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

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Message-ID: <CAAVpQUDRccLyZyaz1iKABHNaw5rHfTBHtrOypmHheOJSaVORLQ@mail.gmail.com>
Date: Tue, 29 Jul 2025 12:41:01 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: Johannes Weiner <hannes@...xchg.org>
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 Tue, Jul 29, 2025 at 7:22 AM Johannes Weiner <hannes@...xchg.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@...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.

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 ?