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

[Eric Dumazet <edumazet@google.com>]

On Tue, Jul 22, 2025 at 8:14 AM Shakeel Butt <shakeel.butt@linux.dev> 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.
> >
>
> Sorry but the above is not reasonable. On a multi-tenant system no
> workload should be able to opt out of memcg accounting if isolation is
> needed. If a workload can opt out then there is no guarantee.

Deployment issue ?

In a multi-tenant system you can not suddenly force all workloads to
be TCP memcg charged. This has caused many OMG.

Also, the current situation of maintaining two limits (memcg one, plus
global tcp_memory_allocated) is very inefficient.

If we trust memcg, then why have an expensive safety belt ?

With this series, we can finally use one or the other limit. This
should have been done from day-0 really.

>
> In addition please avoid adding a per-memcg knob. Why not have system
> level setting for the decoupling. I would say start with a build time
> config setting or boot parameter then if really needed we can discuss if
> system level setting is needed which can be toggled at runtime though
> there might be challenges there.

Built time or boot parameter ? I fail to see how it can be more convenient.