[libvirt] Overhead for a default cpu cg placement scheme

[Andrey Korolyov]

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!