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Some controllers (like the CPU controller) have a performance cost that is non-trivial on certain workloads. While this can be mitigated and improved to an extent, there will for some controllers always be some overheads associated with the benefits gained from the controller. Inside Facebook, the fix applied has been to disable the CPU controller forcibly with `cgroup_disable=cpu` on the kernel command line. This presents a problem: to disable or reenable the controller, a reboot is required, but this is quite cumbersome and slow to do for many thousands of machines, especially machines where disabling/enabling a stateful service on a machine is a matter of several minutes. Currently systemd provides some configuration knobs for these in the form of `[Default]CPUAccounting`, `[Default]MemoryAccounting`, and the like. The limitation of these is that Default*Accounting is overrideable by individual services, of which any one could decide to reenable a controller within the hierarchy at any point just by using a controller feature implicitly (eg. `CPUWeight`), even if the use of that CPU feature could just be opportunistic. Since many services are provided by the distribution, or by upstream teams at a particular organisation, it's not a sustainable solution to simply try to find and remove offending directives from these units. This commit presents a more direct solution -- a DisableControllers= directive that forcibly disallows a controller from being enabled within a subtree.
systemd - System and Service Manager
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