[libvirt] Overhead for a default cpu cg placement scheme

Hi Daniel, would it possible to adopt an optional tunable for a virCgroup mechanism which targets to a disablement of a nested (per-thread) cgroup creation? Those are bringing visible overhead for many-threaded guest workloads, almost 5% in non-congested host CPU state, primarily because the host scheduler should make a much more decisions with those cgroups than without them. We also experienced a lot of host lockups with currently exploited cgroup placement and disabled nested behavior a couple of years ago. Though the current patch is simply carves out the mentioned behavior, leaving only top-level per-machine cgroups, it can serve for an upstream after some adaptation, that`s why I`m asking about a chance of its acceptance. This message is a kind of 'request of a feature', it either can be accepted/dropped from our side or someone may give a hand and redo it from scratch. The detailed benchmarks are related to a host 3.10.y, if anyone is interested in the numbers for latest stable, I can update those. Thanks!