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

[Johannes Weiner <hannes@cmpxchg.org>]

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.