Can we guestimate the free objects based on the number of partial slabs. That number is available.
Yeah, the number of partial slabs is easy to know, but I can't think of a way to
estimate the free objects, since __slab_free() is just double cmpxchg in most cases.
How accurate need the accounting be? We also have fuzzy accounting in the VM counters.
Maybe not need to be very accurate, some delay/fuzzy should be acceptable.
Another direction I think is that we don't distinguish slabs on cpu partial list or
slabs on node partial list anymore (different with current behavior).
Now we have three scopes:
1. SL_ALL: include all slabs
2. SL_PARTIAL: only include partial slabs on node
3. SL_CPU: only include partail slabs on cpu and the using cpu slab
If we change SL_PARTIAL to mean all partial slabs, it maybe simpler.