Re: [kuba@kernel.org: Re: [PATCH net-next v2 0/3] net: introduce rps_default_mask]

[Marcel Apfelbaum <mapfelba@redhat.com>]

Hi Daniel,

On Fri, Nov 20, 2020 at 11:56 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> On 11/20/20 6:39 PM, Marcel Apfelbaum wrote:
> > +netdev
> > [Sorry for the second email, I failed to set the text-only mode]
> > On Fri, Nov 20, 2020 at 7:30 PM Marcel Apfelbaum <mapfelba@redhat.com> wrote:
> [...]
> >>> ---------- Forwarded message ----------
> >>> From: Jakub Kicinski <kuba@kernel.org>
> >>> To: Paolo Abeni <pabeni@redhat.com>
> >>> Cc: Saeed Mahameed <saeed@kernel.org>, netdev@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>, "David S. Miller" <davem@davemloft.net>, Shuah Khan <shuah@kernel.org>, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, Marcelo Tosatti <mtosatti@redhat.com>, Daniel Borkmann <daniel@iogearbox.net>
> >>> Bcc:
> >>> Date: Wed, 4 Nov 2020 11:42:26 -0800
> >>> Subject: Re: [PATCH net-next v2 0/3] net: introduce rps_default_mask
> >>> On Wed, 04 Nov 2020 18:36:08 +0100 Paolo Abeni wrote:
> >>>> On Tue, 2020-11-03 at 08:52 -0800, Jakub Kicinski wrote:
> >>>>> On Tue, 03 Nov 2020 16:22:07 +0100 Paolo Abeni wrote:
> >>>>>> The relevant use case is an host running containers (with the related
> >>>>>> orchestration tools) in a RT environment. Virtual devices (veths, ovs
> >>>>>> ports, etc.) are created by the orchestration tools at run-time.
> >>>>>> Critical processes are allowed to send packets/generate outgoing, it gets a network-interface
> >>>> upstart job just as it does on a real host.
> >>>>
> >>>>>> network traffic - but any interrupt is moved away from the related
> >>>>>> cores, so that usual incoming network traffic processing does not
> >>>>>> happen there.
> >>>>>>
> >>>>>> Still an xmit operation on a virtual devices may be transmitted via ovs
> >>>>>> or veth, with the relevant forwarding operation happening in a softirq
> >>>>>> on the same CPU originating the packet.
> >>>>>>
> >>>>>> RPS is configured (even) on such virtual devices to move away the
> >>>>>> forwarding from the relevant CPUs.
> >>>>>>
> >>>>>> As Saeed noted, such configuration could be possibly performed via some
> >>>>>> user-space daemon monitoring network devices and network namespaces
> >>>>>> creation. That will be anyway prone to some race: the orchestation tool
> >>>>>> may create and enable the netns and virtual devices before the daemon
> >>>>>> has properly set the RPS mask.
> >>>>>>
> >>>>>> In the latter scenario some packet forwarding could still slip in the
> >>>>>> relevant CPU, causing measurable latency. In all non RT scenarios the
> >>>>>> above will be likely irrelevant, but in the RT context that is not
> >>>>>> acceptable - e.g. it causes in real environments latency above the
> >>>>>> defined limits, while the proposed patches avoid the issue.
> >>>>>>
> >>>>>> Do you see any other simple way to avoid the above race?
> >>>>>>
> >>>>>> Please let me know if the above answers your doubts,
> >>>>>
> >>>>> Thanks, that makes it clearer now.
> >>>>>
> >>>>> Depending on how RT-aware your container management is it may or may not
> >>>>> be the right place to configure this, as it creates the veth interface.
> >>>>> Presumably it's the container management which does the placement of
> >>>>> the tasks to cores, why is it not setting other attributes, like RPS?
> >>
> >> The CPU isolation is done statically at system boot by setting Linux kernel parameters,
> >> So the container management component, in this case the Machine Configuration Operator (for Openshift)
> >> or the K8s counterpart can't really help. (actually they would help if a global RPS mask would exist)
> >>
> >> I tried to tweak the rps_cpus mask using the container management stack, but there
> >> is no sane way to do it, please let me get a little into the details.
> >>
> >> The k8s orchestration component that deals with injecting the network device(s) into the
> >> container is CNI, which is interface based and implemented by a lot of plugins, making it
> >> hardly feasible to go over all the existing plugins and change them. Also what about
> >> the 3rd party ones?
> >>
> >> Writing a new CNI plugin and chain it into the existing one is also not an option AFAIK,
> >> they work at the network level and do not have access to sysfs (they handle the network namespaces).
> >> Even if it would be possible (I don't have a deep CNI understanding), it will require a cluster global configuration
> >> that is actually needed only for some of the cluster nodes.
>
> CNI chaining would be ugly, agree, but in a typical setting you'd have the CNI plugin
> itself which is responsible to set up the Pod for communication to the outside world;
> part of it would be creation of devices and moving them into the target netns, and
> then you also typically have an agent running in kube-system namespace in the hostns
> to which the CNI plugin talks to via IPC e.g. to set up IPAM and other state. Such
> agent usually sets up all sort of knobs for the networking layer upon bootstrap.

The main issue is that CNI is networking related, but the way to set
the RPS is by writing to /sys which is not considered network namespace
related and is read only inside the containers.

> Assuming you have a cluster where only some of the nodes have RT kernel, these would
> likely have special node annotations in K8s so you could select them to run certain
> workloads on it.. couldn't such agent be taught to be RT-aware and set up all the
> needed knobs?

I do agree this part may be doable, sadly is by far not the biggest problem.

> Agree it's a bit ugly to change the relevant CNI plugins to be RT-aware,
> but what if you also need other settings in future aside from RPS mask for RT? At some
> point you'd likely end up having to extend these anyway, no?
>

All networking changes are fair play, however setting the RPS mask
is related to networking but not a networking operation per se - is a
cross-domain operation (network namespace/mount namespace).

Thank you for your response,
Marcel

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