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 22, 2025 at 12:56 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Tue, Jul 22, 2025 at 12:03:48PM -0700, Kuniyuki Iwashima wrote:
> > On Tue, Jul 22, 2025 at 11:48 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > >
> > > On Tue, Jul 22, 2025 at 11:18:40AM -0700, Kuniyuki Iwashima wrote:
> > > > >
> > > > > I expect this state of jobs with different network accounting config
> > > > > running concurrently is temporary while the migrationg from one to other
> > > > > is happening. Please correct me if I am wrong.
> > > >
> > > > We need to migrate workload gradually and the system-wide config
> > > > does not work at all.  AFAIU, there are already years of effort spent
> > > > on the migration but it's not yet completed at Google.  So, I don't think
> > > > the need is temporary.
> > > >
> > >
> > > From what I remembered shared borg had completely moved to memcg
> > > accounting of network memory (with sys container as an exception) years
> > > ago. Did something change there?
> >
> > AFAICS, there are some workloads that opted out from memcg and
> > consumed too much tcp memory due to tcp_mem=UINT_MAX, triggering
> > OOM and disrupting other workloads.
> >
>
> What were the reasons behind opting out? We should fix those
> instead of a permanent opt-out option.
>
> > >
> > > > >
> > > > > My main concern with the memcg knob is that it is permanent and it
> > > > > requires a hierarchical semantics. No need to add a permanent interface
> > > > > for a temporary need and I don't see a clear hierarchical semantic for
> > > > > this interface.
> > > >
> > > > I don't see merits of having hierarchical semantics for this knob.
> > > > Regardless of this knob, hierarchical semantics is guaranteed
> > > > by other knobs.  I think such semantics for this knob just complicates
> > > > the code with no gain.
> > > >
> > >
> > > Cgroup interfaces are hierarchical and we want to keep it that way.
> > > Putting non-hierarchical interfaces just makes configuration and setup
> > > hard to reason about.
> >
> > Actually, I tried that way in the initial draft version, but even if the
> > parent's knob is 1 and child one is 0, a harmful scenario didn't come
> > to my mind.
> >
>
> It is not just about harmful scenario but more about clear semantics.
> Check memory.zswap.writeback semantics.

zswap checks all parent cgroups when evaluating the knob, but
this is not an option for the networking fast path as we cannot
check them for every skb, which will degrade the performance.

Also, we don't track which sockets were created with the knob
enabled and how many such sockets are still left under the cgroup,
there is no way to keep options consistent throughout the hierarchy
and no need to try hard to make the option pretend to be consistent
if there's no real issue.


>
> >
> > >
> > > >
> > > > >
> > > > > I am wondering if alternative approches for per-workload settings are
> > > > > explore starting with BPF.
> > > > >
> > >
> > > Any response on the above? Any alternative approaches explored?
> >
> > Do you mean flagging each socket by BPF at cgroup hook ?
>
> Not sure. Will it not be very similar to your current approach? Each
> socket is associated with a memcg and the at the place where you need to
> check which accounting method to use, just check that memcg setting in
> bpf and you can cache the result in socket as well.

The socket pointer is not writable by default, thus we need to add
a bpf helper or kfunc just for flipping a single bit.  As said, this is
overkill, and per-memcg knob is much simpler.


>
> >
> > I think it's overkill and we don't need such finer granularity.
> >
> > Also it sounds way too hacky to use BPF to correct the weird
> > behaviour from day0.
>
> What weird behavior? Two accounting mechanisms. Yes I agree but memcgs
> with different accounting mechanisms concurrently is also weird.

Not that weird given the root cgroup does not allocate sk->sk_memcg
and are subject to the global tcp memory accounting.  We already have
a mixed set of memcgs.

Also, not every cgroup sets memory limits.  systemd puts some
processes to a non-root cgroup by default without setting memory.max.
In such a case we definitely want the global memory accounting to take
place.

Having to set memory.max to every non-root cgroup is less flexible
and too restricted.