Re: [RFC 0/5] kernel: Introduce CPU Namespace

Peter Zijlstra
2021-10-09T22:41:46Z

On Sat, Oct 09, 2021 at 08:42:38PM +0530, Pratik R. Sampat wrote:

> Current shortcomings in the prototype:
> --------------------------------------
> 1. Containers also frequently use cfs period and quotas to restrict CPU
> runtime also known as millicores in modern container runtimes.
> The RFC interface currently does not account for this in
> the scheme of things.
> 2. While /proc/stat is now namespace aware and userspace programs like
> top will see the CPU utilization for their view of virtual CPUs;
> if the system or any other application outside the namespace
> bumps up the CPU utilization it will still show up in sys/user time.
> This should ideally be shown as stolen time instead.
> The current implementation plugs into the display of stats rather
> than accounting which causes incorrect reporting of stolen time.
> 3. The current implementation assumes that no hotplug operations occur
> within a container and hence the online and present cpus within a CPU
> namespace are always the same and query the same CPU namespace mask
> 4. As this is a proof of concept, currently we do not differentiate
> between cgroup cpus_allowed and effective_cpus and plugs them into
> the same virtual CPU map of the namespace
> 5. As described in a fair use implication earlier, knowledge of the
> CPU topology can potentially be taken an misused with a flood.
> While scrambling the CPUset in the namespace can help by
> obfuscation of information, the topology can still be roughly figured
> out with the use of IPI latencies to determine siblings or far away
> cores

6. completely destroys and ignores any machine topology information.