that it includes the top-level zombies makes it
less valuable for this use case, as zombie memory is considered system
overhead as well. So we need to iterate the top level memcgs anyway.