On Thu 31-12-20 18:39:55, Shakeel Butt wrote:

This patch adds swapcache stat for the cgroup v2. The swapcache
represents the memory that is accounted against both the memory and the
swap limit of the cgroup. The main motivation behind exposing the
swapcache stat is for enabling users to gracefully migrate from cgroup
v1's memsw counter to cgroup v2's memory and swap counters.

Cgroup v1's memsw limit allows users to limit the memory+swap usage of a
workload but without control on the exact proportion of memory and swap.
Cgroup v2 provides separate limits for memory and swap which enables
more control on the exact usage of memory and swap individually for the
workload.

With some little subtleties, the v1's memsw limit can be switched with
the sum of the v2's memory and swap limits. However the alternative for
memsw usage is not yet available in cgroup v2. Exposing per-cgroup
swapcache stat enables that alternative. Adding the memory usage and
swap usage and subtracting the swapcache will approximate the memsw
usage. This will help in the transparent migration of the workloads
depending on memsw usage and limit to v2' memory and swap counters.

Could you expand a bit more on why memsw usage is important even in
cgroup v2 land? How are you going to use the approximated value?

I am not really objecting to adding this counter. We do export it for
the global case and having a memcg view sounds useful for analysis.
-- 
Michal Hocko
SUSE Labs