Linux Developers Look To Drop RT_GROUP_SCHED As It's "Broken Beyond Repair"

Linux kernel developers are looking at dropping the RT_GROUP_SCHED feature that is used for group scheduling of real-time tasks using cgroups. RT_GROUP_SCHED was designed for cgroups v1 rather than the newer v2 and in the proposed removal patch the feature is described as "broken beyond repair". All major Linux distribution kernels have the feature disabled too. Linutronix engineer Sebastian Andrzej Siewior explained with the proposed removal of RT_GROUP_SCHED: "The RT_GROUP_SCHED is broken beyond repair and it is to not enable it during compile. If enabled it is recommended to disable it at boot time with the rt_group_sched= option. With it enabled it is not possible to set the policy of a task to SCHED_{FIFO|RR} because its cgroup has no runtime assigned. The runtime can not be assigned with cgroup v2. The root group has however all the runtime assigned meaning the global sysctl runtime parameter (sched_rt_runtime_us) can no be reduced because the new bandwidth will be less than what is current available. It is possible to schedule SCHED_DEADLINE tasks but the admission control is stuck can only be disabled via -1 (same sysctl parameter). As a result all major distors (Debian, Open Suse, Fedora, Archlinux) have it disabled. I don't think it is needed to somehow fix RT_GROUP_SCHED. The DL fair server already handles the RT throttling mechanism. There is work on Hierarchical constant bandwidth server (HCBS) which aim to group multiple real-time tasks within one deadline configuration. Remove CONFIG_RT_GROUP_SCHED." The patch will likely move forward and ultimately reach mainline for dropping this broken functionality. In response to the patch, it was raised there are apparently some unspecified users left of the code. It was also raised whether the option should first be deprecated in the kernel a while prior to its removal. We'll see, but per the mailing list there is some "love" to get RT_GROUP_SCHED removed.