The search for the correct amount of split-lock misery

Not sure cgroups are appropriate either, since the kernel cannot isolate the impact of the split-locking only to the members of the cgroup. I guess you could make some mechanism like "at most N split locks per second for this cgroup, after that split lock usage will be throttled", though I'm not sure if it would actually be useful for any practical situation. The sysctl knob sounds much simpler and should cover most usecases.