Re: [PATCH v1 net-next 13/13] net-memcg: Allow decoupling memcg from global protocol memory accounting.

[Kuniyuki Iwashima <kuniyu@google.com>]

On Tue, Jul 29, 2025 at 7:22 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Mon, Jul 28, 2025 at 02:41:38PM -0700, Kuniyuki Iwashima wrote:
> > On Mon, Jul 28, 2025 at 9:07 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > >
> > > On Mon, Jul 21, 2025 at 08:35:32PM +0000, Kuniyuki Iwashima wrote:
> > > > Some protocols (e.g., TCP, UDP) implement memory accounting for socket
> > > > buffers and charge memory to per-protocol global counters pointed to by
> > > > sk->sk_proto->memory_allocated.
> > > >
> > > > When running under a non-root cgroup, this memory is also charged to the
> > > > memcg as sock in memory.stat.
> > > >
> > > > Even when memory usage is controlled by memcg, sockets using such protocols
> > > > are still subject to global limits (e.g., /proc/sys/net/ipv4/tcp_mem).
> > > >
> > > > This makes it difficult to accurately estimate and configure appropriate
> > > > global limits, especially in multi-tenant environments.
> > > >
> > > > If all workloads were guaranteed to be controlled under memcg, the issue
> > > > could be worked around by setting tcp_mem[0~2] to UINT_MAX.
> > > >
> > > > In reality, this assumption does not always hold, and a single workload
> > > > that opts out of memcg can consume memory up to the global limit,
> > > > becoming a noisy neighbour.
> > >
> > > Yes, an uncontrolled cgroup can consume all of a shared resource and
> > > thereby become a noisy neighbor. Why is network memory special?
> > >
> > > I assume you have some other mechanisms for curbing things like
> > > filesystem caches, anon memory, swap etc. of such otherwise
> > > uncontrolled groups, and this just happens to be your missing piece.
> >
> > I think that's the tcp_mem[] knob, limiting tcp mem globally for
> > the "uncontrolled" cgroup.  But we can't use it because the
> > "controlled" cgroup is also limited by this knob.
>
> No, I was really asking what you do about other types of memory
> consumed by such uncontrolled cgroups.
>
> You can't have uncontrolled groups and complain about their resource
> consumption.

Only 10% of physical memory is allowed to be used globally for TCP.
How is it supposed to work if we don't enforce limits on uncontrolled
cgroups ?