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

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Message-ID: <20250729142246.GF54289@cmpxchg.org>
Date: Tue, 29 Jul 2025 10:22:46 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Kuniyuki Iwashima <kuniyu@...gle.com>
Cc: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Neal Cardwell <ncardwell@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Matthieu Baerts <matttbe@...nel.org>,
	Mat Martineau <martineau@...nel.org>,
	Michal Hocko <mhocko@...nel.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Simon Horman <horms@...nel.org>, Geliang Tang <geliang@...nel.org>,
	Muchun Song <muchun.song@...ux.dev>,
	Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org,
	mptcp@...ts.linux.dev, cgroups@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 net-next 13/13] net-memcg: Allow decoupling memcg from
 global protocol memory accounting.

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@...xchg.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.