Re: [PATCH v2 3/3] mm: memcg: add swapcache stat for memcg v2

[Roman Gushchin <guro@fb.com>]

On Fri, Jan 08, 2021 at 07:58:13AM -0800, 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.
> 
> The reasons these applications are still interested in this approximate
> memsw usage are: (1) these applications are not really interested in two
> separate memory and swap usage metrics. A single usage metric is more
> simple to use and reason about for them.
> 
> (2) The memsw usage metric hides the underlying system's swap setup from
> the applications. Applications with multiple instances running in a
> datacenter with heterogeneous systems (some have swap and some don't)
> will keep seeing a consistent view of their usage.
> 
> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> Acked-by: Michal Hocko <mhocko@suse.com>

Reviewed-by: Roman Gushchin <guro@fb.com>

Thanks!