You are changing semantic of the existing user interface. This knob has
never been memcg aware and it is supposed to have a global impact. I do
not think we can simply change that without some users being surprised
or even breaking them.

Yes, do you think add new interface to sysfs is a good way? such as 
/sys/fs/cgroup/memory/lxc/i-vbe1u8o7/memory.kmem.drop_caches

I will rewrite this patch later.