How do I report the max RSS of a cgroup?

I would like to monitor the peak RSS a cgroup has used since it was created. By "peak RSS" I mean: the sum of all processes' RSS at the point in time where that sum is the greatest. I believe memory.max_usage_in_bytes reports RSS+CACHE, and AFAIK there isn't a "max_cache_in_bytes" metric that I can use to derive a "max_rss_in_bytes". Basically: I would love the cgroup equivalent of time -f %M. Does anyone know of a way?